A study of peptide--peptide interaction by matrix-assisted laser desorption/ionization

Fluorovinylsulfones and -Sulfonates as Potent Covalent Reversible Inhibitors of the Trypanosomal Cysteine Protease Rhodesain: Structure-Activity Relationship, Inhibition Mechanism, Metabolism, and In Vivo Studies

Rhodesain is a major cysteine protease of Trypanosoma brucei rhodesiense, a pathogen causing Human African Trypanosomiasis, and a validated drug target. Recently, we reported the development of α-halovinylsulfones as a new class of covalent reversible cysteine protease inhibitors. Here, α-fluorovinylsulfones/-sulfonates were optimized for rhodesain based on molecular modeling approaches. 2d, the most potent and selective inhibitor in the series, shows a single-digit nanomolar affinity and high selectivity toward mammalian cathepsins B and L. Enzymatic dilution assays and MS experiments indicate that 2d is a slow-tight binder (K_i = 3 nM). Furthermore, the nonfluorinated 2d-(H) shows favorable metabolism and biodistribution by accumulation in mice brain tissue after intraperitoneal and oral administration. The highest antitrypanosomal activity was observed for inhibitors with an N-terminal 2,3-dihydrobenzo[b][1,4]dioxine group and a 4-Me-Phe residue in P2 (2e/4e) with nanomolar EC₅₀ values (0.14/0.80 μM). The different mechanisms of reversible and irreversible inhibitors were explained using QM/MM calculations and MD simulations.

Kappa opioid receptors mediate an initial aversive component of paclitaxel-induced neuropathy

RATIONALE

Cancer patients receiving the antineoplastic drug paclitaxel report higher incidences and longer duration of treatment-resistant depression than patients receiving other classes of chemotherapeutics. Rodents treated with paclitaxel exhibit a suite of changes in affect-like behaviors. Further, paclitaxel causes chemotherapy-induced peripheral neuropathy (CIPN) in humans and rodents. Kappa opioid receptors (KOR) have a well-established role in depression and neuropathy. The contributions of KOR signaling to paclitaxel-induced aversive-like state and CIPN in rodents remain to be explored.

OBJECTIVES

We aimed to investigate whether dysregulation of the KOR/dynorphin system is associated with paclitaxel-mediated pain-like behavior and depression-like behavior.

METHODS

Cancer-free male C57BL/6J mice were treated with four injections of vehicle or paclitaxel (32 mg/kg cumulative). The effects of the selective KOR antagonist norbinaltorphimine (norBNI) on paclitaxel-induced sucrose preference deficits and mechanical hypersensitivity were measured. Prodynorphin mRNA and receptor-mediated G protein activation were measured at two time points following the last paclitaxel injection using quantitative real-time polymerase chain reaction and agonist-stimulated [³⁵S]guanosine-5'-O'-(γ-thio)-triphosphate ([³⁵S]GTPγS) binding, respectively, in the nucleus accumbens (NAc), caudate-putamen, amygdala, and spinal cord.

RESULTS

Paclitaxel produced a norBNI-reversible sucrose preference deficit, whereas mechanical hypersensitivity was not reversed by norBNI. Paclitaxel treatment increased the levels of mRNA for prodynorphin, a precursor for endogenous KOR agonists, in the NAc. Paclitaxel also had time-dependent effects on KOR-mediated G protein activation in the NAc.

CONCLUSIONS

These results suggest that KOR signaling mediates an initial aversive component of paclitaxel, but not necessarily paclitaxel-induced mechanical hypersensitivity.

Emerging role of clinical mass spectrometry in pathology

Mass spectrometry-based assays have been increasingly implemented in various disciplines in clinical diagnostic laboratories for their combined advantages in multiplexing capacity and high analytical specificity and sensitivity. It is now routinely used in areas including reference methods development, therapeutic drug monitoring, toxicology, endocrinology, paediatrics, immunology and microbiology to identify and quantify biomolecules in a variety of biological specimens. As new ionisation methods, instrumentation and techniques are continuously being improved and developed, novel mass spectrometry-based clinical applications will emerge for areas such as proteomics, metabolomics, haematology and anatomical pathology. This review will summarise the general principles of mass spectrometry and specifically highlight current and future clinical applications in anatomical pathology.

A recombinant fungal defensin-like peptide-P2 combats multidrug-resistant Staphylococcus aureus and biofilms

There is an urgent need to discover new active drugs to combat methicillin-resistant Staphylococcus aureus, which is a serious threat to humans and animals and incompletely eliminated by antibiotics due to its intracellular accumulation in host cells, production of biofilms, and persisters. Fungal defensin-like peptides (DLPs) are emerging as a potential source of new antibacterial drugs due to their potent antibacterial activity. In this study, nine novel fungal DLPs were firstly identified by querying against UniProt databases and expressed in Pichia pastoris, and their antibacterial and anti-biofilm ability were tested against multidrug-resistant (MDR) S. aureus. Results showed that among them, P2, the highest activity and expression level, showed low toxicity, no resistance, and high stability. Minimal inhibitory concentrations (MICs) of P2 against Gram-positive bacteria were < 2 μg/mL. P2 exhibited the potent activity against intracellular MDR S. aureus (bacterial reduction in 80-97%) in RAW264.7 macrophages. P2 bound to/disrupted bacterial DNA, wrinkled outer membranes and permeabilized cytoplasmic membranes, but maintained the integrity of bacterial cells. P2 inhibited/eradicated the biofilm and killed 99% persister bacteria, which were resistant to 100× MIC vancomycin. P2 upregulated the anti-inflammatory cytokine (IL-10) and downregulated pro-inflammatory cytokines (TNF-α/IL-1β) and chemokine (MCP-1) levels in RAW 264.7 macrophages and in mice, respectively. Five milligram per kilogram P2 enhanced the survival of S. aureus-infected mice (100%), superior to vancomycin (30 mg/kg), inhibited the bacterial translocation, and alleviated multiple-organ injuries (liver, spleen, kidney, and lung). These data suggest that P2 may be a candidate for novel antimicrobial agents against MDR staphylococcal infections.

Histidine, the less interactive cousin of arginine

Electrostatic interactions are one of the main factors influencing biomolecular conformation. The formation of noncovalent complexes by electrostatic interactions is governed by certain amino acid residues and post-translational modifications. It has been demonstrated that adjacent arginine forms noncovalent complex with phosphate; however, histidine noncovalent complexes have rarely been investigated. In the present work, we compare the interaction between basic epitopes (NLRRITRVN, SHHGLHSTPD) and diverse acidic and aromatic-rich peptides using both MALDI and ESI Mass spectrometry. We show that adjacent histidines can also form stable noncovalent bonds and that those bonds are probably formed by a salt bridge between the phosphate or the acid residues and the histidines. However, noncovalent complexes with the arginine epitopes form more readily and are stronger than those with histidine-containing epitopes.

Liquid Native MALDI Mass Spectrometry for the Detection of Protein-Protein Complexes

Native mass spectrometry (MS) encompasses methods to keep noncovalent interactions of biomolecular complexes intact in the gas phase throughout the instrument and to measure the mass-to-charge ratios of supramolecular complexes directly in the mass spectrometer. Electrospray ionization (ESI) in nondenaturing conditions is now an established method to characterize noncovalent systems. Matrix-assisted laser desorption/ionization (MALDI), on the other hand, consumes low quantities of samples and largely tolerates contaminants, making it a priori attractive for native MS. However, so-called native MALDI approaches have so far been based on solid deposits, where the rapid transition of the sample through a solid state can engender the loss of native conformations. Here we present a new method for native MS based on liquid deposits and MALDI ionization, unambiguously detecting intact noncovalent protein complexes by direct desorption from a liquid spot for the first time. To control for aggregation, we worked with HUαβ, a heterodimer that does not spontaneously rearrange into homodimers in solution. Screening through numerous matrix solutions to observe first the monomeric protein, then the dimer complex, we settled on a nondenaturing binary matrix solution composed of acidic and basic organic matrices in glycerol, which is stable in vacuo. The role of temporal and spatial laser irradiation patterns was found to be critical. Both a protein-protein and a protein-ligand complex could be observed free of aggregation. To minimize gas-phase dissociation, source parameters were optimized to achieve a conservation of complexes above 50% for both systems. Graphical Abstract ᅟ.

Cracking the Sugar Code by Mass Spectrometry : An Invited Perspective in Honor of Dr. Catherine E. Costello, Recipient of the 2017 ASMS Distinguished Contribution Award

The structural study of glycans and glycoconjugates is essential to assign their roles in homeostasis, health, and disease. Once dominated by nuclear magnetic resonance spectroscopy, mass spectrometric methods have become the preferred toolbox for the determination of glycan structures at high sensitivity. The patterns of such structures in different cellular states now allow us to interpret the sugar codes in health and disease, based on structure-function relationships. Dr. Catherine E. Costello was the 2017 recipient of the American Society for Mass Spectrometry's Distinguished Contribution Award. In this Perspective article, we describe her seminal work in a historical and geographical context and review the impact of her research accomplishments in the field.8 ᅟ Graphical abstract.

Positive and negative ion mode comparison for the determination of DNA/peptide noncovalent binding sites through the formation of "three-body" noncovalent fragment ions

Gas-phase fragmentation of single strand DNA-peptide noncovalent complexes is investigated in positive and negative electrospray ionization modes.Collision-induced dissociation experiments, performed on the positively charged noncovalent complex precursor ions, have confirmed the trend previously observed in negative ion mode, i.e. a high stability of noncovalent complexes containing very basic peptidic residues (i.e. R > K) and acidic nucleotide units (i.e. Thy units), certainly incoming from the existence of salt bridge interactions. Independent of the ion polarity, stable noncovalent complex precursor ions were found to dissociate preferentially through covalent bond cleavages of the partners without disrupting noncovalent interactions. The resulting DNA fragment ions were found to be still noncovalently linked to the peptides. Additionally, the losses of an internal nucleic fragment producing "three-body" noncovalent fragment ions were also observed in both ion polarities, demonstrating the spectacular salt bridge interaction stability. The identical fragmentation patterns (regardless of the relative fragment ion abundances) observed in both polarities have shown a common location of salt bridge interaction certainly preserved from solution. Nonetheless, most abundant noncovalent fragment ions (and particularly three-body ones) are observed from positively charged noncovalent complexes. Therefore, we assume that, independent of the preexisting salt bridge interaction and zwitterion structures, multiple covalent bond cleavages from single-stranded DNA/peptide complexes rely on an excess of positive charges in both electrospray ionization ion polarities.

An Electrospray Ionization Mass Spectrometry Study on the "In Vacuo" Hetero-Oligomers Formed by the Antimicrobial Peptides, Surfactin and Gramicidin S

It was previously observed that the lipopeptide surfactants in surfactin (Srf) have an antagonistic action towards the highly potent antimicrobial cyclodecapeptide, gramicidin S (GS). This study reports on some of the molecular aspects of the antagonism as investigated through complementary electrospray ionization mass spectrometry techniques. We were able to detect stable 1:1 and 2:1 hetero-oligomers in a mixture of surfactin and gramicidin S. The noncovalent interaction between GS and Srf, with the proposed equilibrium: GS~Srf↔GS+Srf correlated to apparent K _d values of 6-9 μM in gas-phase and 1 μM in aqueous solution. The apparent K _d values decreased with a longer incubation time and indicated a slow oligomerization equilibrium. Furthermore, the low μM K _d^app values of GS~Srf↔GS+Srf fell within the biological concentration range and related to the 2- to 3-fold increase in [GS] needed for bacterial growth inhibition in the presence of Srf. Competition studies indicated that neither Na⁺ nor Ca²⁺ had a major effect on the stability of preformed heterodimers and that GS in fact out-competed Ca²⁺ and Na⁺ from Srf. Traveling wave ion mobility mass spectrometry revealed near symmetrical peaks of the heterodimers correlating to a compact dimer conformation that depend on specific interactions. Collision-induced dissociation studies indicated that the peptide interaction is most probably between one Orn residue in GS and the Asp residue, but not the Glu residue in Srf. We propose that flanking hydrophobic residues in both peptides stabilize the antagonistic and inactive peptide hetero-oligomers and shield the specific polar interactions in an aqueous environment. Graphical Abstract ᅟ.

New Insights into the Wavelength Dependence of MALDI Mass Spectrometry

The interplay between the wavelength of the laser and the absorption profile of the matrix constitutes a crucial factor in matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). Numerous studies have shown that typically best analytical results are obtained if the laser wavelength matches the UV absorption band of the matrix in the solid state well. However, many powerful matrices exhibit peak absorptions which differ notably from the standard MALDI laser wavelengths of 337, 349, and 355 nm, respectively. Here we used two wavelength-tunable lasers to investigate the MALDI wavelength dependence with a selected set of such matrices. We studied 3-hydroxypicolinic acid (3-HPA), 2,4,6-trihydroxyacetophenon (THAP), dithranol (1,8-dihydroxy-10H-anthracen-9-on), 2-(4'-hydroxybenzeneazo)benzoic acid (HABA), and 6-aza-2-thiothymine (ATT). For analyte systems we investigated DNA oligomers (3-HPA), phospholipids (dithranol, THAP, HABA), and non-covalent peptide-peptide and protein-peptide complexes (ATT). We recorded analyte ion and total ion counts as a function of wavelength and laser fluence between 213 and 600 nm. Although the so-generated comprehensive heat maps generally corroborated the previously made findings, several fine features became notable. For example, despite a still high optical absorption exhibited by some of the matrices in the visible wavelength range, ion yields generally dropped strongly, indicating a change in ionization mechanism. Moreover, the non-covalent complexes were optimally detected at wavelengths corresponding to a relatively low optical absorptivity of the ATT matrix, presumably because of ejection of a particular cold MALDI plume. Our comprehensive data shed useful light into the MALDI mechanisms and could assist in further methodological advancement of the technique.

Negative Ion In-Source Decay Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry for Sequencing Acidic Peptides

Matrix-assisted laser desorption/ionization (MALDI) in-source decay was studied in the negative ion mode on deprotonated peptides to determine its usefulness for obtaining extensive sequence information for acidic peptides. Eight biological acidic peptides, ranging in size from 11 to 33 residues, were studied by negative ion mode ISD (nISD). The matrices 2,5-dihydroxybenzoic acid, 2-aminobenzoic acid, 2-aminobenzamide, 1,5-diaminonaphthalene, 5-amino-1-naphthol, 3-aminoquinoline, and 9-aminoacridine were used with each peptide. Optimal fragmentation was produced with 1,5-diaminonphthalene (DAN), and extensive sequence informative fragmentation was observed for every peptide except hirudin(54-65). Cleavage at the N-Cα bond of the peptide backbone, producing c' and z' ions, was dominant for all peptides. Cleavage of the N-Cα bond N-terminal to proline residues was not observed. The formation of c and z ions is also found in electron transfer dissociation (ETD), electron capture dissociation (ECD), and positive ion mode ISD, which are considered to be radical-driven techniques. Oxidized insulin chain A, which has four highly acidic oxidized cysteine residues, had less extensive fragmentation. This peptide also exhibited the only charged localized fragmentation, with more pronounced product ion formation adjacent to the highly acidic residues. In addition, spectra were obtained by positive ion mode ISD for each protonated peptide; more sequence informative fragmentation was observed via nISD for all peptides. Three of the peptides studied had no product ion formation in ISD, but extensive sequence informative fragmentation was found in their nISD spectra. The results of this study indicate that nISD can be used to readily obtain sequence information for acidic peptides.

Dynorphin A analogs for the treatment of chronic neuropathic pain

Chronic pain is one of the most ubiquitous diseases in the world, but treatment is difficult with conventional methods, due to undesirable side effects of treatments and unknown mechanisms of pathological pain states. The endogenous peptide, dynorphin A has long been established as a target for the treatment of pain. Interestingly, this unique peptide has both inhibitory (opioid in nature) and excitatory activities (nonopioid) in the CNS. Both of these effects have been found to play a role in pain and much work has been done to develop therapeutics to enhance the inhibitory effects. Here we will review the dynorphin A compounds that have been designed for the modulation of pain and will discuss where the field stands today.

Photoactive Spatial Proximity Probes for Binding Pairs with Epigenetic Marks

A new strategy for encoding polypeptide libraries with photolabile tags is developed. The photoassisted assay, based on conditional release of encoding tags only from bound pairs, can differentiate between peptides which have minor differences in a form of post-translational modifications with epigenetic marks. The encoding strategy is fully compatible with automated peptide synthesis. The encoding pendants are compact and do not perturb potential binding interactions.

Functional characterization of G-protein-coupled receptors: a bioinformatics approach

Complex molecular and cellular mechanisms regulate G protein-coupled receptors (GPCRs). It is suggested that proteins intrinsically disordered regions (IDRs) are to play a role in GPCR's intra and extracellular regions plasticity, due to their potential for post-translational modification and interaction with other proteins. These regions are defined as lacking a stable three-dimensional (3D) structure. They are rich in hydrophilic and charged, amino acids and are capable to assume different conformations which allow them to interact with multiple partners. In this study we analyzed 75 GPCR involved in synaptic transmission using computational tools for sequence-based prediction of IDRs within a protein. We also evaluated putative ligand-binding motifs using receptor sequences. The disorder analysis indicated that neurotransmitter GPCRs have a significant amount of disorder in their N-terminus, third intracellular loop (3IL) and C-terminus. About 31%, 39% and 53% of human GPCR involved in synaptic transmission are disordered in these regions. Thirty-three percent of receptors show at least one predicted PEST motif, this being statistically greater than the estimate for the rest of human GPCRs. About 90% of the receptors had at least one putative site for dimerization in their 3IL or C-terminus. ELM instances sampled in these domains were 14-3-3, SH3, SH2 and PDZ motifs. In conclusion, the increased flexibility observed in GPCRs, added to the enrichment of linear motifs, PEST and heteromerization sites, may be critical for the nervous system's functional plasticity.

Recent methodological advances in MALDI mass spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is widely used for characterization of large, thermally labile biomolecules. Advantages of this analytical technique are high sensitivity, robustness, high-throughput capacity, and applicability to a wide range of compound classes. For some years, MALDI-MS has also been increasingly used for mass spectrometric imaging as well as in other areas of clinical research. Recently, several new concepts have been presented that have the potential to further advance the performance characteristics of MALDI. Among these innovations are novel matrices with low proton affinities for particularly efficient protonation of analyte molecules, use of wavelength-tunable lasers to achieve optimum excitation conditions, and use of liquid matrices for improved quantification. Instrumental modifications have also made possible MALDI-MS imaging with cellular resolution as well as an efficient generation of multiply charged MALDI ions by use of heated vacuum interfaces. This article reviews these recent innovations and gives the author's personal outlook of possible future developments.

Antihypertensive mechanism of lactoferrin-derived peptides: angiotensin receptor blocking effect

Looking for antihypertensive mechanisms beyond ACE inhibition, we assessed whether lactoferrin (LF)-derived peptides can act as receptor blockers to inhibit vasoconstriction induced by angiotensin II or endothelin-1. The lactoferricin B (LfcinB)-derived peptide LfcinB20-25 (RRWQWR), the low molecular weight LF hydrolysate (LFH < 3 kDa), and two peptides identified in LFH < 3 kDa (LIWKL and RPYL) were tested in ex vivo assays of vasoactive responses. The peptide RPYL was tested in radioligand receptor binding assays. Both LFH < 3 kDa and individual peptides inhibited angiotensin II-induced vasoconstriction. RPYL showed the highest ex vivo inhibitory effect and also inhibited binding of [(125)I]-(Sar(1),Ile(8))-angiotensin II to AT1 receptors. By contrast, neither LFH < 3 kDa nor RPYL inhibited endothelin-1 and depolarization-induced vasoconstrictions. In conclusion, LF-derived peptides selectively inhibit angiotensin II-induced vasoconstriction by blocking angiotensin AT1 receptors. Therefore, inhibition of angiotensin II-induced vasocontriction is suggested as a mechanism contributing along with ACE inhibition to the antihypertensive effect of some LF-derived peptides.

ETD and sequential ETD localize the residues involved in D2-A2A heteromerization

ETD²to identify binding site in NCX.

Imaging of noncovalent complexes by MALDI-MS

Noncovalent interactions govern how molecules communicate. Mass spectrometry is an important and versatile tool for the analysis of noncovalent complexes (NCX). Electrospray mass spectrometry (ESI-MS) is the most widely used MS technique for the study of NCXs because of its softer ionization and easy compatibility with the solution phase of NCX mixtures. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has also been used to study NCXs. However, successful analysis depends upon several experimental factors, such as matrix selection, solution pH, and instrumental parameters. In this study, we employ MALDI imaging mass spectrometry to investigate the location and formation of NCXs, involving both peptides and proteins, in a MALDI sample spot.

MALDI/post ionization-ion mobility mass spectrometry of noncovalent complexes of dopamine receptors' epitopes

Protein domains involved in receptor heteromer formation are disordered and rich in the amino acids necessary for the formation of noncovalent complexes (NCX). We present mass spectral NCX data from proteins and protein receptors' epitopes obtained by combining ion mobility (IM) and MALDI. We focus on NCX involved in heteromer formation occurring between epitopes of the Dopamine D2 (D2R) and Adenosine A2A receptors (A2AR) as well as D2R and the α2 nicotinic (NR) receptor's subunit. The IM data yield information on the gas phase conformation of the singly charged NCX which are observed either directly from MALDI or as codesorbed neutrals that are subsequently postionized by a time-delayed excimer laser pulse directed onto a portion of the neutral plume created by the MALDI desorption laser. Imaging mass spectrometry of the matrix/epitope dried droplet surface shows that the acidic and basic epitopes and their NCX are found to be spatially collocated within regions as small as 25 × 50 μm(2). Subtle differences in the relative abundance of protonated and cationized NCX and epitopes are measured in spatial regions near the sodium-rich outer border of the droplet.

MALDI-ToF mass spectrometry for studying noncovalent complexes of biomolecules

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has been demonstrated to be a valuable tool to investigate noncovalent interactions of biomolecules. The direct detection of noncovalent assemblies is often more troublesome than with electrospray ionization. Using dedicated sample preparation techniques and carefully optimized instrumental parameters, a number of biomolecule assemblies were successfully analyzed. For complexes dissociating under MALDI conditions, covalent stabilization with chemical cross-linking is a suitable alternative. Indirect methods allow the detection of noncovalent assemblies by monitoring the fading of binding partners or altered H/D exchange patterns.

Heparin-like glycosaminoglycan/amine salt-bridge interactions: a new potential tool for HLGAGs analysis using mass spectrometry

Characterization of glycosaminoglycans poses a challenge for current analytical techniques, as they are highly acidic, polydisperse and heterogeneous compounds. The purpose of this study is the separation and analysis of a partially depolymerized heparin-like glycosaminoglycan by on-line ion-pairing reversed-phase high-performance liquid chromatography/electrospray mass spectrometry. The gas-phase behavior of two synthesized glycosaminoglycans has been investigated. Dibutylamine was found to be the best suited ion-pairing reagents for mass spectrometry analysis. The optimized ion-pairing conditions provide reproducible and easily interpretable electrospray mass spectra in both negative and positive ESI modes. The glycosaminoglycans are detected as a non-covalent complex with amines. In fact, the observed ionic species and their gas-phase dissociation under CID conditions revealed the presence of salt bridge interactions in the gas phase.

Non-covalent dimers of the lysine containing protonated peptide ions in gaseous state: electrospray ionization mass spectrometric study

Study of the non-covalent molecular complexes in gas phase by electrospray ionization mass spectrometry (ESI-MS) represents a promising strategy to probe the intrinsic nature of these complexes. ESI-MS investigation of a series of synthetic octapeptides containing six alanine and two lysine residues differing only by their positions showed the formation of non-covalent dimers, which were preserved in the gas phase. Unlike the monomers, the dimers were found to show only singly protonated state. The decrease in the solvent polarity from water to alcohol showed enhanced propensity of formation of the dimer indicating that the electrostatic interaction plays a crucial role to stabilize the dimer. Selective functionalization studies showed that ε-NH(2) of lysine and C-terminal amide (-CONH(2)) facilitate the dimerization through intermolecular hydrogen bonding network.

Interactions between calmodulin, adenosine A2A, and dopamine D2 receptors

The Ca(2+)-binding protein calmodulin (CaM) has been shown to bind directly to cytoplasmic domains of some G protein-coupled receptors, including the dopamine D(2) receptor. CaM binds to the N-terminal portion of the long third intracellular loop of the D(2) receptor, within an Arg-rich epitope that is also involved in the binding to G(i/o) proteins and to the adenosine A(2A) receptor, with the formation of A(2A)-D(2) receptor heteromers. In the present work, by using proteomics and bioluminescence resonance energy transfer (BRET) techniques, we provide evidence for the binding of CaM to the A(2A) receptor. By using BRET and sequential resonance energy transfer techniques, evidence was obtained for CaM-A(2A)-D(2) receptor oligomerization. BRET competition experiments indicated that, in the A(2A)-D(2) receptor heteromer, CaM binds preferentially to a proximal C terminus epitope of the A(2A) receptor. Furthermore, Ca(2+) was found to induce conformational changes in the CaM-A(2A)-D(2) receptor oligomer and to selectively modulate A(2A) and D(2) receptor-mediated MAPK signaling in the A(2A)-D(2) receptor heteromer. These results may have implications for basal ganglia disorders, since A(2A)-D(2) receptor heteromers are being considered as a target for anti-parkinsonian agents.

Selective detection of specific protein-ligand complexes by electrosonic spray-precursor ion scan tandem mass spectrometry

A novel mass spectrometric method for the selective detection of specific protein-ligand complexes is presented. The new method is based on electrosonic spray ionization of samples containing protein and ligand molecules, and mass spectrometric detection using the precursor ion scanning function on a triple quadrupole instrument. Mass-selected intact protein-ligand complex ions are subjected to fragmentation by means of collision-induced dissociation in the collision cell of the instrument, while the second mass analyzer is set to the m/z of protonated ligand ions or their alkali metal adducts. The method allows for the detection of only those ions which yield ions characteristic of the ligand molecules upon fragmentation. Since the scan range of first analyzer is set well above the m/z of the ligand ion, and the CID conditions are established to permit fragmentation of only loosely bound, noncovalent complexes, the method is specific to the detection of protein-ligand complexes under described conditions. Behavior of biologically specific and nonspecific complexes was compared under various instrumental settings. Parameters were optimized to obtain maximal selectivity for specific complexes. Specific and nonspecific complexes were found to show markedly different fragmentation characteristics, which can be a basis for selective detection of complexes with biological relevance. Preparation of specific and nonspecific complexes containing identical building blocks was attempted. Complex ions with identical stoichiometry but different origin showed the expected difference in fragmentation characteristics, which gives direct evidence for the different mechanism of specific versus nonspecific complex ion formation.

The use of ECD/ETD to identify the site of electrostatic interaction in noncovalent complexes

Electrostatic interactions play an important role in the formation of noncovalent complexes. Our previous work has highlighted the role of certain amino acid residues, such as arginine, glutamate, aspartate, and phosphorylated/sulfated residues, in the formation of salt bridges resulting in noncovalent complexes between peptides. Tandem mass spectrometry (MS) studies of these complexes using collision-induced dissociation (CID) have provided information on their relative stability. However, product-ion spectra produced by CID have been unable to assign specifically the site of interaction for the complex. In this work, tandem MS experiments were conducted on noncovalent complexes using both electron capture dissociation (ECD) and electron-transfer dissociation (ETD). The resulting spectra were dominated by intramolecular fragments of the complex with the electrostatic interaction site intact. Based upon these data, we were able to assign the binding site for the peptides forming the noncovalent complex.

How calmodulin interacts with the adenosine A(2A) and the dopamine D(2) receptors

Receptor heteromerization is a mechanism used by G protein-coupled receptors to diversify their properties and function. We previously demonstrated that these interactions occur through salt bridge formation between epitopes of the involved receptors. Recent studies claim that calmodulin (CaM) binds to an Arg-rich epitope located in the amino-terminus of the dopamine D(2) receptor third intracellular loop. This is the same epitope involved in adenosine A(2A)-D(2) receptor heteromerization, through Coulombic interaction between the Arg residues and a phosphorylated serine (pS) located in the medial segment of the C-terminus of the A(2A) receptor. Mass spectrometric analysis indicates that an electrostatic interaction involving the D(2) receptor Arg-rich epitope and several CaM acidic epitopes are mainly responsible for the D(2) receptor-CaM binding. CaM could also form multiple noncovalent complexes by means of electrostatic interactions with an epitope localized in the proximal segment of the C-terminus of the A(2A) receptor. Ca(2+) disrupted the binding of CaM to the D(2) but not to the A(2A) receptor epitope, and CaM disrupted the electrostatic interactions between the D(2) receptor epitope and the more distal A(2A) receptor epitope. A model is introduced with the possible functional implications of A(2A)-D(2)-CaM interactions. These in vitro findings imply a possible regulatory role for CaM in receptor heteromers formation.

Charge state effect on the zwitterion influence on stability of non-covalent interaction of single-stranded DNA with peptides

Negative ion ESI mass spectrometry was used to study the gas-phase stability and dissociation pathways of peptide-DNA complexes. We show that bradykinin and three modified peptides containing the basic residue arginine or lysine form stable interactions with single-stranded oligonucleotides. ESI-MS/MS of complexes of T(8) with PPGFSPFRR resulted in a major dissociation pathway through cleavage of the peptide covalent bond. The stability of the complex is due to electrostatic interaction between the negatively charged phosphate group and the basic side chain of the arginine and lysine residues as demonstrated by Vertes et al. and Woods et al. In fact, the present work establishes the role played by zwitterions on complex stabilisation. The presence of protons in nucleobase and/or amino acid contributes in reinforcing the strength of the salt bridge (SB) interaction. The zwitterionic form of the most basic of amino acid residues, arginine, is assumed to form a strong SB interaction to the negatively charged phosphate groups of DNA. This non-covalent complex is stable enough to withstand disruption of the non-covalent interaction and to first break the covalent bond. Moreover, the dependence of fragmentation patterns upon the complex charge state is explained by the fact that the net number of negative charges modulates the number of zwitterionic sites, which stabilise the complexes. Finally, the weak influence of the nucleobase is assumed by the existence of competition for proton addition between the nucleobase and the R/K side chain leading to a decrease in the stabilisation of the SB interaction.

Noncovalent complexes between DNA and basic polypeptides or polyamines by MALDI-TOF

MALDI-MS was evaluated as a method for the study of noncovalent complexes involving DNA oligonucleotides and various polybasic compounds (basic polypeptides and polyamines). Complexes involving single-stranded DNA were successfully detected using DHAP matrix in the presence of an ammonium salt. Control experiments confirmed that the interactions involved basic sites of the polybasic compounds and that the complexes were not formed in the gas phase but were pre-existing in the matrix crystals. Moreover, the pre-existence in solution was probed by isothermal titration calorimetry at concentration and ionic strength similar to those used for mass spectrometry. Spectra showed no important difference between negative and positive ion modes. The influence of nature and size of DNA and polybasic compound on the relative intensities and stoichiometries of the complexes was investigated. Despite the fact that relative intensities can be affected by ionization yields and the gas-phase stabilities of the different species, numerous trends observed in the MALDI study were consistent with the expected in-solution behaviors. Experimental conditions related to sample preparation were investigated also. Complex abundance generally decreased when increasing the ammonium acetate concentration. It was dramatically decreased when using ATT instead of DHAP. Penta-L-arginine is an exception to these observations. Lastly, in the case of complexes involving DNA duplex, the ATT matrix was shown to favor the observation of specific DNA duplex but not that of its complex with polybasic compounds. Inversely, DHAP was appropriate for the conservation of DNA-polybasic compound interaction but not for the transfer of intact duplex.

A study of noncovalent complexes involving single-stranded DNA and polybasic compounds using nanospray mass spectrometry

Noncovalent complexes involving a single-stranded DNA oligonucleotide and a polybasic compound (spermine, penta-L-lysine, penta-L-arginine, or polydisperse poly-L-lysine) were detected by nanospray-MS. Several control experiments tended to show that these complexes preexisted in solution and that the interactions were initially ionic ones between oligonucleotide phosphates and protonated basic sites of the polybasic compound. Collision-induced dissociation (CID) experiments carried out with these complexes allowed us to identify some differences in the nature of the interactions between the solution and the gas phase, arising from possible proton transfers. Different dissociation pathways were observed according to the nature of the polybasic compound and to the initial charge state of the complex. The complex involving spermine dissociated by cleavage of noncovalent bonds leading to the separation of the two components, whereas the one involving penta-L-arginine underwent fragmentations of covalent bonds. Both behaviors were independent of the initial charge state of the complex. On the other hand, the dissociation pathway of the complex involving penta-L-lysine has been shown to be clearly charge state dependent. Noncovalent dissociation (separation of the two components) driven by coulomb repulsion occurred for the higher charged complexes, whereas fragmentation of covalent bonds was the main pathway of the lower charged complexes. In the latter case, differences in CID behavior were observed for different lengths of poly-L-lysine.

Exploring the "intensity fading" phenomenon in the study of noncovalent interactions by MALDI-TOF mass spectrometry

The difficulties to detect intact noncovalent complexes involving proteins and peptides by MALDI-TOF mass spectrometry have hindered a widespread use of this approach. Recently, "intensity fading MS" has been presented as an alternative strategy to detect noncovalent interactions in solution, in which a reduction in the relative signal intensity of low molecular mass binding partners (i.e., protease inhibitors) can be observed when their target protein (i.e., protease) is added to the sample. Here we have performed a systematic study to explore how various experimental conditions affect the intensity fading phenomenon, as well as a comparison with the strategy based on the direct detection of intact complexes by MALDI MS. For this purpose, the study is focused on two different protease-inhibitor complexes naturally occurring in solution, together with a heterogeneous mixture of nonbinding molecules derived from a biological extract, to examine the specificity of the approach, i.e., those of carboxypeptidase A (CPA) bound to potato carboxypeptidase inhibitor (PCI) and of trypsin bound to bovine pancreatic trypsin inhibitor (BPTI). Our results show that the intensity fading phenomenon occurs when the binding assay is carried out in the sub-muM range and the interacting partners are present in complex mixtures of nonbinding compounds. Thus, at these experimental conditions, the specific inhibitor-protease interaction causes a selective reduction in the relative abundance of the inhibitor. Interestingly, we could not detect any gaseous noncovalent inhibitor-protease ions at these conditions, presumably due to the lower high-mass sensitivity of MCP detectors.

Sulfation, the up-and-coming post-translational modification: its role and mechanism in protein-protein interaction

Tyrosine sulfation is a post-translational modification entailing covalent attachment of sulfate to tyrosine residues. It takes place in the trans-Golgi, is necessary for the bioactivity of some proteins, and improves their ability to interact with other proteins. In the present work, we show that a protein containing a sulfated tyrosine with a delocalized negative charge forms a salt bridge with another protein if it has two or more adjacent arginine residues containing positive delocalized charges. These noncovalent complexes are so stable that, when submitted to collision induced dissociation, the peptides forming the complex dissociate. Just one covalent bond fragments, the covalent bond between the tyrosine oxygen and the SO3 sulfur, and is represented by the appearance of a new peak (basic peptide + SO3), suggesting that in some instances covalent bonds will break down before the noncovalent bonds between the arginine guanidinium and SO3 dissociate. The data implies that the dissociation pathway is preferred; however, fragmentation between tyrosine and the sulfate residue is a major pathway.

Investigation of molecular interaction within biological macromolecular complexes by mass spectrometry

The advent of electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) has accelerated structural studies of biological macromolecular complexes. At present, mass spectrometry can provide accurate mass values not only of individual biological macromolecules but also of their assemblies. Furthermore, it can also give information on the interface sites of the biological macromolecular complexes. The present article focuses on the role of mass spectrometry in the investigation of biological molecular interactions, such as protein-protein, protein-DNA, and protein-ligand interactions, which play essential roles in various biological events.

Localization of noncovalent complexes in MALDI-preparations by CLSM

The unambiguous detection of noncovalent complexes (NCCs) by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is still a far cry from being routine. For protein NCCs such as their quaternary structure it has been reported that signals of the intact complex are only obtained for the first or at most the first few laser exposures of a given sample area. This observation was called the first-shot phenomenon. In the present study, this first-shot phenomenon has been investigated for the hexameric protein complex allophycocyanine (APC) by two independent methods, MALDI-MS with a (nearly) pH-neutral matrix 6-aza-2-thiothymine (6-ATT) and by imaging the fluorescence of the complex in APC-6-ATT preparations by confocal laser scan microscopy (CLSM). The intact APC heterohexamer loses its visible fluorescence upon dissociation into its subunits. Both methods consistently show that intact APC complexes are precipitated at the matrix crystal surface, but dissociate upon incorporation into the matrix crystals.

Phosphate stabilization of intermolecular interactions

Receptor heteromerization is an important phenomenon that results from the interaction of epitopes on two receptors. Previous studies have suggested the possibility of Dopamine D2-NMDA receptors' interaction. We believe that the interaction is through an acidic epitope of the NMDA NR1 subunit (KVNSEEEEEDA) and a basic epitope of the D2 third intracellular loop (VLRRRRKRVN), which was shown to also interact with the Adenosine A2A receptor. In previous work, we highlighted the role of certain amino acid residues, mainly two or more adjacent arginine on one peptide and two or more adjacent glutamate, or aspartate, or a phosphorylated residue on the other in the formation of noncovalent complexes (NCX) between epitopes. In the present work, we use the phosphorylated (KVNSpEEEEEDA), nonphosphorylated (KVNSEEEEEDA) and modified (KVNpSAAAAAAA) forms of the NMDA epitope that possibly interact with the D2 epitope to investigate the gas-phase stability of the NCXs as a function of the nominal energy given to the NCX ion as it enters the collision cell. In addition to theoretical calculations, the experimental data was used to calculate the stability of each electrostatic complex versus that of the dimer of KVNSpEEEEEDA. Our results demonstrate the importance of the phosphate group in stabilizing molecular interactions and that appreciably higher collision energies are required to completely dissociate any of the three different NCX ions that are formed through electrostatic interaction in comparison to the energy required to dissociate the KVNpSEEEEEDA dimer ion, which is mainly kept together by hydrogen bonding. This study emphasizes ionic bonds stability and their importance to protein structure as their potent electrostatic attractions can in the gas-phase surpass the strength of covalent bonds.

Characterization of specific noncovalent complexes between guanidinium derivatives and single-stranded DNA by MALDI

Noncovalently bound complexes between highly basic sites of 12 guanidinium compounds and single-stranded DNA were studied using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. 6-Aza-2-thiothymine (ATT) was used as the matrix in the presence of ammonium citrate, and spectra were recorded in the positive ion mode. Detailed control experiments confirmed unambiguously the high selectivity and specificity of the guanidinium moiety for phosphate groups of DNA. The results verify the binding stoichiometry and show preferential binding of hydrophobic binders (pyrene and anthracene guanidinium derivatives) to all sequences examined. In addition, we demonstrate that electrostatic noncovalent interactions are strengthened with phosphorothioate analogs of DNA. These results clearly highlight the structure-directing role of the self-assembling organic species and strongly emphasize the significance of concentration, hydrophobicity, hydrogen-bonding, and pi-pi interactions of the artificial receptor in the formation of these noncovalent complexes. Because of the ability of DNA-binding compounds to influence gene expression, and therefore cell proliferation and differentiation, the interactions described above could be important in providing a better understanding of the mechanism of action of these noncovalent genetic regulators.

Study of the fragmentation patterns of the phosphate-arginine noncovalent bond

Our previous work has highlighted the role of certain amino acid residues, mainly two or more adjacent arginine on one peptide and two or more adjacent glutamate, or aspartate, or a phosphorylated residue on the other in the formation of noncovalent complexes (NCX) between peptides. In the present study, we employ ESI-MS to investigate the gas-phase stability and dissociation pathways of the NCX of a basic peptide VLRRRRKRVN, an epitope from the third intracellular loop of the dopamine D(2) receptor, with the phosphopetide SVSTDpTpSAE, an epitope from the cannabinoid CB1 carboxyl terminus. ESI-MS/MS analysis of the NCX between VLRRRRKRVN and SVSTDpTpSAE suggests two dissociation pathways for the NCX. The major pathway is the disruption of the electrostatic interactions between the Arg residues and the phosphate groups, while an alternative pathway is also recorded, in which the complex is dissociated along the covalent bond between the oxygen from either Thr or Ser and HPO(3). To verify the alternative pathway, we have used an ion trap instrument to conduct MS(3) analysis on the product ions of both dissociation pathways.

Amazing stability of the arginine-phosphate electrostatic interaction

Electrostatic interactions between a basic epitope containing adjacent arginine residues and an acidic epitope containing a phosphorylated serine are involved in receptor heteromerization. In the present study, we demonstrate that this arginine-phosphate electrostatic interaction possesses a "covalent-like" stability. Hence, these bonds can withstand fragmentation by mass spectrometric collision-induced dissociation at energies similar to those that fragment covalent bonds and they demonstrate an extremely low dissociation constant by plasmon resonance. The present work also highlights the importance of phosphorylation-dephosphorylation events in the modulation of this electrostatic attraction. Phosphorylation of the acidic epitope, a casein kinase one consensus site, makes it available to interact with the basic epitope. On the other hand, phosphorylation of serine and/or threonine residues adjacent to the basic epitope, a protein kinase A consensus site, slows down the attraction between the epitopes. Although analyzed here in the frame of receptor heteromerization, the arginine-phosphate electrostatic interaction most likely represents a general mechanism in protein-protein interactions.

Direct tissue analysis of phospholipids in rat brain using MALDI-TOFMS and MALDI-ion mobility-TOFMS

After water, lipids are the most common biomolecules found in the brain (12%). A brief perusal of the physiology, anatomy, and pathophysiology of the brain illustrates the importance of lipids. Recent advances in mass spectrometry have allowed the direct probing of tissues. However, most studies have focused on proteins. In the present work, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) and MALDI-ion mobility (IM)-TOFMS were employed for direct analysis of phospholipids in rat brain tissue. Molecular ions (MH+) corresponding to phosphatidylcholines, phosphatidylethanolamines, and sphingomyelin, were recorded. When studying pharmacology, we learn that many therapeutic compounds are stored in the body's adipose tissue. MALDI-TOFMS and MALDI- IM-TOFMS were thus used to analyze rat brain tissue with chlorisondamine added directly onto the tissue slice. With both techniques, noncovalent complexes between the tissue phospholipids and chlorisondamine were detected. In addition, MALDI-IM-TOFMS of noncovalent complexes between phospholipids and chlorisondamine displayed a mobility between that of an isobaric lipid and peptide.

Combining mass spectrometry and pull-down techniques for the study of receptor heteromerization. Direct epitope-epitope electrostatic interactions between adenosine A2A and dopamine D2 receptors

Previous results from FRET and BRET experiments and computational analysis (docking simulations) have suggested that a portion of the third intracellular loop (I3) of the human dopamine D2 receptor (D2R) and the C-tail from the human adenosine A2A receptor (A2AR) are involved in A2AR-D2R heteromerization. The results of the present studies, using pull-down and mass spectrometry experiments, suggest that A2AR-D2R heteromerization depends on an electrostatic interaction between an Arg-rich epitope from the I3 of the D2R (217RRRRKR222) and two adjacent Asp residues (DD401-402) or a phosphorylated Ser (S374) residue in the C-tail of the A2AR. A GST-fusion protein containing the C-terminal domain of the A2AR (GST-A2ACT) was able to pull down the whole D2R solubilized from D2R-tranfected HEK-293 cells. Second, a peptide corresponding to the Arg-rich I3 region of the D2R (215VLRRRRKRVN224) and bound to Sepharose was able to pull down both GST-A2ACT and the whole A2AR solubilized from A2AR-tranfected HEK-293 cells. Finally, mass spectometry and pull-down data showed that the Arg-rich D2R epitope binds to two different epitopes from the C-terminal part of the A2AR, containing the two adjacent Asp residues or the phosphorylated Ser residue (388HELKGVCPEPPGLDDPLAQDGAVGS412 and 370SAQEpSQGNT378). The present results are the first example of epitope-epitope electrostatic interaction underlying receptor heteromerization, a new, expanding area of protein-protein interactions.

Affinity chromatography techniques based on the immobilisation of peptides exhibiting specific binding activity

Affinity chromatography is one of the powerful techniques in selective purification and isolation of a great number of compounds. New challenges in scientific research, such as high-throughput systems, isolation procedures that allow to obtain a single substance from a complex matrix in high degree of purity, low costs and wide availability, have led to the discovery of new tailor-made synthetic recognition systems. In this review the design, synthesis, purification and characterisation of peptides with recognition properties are discussed. Applications of peptide ligands are described and analytical tools mentioned.

Biochemical and spectroscopic characterization of the covalent binding of heme to cytochrome b6

The three-dimensional structure of the cytochrome b(6)f complex disclosed the unexpected presence of a new heme c(i) [Stroebel, D., Choquet, Y., Popot, J.-L., and Picot, D. (2003) Nature 426, 413-418; Kurisu, G., Zhang, H., Smith, J. L., and Cramer, W. A. (2003) Science 302, 1009-1014]. Here we present a biochemical, spectroscopic, and mutagenesis study of this unusual heme binding in Chlamydomonas reinhardtii. As predicted by the structure data, we identify a Cys(35)-containing proteolytic fragment (Tyr(25)-Lys(111)) from cytochrome b(6) as a peptide that covalently binds a heme. Resonance Raman spectra of cyt b(6)f complexes show particular frequencies in nu(2), nu(3), nu(4), and nu(8) regions that identify this extra heme as a ferrous c'-like heme under a five-coordinated high-spin state. The set of frequencies is consistent with a coordination by either a water molecule or a hydroxide ion. Other changes in resonance Raman bands, observed in the mid- and low-frequency regions, point to a modification in conformation and/or environment of at least one b heme methyl and/or propionate group. Site-directed mutagenesis of apocytochrome b(6), leading to a Cys(35)Val substitution, generates Chlamydomonas strains that are unable to assemble cytochrome b(6)f complexes. On the basis of the mutant phenotype, we discuss the participation, in the covalent binding of heme c(i), of the nuclear CCB factors that we identified previously as controlling the apo to holo conversion of cytochrome b(6) [Kuras, R., de Vitry, C., Choquet, Y., Girard-Bascou, J., Culler, D., Büschlen, S., Merchant, S., and Wollman, F.-A. (1997) J. Biol. Chem. 272, 32427-32435].

Adenosine A2A-dopamine D2 receptor–receptor heteromers. Targets for neuro-psychiatric disorders

Emerging evidence shows that G protein-coupled receptors can form homo- and heteromers. These include adenosine A(2A) receptor-dopamine D(2) receptor heteromers, which are most probably localized in the dendritic spines of the striatopallidal GABAergic neurons, where they are in a position to modulate glutamatergic neurotransmission. The discovery of A(2A) receptor-dopamine D(2) receptor heteromers gives a frame for the well-known antagonistic interaction between both receptors, which is the bases for a new therapeutic approach for neuro-psychiatric disorders, such as Parkinson's disease and schizoprenia. The present review deals mainly with the biochemical and molecular aspects of A(2A) receptor-dopamine D(2) receptor interactions. Recent results at the molecular level show that A(2A) receptor-dopamine D(2) receptor heteromers represent the first example of epitope-epitope electrostatic interaction underlying receptor heteromerization. Most probably A(2A) receptor-D(2) receptor heteromerization is not static, but subject to a dynamic regulation, related to the phosphorylation dependence of the A(2A) receptor epitope and to the ability of the D(2) receptor epitope to bind different partners. Finding out the mechanisms involved in this dynamic regulation can have important implications for the treatment of basal ganglia disorders, schizophrenia and drug addiction.

The Mighty Arginine, the Stable Quaternary Amines, the Powerful Aromatics, and the Aggressive Phosphate: Their Role in the Noncovalent Minuet

In the age of proteomics, the role of certain amino acid residues and some post-translational modifications in noncovalent complex formation are gaining in importance, as the understanding of interactions between biological molecules, is at the heart of the structure function relationship puzzle. In this work, mass spectrometry is used to highlight ammonium- or guanidinium-aromatic interactions through Cation-pi bonds and ammonium- or guanidinium-phosphate interactions through salt bridge formation. Such interactions are crucial factors in certain ligand-receptor interactions and receptor-receptor interactions. In addition, the ability of phosphorylated residues and phosphorylated lipids to form noncovalent complexes with guanidinium and quaternary ammonium (mostly through Coulombic interactions) is demonstrated, and could explain the stability of certain membrane embedded protein, or a possible role for phosphorylation in protein-protein interactions. Dougherty's work demonstrates cation-pi interactions in intra-protein interactions and folding, the present work explores inter-peptide interactions, i.e., the formation of noncovalent complexes between peptides' epitopes containing adjacent aromatic residues and ones containing adjacent Arg as a model to better understand the role of cation-pi complexes in protein-protein interaction. Complexes of peptides containing aromatic residues with quaternary amines as well as the interaction of aromatic compounds, with the guanidinium group of Arg are also investigated. Considering that an inordinate number of therapeutic compounds contain aromatic rings and quaternary amines, the above-described interactions could possibly be of great importance in better understanding their mechanism of action.

Interaction of chlorisondamine with the neuronal nicotinic acetylcholine receptor

An epitope was found on the alpha2-nicotinic isoform of the neuronal nicotinic acetylcholine receptor that would likely form salt bridges with quaternary ammonium compounds and a cation-pi interaction with the pi-cloud of an aromatic ring. Chlorisondamine, a nicotinic antagonist, exerts a long-lasting, if not permanent, blockade of the ion channel gated by acetylcholine. Blocking of the ion channel prevents nicotine from exerting its rewarding effect on the CNS. Chlorisondamine contains two quaternary ammonium groups and a tetrachloroisoindoline ring. We propose that chlorisondamine interacts with an epitope on the alpha2 isoform of the rat neuronal nicotinic receptor (residues 388-402, GEREETEEEEEEEDE), where one or both of the quaternary ammonium groups of chlorisondamine form a salt bridge with dither a glutamic acid side chain or a phosphate group, whereas the tetrachlorobenzene portion of the tetrachloroisoindoline ring interacts with the guanidinium group of arginine in a cation-pi association: In this work, a new way of probing the interaction of a receptor epitope (alpha2) with organic molecules (chlorisondamine and hexachlorobenzene) was undertaken using matrix assisted laser desorption/ionization mass spectrometry.

Investigation of sample preparation and instrumental parameters in the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of noncovalent peptide/peptide complexes

The application of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) to the direct detection and investigation of noncovalent solution-phase complexes is far from being routine and some principal problems and questions still exist. Therefore, this study systematically investigates several main problems, namely, the effect of sample preparation and some instrument-related parameters on the stability of the noncovalent complexes as well as the formation of nonspecific cluster ions in the case of the MALDI-MS analysis of specific peptide/peptide complexes. The complexes formed between biologically active fragments of human gastrin I, which contain the sequence motif EEEEE, and different peptides, which contain the interacting sequence motifs RR and RKR, were chosen as examples. A broad variety of MALDI matrices and sample preparation protocols were screened systematically and evaluated. The two 'less acidic' matrices 2,4,6-trihydroxyacetophenone and 6-aza-2-thiothymine, in combination with carefully selected solvents and additives, turned out to allow the reproducible detection of the solution-phase peptide/peptide complexes with good intensity, whereas the classical MALDI matrices could not be applied with the same success. Because both matrices also tend to induce the formation of nonspecific cluster ions, control experiments using nonbinding peptides were performed to definitely prove the specificity of the detected complexes. In contrast to the sensitivity of the peptide/peptide complexes to solution-phase conditions, the gas-phase stability during desorption/ionization was found to be extraordinary high. Neither the application of high laser fluence nor switching from continuous to delayed extraction mode as well as variation of the delay time up to 520 ns had considerable effect on the relative intensities of the specific peptide/peptide complexes.