IRMPD spectroscopy and quantum-chemical simulations of the reaction products of cisplatin with the dipeptide CysGly

The inorganic antineoplastic drug cisplatin was made to react in solution with the dipeptide cysteinylglycine (CysGly), chosen as a functional model of glutathione, and the reaction products were analyzed using electrospray ionization mass spectrometry (ESI-MS). Selected complexes, i.e., the primary substitution product cis-[PtCl(NH3)2(CysGly)]+ and the chelate cis-[PtCl(NH3)(CysGly)]+, were submitted to IR multiple photon dissociation (IRMPD) spectroscopy obtaining their vibrational features. The experimental IR ion spectra were compared with the calculated IR absorptions of different plausible isomeric families, finding CysGly to bind preferentially platinum(II) via its deprotonated thiolic group in the monovalent complex, cis-[PtCl(NH3)2(CysGly)]+, and to evolve in the S,N-bound chelate structure cis-[PtCl(NH3)(CysGly)]+ through the SH and NH2 functionality of the cysteine residue. Moreover, our findings indicate that the platination reaction does not affect the CysGly peptide bond, which remains in its trans configuration. These results provide additional insights into the reactivity of Pt(II)-complexes with glutathione which is involved in cellular cisplatin resistance.

Binding Motifs of Carboplatin and Oxaliplatin with Guanine: A Combined MS/MS, IRMPD, and Theoretical Study

Complexes generated in the gas phase involving the purine nucleobase guanine bound to second and third generation platinum drugs, namely, carboplatin (CarboPt) and oxaliplatin (OxaliPt), were investigated by combining tandem mass spectrometry, collision-induced dissociation (CID), infrared multiple photon dissociation spectroscopy (IRMPD), and density functional theory (DFT) calculations. As the first step, a spectroscopic characterization of the protonated platinum drugs was accomplished. Protonation of both CarboPt and OxaliPt in the gas phase occurs on one of the two carbonyl groups of the cyclobutanedicarboxylate and oxalate ligand, respectively. Such protonation has been postulated by several theoretical studies as a key preliminary step in the hydrolysis of Pt drugs under acidic conditions. Subsequently, the protonated drugs react with guanine in solution to generate a complex of general formula [Pt drug + H + guanine]+, which was then mass-selected. CID experiments provided evidence of the presence of strong binding between guanine and platinum-based drugs within the complexes. The structures of the two complexes have also been examined by comparing the experimental IRMPD spectra recorded in two spectral regions with DFT-computed IR spectra. For each system, the IRMPD spectra agree with the vibrational spectra calculated for the global minimum structures, which present a monodentate complexation of Pt at the N7 position of canonical guanine. This binding scheme is therefore akin to that observed for cisplatin, while other coordination sites yield substantially less stable species. Interestingly, in the case of oxaliplatin, the IRMPD spectra are consistent with the presence of two isomeric forms very close in energy.

Protonated Forms of Naringenin and Naringenin Chalcone: Proteiform Bioactive Species Elucidated by IRMPD Spectroscopy, IMS, CID-MS, and Computational Approaches

Naringenin (Nar) and its structural isomer, naringenin chalcone (ChNar), are two natural phytophenols with beneficial health effects belonging to the flavonoids family. A direct discrimination and structural characterization of the protonated forms of Nar and ChNar, delivered into the gas phase by electrospray ionization (ESI), was performed by mass spectrometry-based methods. In this study, we exploit a combination of electrospray ionization coupled to (high-resolution) mass spectrometry (HR-MS), collision-induced dissociation (CID) measurements, IR multiple-photon dissociation (IRMPD) action spectroscopy, density functional theory (DFT) calculations, and ion mobility-mass spectrometry (IMS). While IMS and variable collision-energy CID experiments hardly differentiate the two isomers, IRMPD spectroscopy appears to be an efficient method to distinguish naringenin from its related chalcone. In particular, the spectral range between 1400 and 1700 cm^-1 is highly specific in discriminating between the two protonated isomers. Selected vibrational signatures in the IRMPD spectra have allowed us to identify the nature of the metabolite present in methanolic extracts of commercial tomatoes and grapefruits. Furthermore, comparisons between experimental IRMPD and calculated IR spectra have clarified the geometries adopted by the two protonated isomers, allowing a conformational analysis of the probed species.

Elusive intermediates in cisplatin reaction with target amino acids: Platinum(II)-cysteine complexes assayed by IR ion spectroscopy and DFT calculations

The reactivity of a widely used metal based antineoplastic drug, cisplatin, cis-PtCl₂(NH₃)₂, with L-cysteine (Cys) has been investigated using a combination of electrospray ionization mass spectrometry (ESI-MS), IRMPD gas phase ion spectroscopy and DFT calculations. The cysteine lateral chain represents one of the main platination sites in proteins, which is believed to be related to the resistance mechanisms to cisplatin. The vibrational features of the mass-selected substitution product cis-[PtCl(NH₃)₂(Cys)]⁺ and the intercepted cis-[PtCl(NH₃)₂(H₂O)(Cys)]⁺ intermediate complex were compared to calculated IR spectra, enabling the assessment of the sampled ions structures. In cis-[PtCl(NH₃)₂(Cys)]⁺, cysteine was found to bind platinum through the sulfur atom as a thiolate zwitterion, highlighting the enhanced acidity of the cysteine thiol group upon metal coordination. The cis-[PtCl(NH₃)₂(H₂O)(Cys)]⁺ structure complies with the non-covalent encounter complex, formed by cis-[PtCl(NH₃)₂(H₂O)]⁺ and neutral cysteine. This species is able to undergo the substitution process to produce cis-[PtCl(NH₃)₂(Cys)]⁺ when activated as a mass-isolated ion suggesting its participation in the reaction mechanism of cisplatin with cysteine in solution. Finally, the DFT-calculated energy profile for the substitution reaction was correlated with the peculiar gas-phase reactivity of this non-covalent complex, resulting to be 10-fold less reactive toward substitution than the corresponding methionine complex.

IRMPD Spectroscopy of Bare Monodeprotonated Genistein, an Antioxidant Flavonoid

Genistein is a naturally occurring polyphenol belonging to the family of flavonoids with estrogenic properties and proven antioxidant, anti-inflammatory, and hormonal effects. Genistein and its derivatives are involved in radical scavenging activity by way of mechanisms based on sequential proton-loss electron transfer. In view of this role, a detailed structural characterization of its bare deprotonated form, [geni-H]^-, generated by electrospray ionization, has been performed by tandem mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy in the 800-1800 cm^-1 spectral range. Quantum chemical calculations at the B3LYP/6-311+G(d,p) level of theory were carried out to determine geometries, thermochemical data, and anharmonic vibrational properties of low-lying isomers, enabling to interpret the experimental spectrum. Evidence is gathered that the conjugate base of genistein exists as a single isomeric form, which is deprotonated at the most acidic site (7-OH) and benefits from a strong intramolecular H-bond interaction between 5-OH and the adjacent carbonyl oxygen in the most stable arrangement.

Cation-π Interactions between a Noble Metal and a Polyfunctional Aromatic Ligand: Ag+ (benzylamine)

The structure of an isolated Ag⁺ (benzylamine) complex is investigated by infrared multiple photon dissociation (IRMPD) spectroscopy complemented with quantum chemical calculations of candidate geometries and their vibrational spectra, aiming to ascertain the role of competing cation-N and cation-π interactions potentially offered by the polyfunctional ligand. The IRMPD spectrum has been recorded in the 800-1800 cm^-1 fingerprint range using the IR free electron laser beamline coupled with an FT-ICR mass spectrometer at the Centre Laser Infrarouge d'Orsay (CLIO). The resulting IRMPD pattern points toward a chelate coordination (N-Ag⁺ -π) involving both the amino nitrogen atom and the aromatic π-system of the phenyl ring. The gas-phase reactivity of Ag⁺ (benzylamine) with a neutral molecular ligand (L) possessing either an amino/aza functionality or an aryl group confirms N- and π-binding affinity and suggests an augmented silver coordination in the product adduct ion Ag + ( benzylamine ) ( L ) .

Molecular Basis for the Remarkably Different Gas-Phase Behavior of Deprotonated Thyroid Hormones Triiodothyronine (T3) and Reverse Triiodothyronine (rT3): A Clue for Their Discrimination?

Thyroid hormones are biologically active small molecules responsible for growth and development regulation, basal metabolic rate, and lipid and carbohydrate metabolism. Liquid chromatography mass spectrometry (LC–MS) can be used to quantify thyroid hormones blood level with high speed and selectivity, aiming to improve the diagnosis and treatment of the severe pathological conditions in which they are implicated, i.e., hypo- and hyperthyroidism. In this work, the gas-phase behavior of the isomeric thyroid hormones triiodothyronine (T3) and reverse triiodothyronine (rT3) in their deprotonated form was studied at a molecular level using MS-based techniques. Previously reported collision-induced dissociation experiments yielded distinct spectra despite the high structural similarity of the two compounds, suggesting different charge sites to be responsible. Infrared multiple photon dissociation spectroscopy on [T3-H]⁻ and [rT3-H]⁻ was performed, and the results were interpreted using DFT and MP2 calculations, assessing the prevalence of T3 in the carboxylate form and rT3 as a phenolate isomer. The different deprotonation sites of the two isomers were also found to drive their ion-mobility behavior. In fact, [T3-H]⁻ and [rT3-H]⁻ were successfully separated. Drift times were correlated with collisional cross section values of 209 and 215 Ų for [T3-H]⁻ and [rT3-H]⁻, respectively. Calculations suggested the charge site to be the main parameter involved in the different mobilities of the two anions. Finally, bare [T3-H]⁻ and [rT3-H]⁻ were made to react with neutral acetylacetone and trifluoroacetic acid, confirming rT3 to be more acidic than T3 in agreement with the calculated gas-phase acidities of T3 and rT3 equal to 1345 and 1326 kJ mol^–1, respectively.

From Preassociation to Chelation: A Survey of Cisplatin Interaction with Methionine at Molecular Level by IR Ion Spectroscopy and Computations

Methionine (Met) plays an important role in the metabolism of cisplatin anticancer drug. Yet, methionine platination in aqueous solution presents a highly complex pattern of interconnected paths and intermediates. This study reports on the reaction of methionine with the active aqua form of cisplatin, cis-[PtCl(NH₃)₂(H₂O)]⁺, isolating the encounter complex of the reactant pair, {cis-[PtCl(NH₃)₂(H₂O)]⁺·Met}, by electrospray ionization. In the unsolvated state, charged intermediates are characterized for their structure and photofragmentation behavior by IR ion spectroscopy combined with quantum-chemical calculations, obtaining an outline of the cisplatin-methionine reaction at a molecular level. To summarize the major findings: (i) the {cis-[PtCl(NH₃)₂(H₂O)]⁺·Met} encounter complex, lying on the reaction coordinate of the Eigen-Wilkins preassociation mechanism for ligand substitution, is delivered in the gas phase and characterized by IR ion spectroscopy; (ii) upon vibrational excitation, ligand exchange occurs within {cis-[PtCl(NH₃)₂(H₂O)]⁺·Met}, releasing water and cis-[PtCl(NH₃)₂(Met)]⁺, along the calculated energy profile; (iii) activated cis-[PtCl(NH₃)₂(Met)]⁺ ions undergo NH₃ departure, forming a chelate complex, [PtCl(NH₃)(Met)]⁺, whose structure is congruent with overwhelming S-Met ligation as the primary coordination step. The latter process involving ammonia loss marks a difference with the prevailing chloride replacement in protic solvent, pointing to the effect of a low-polarity environment.

Unprotected Galactosamine as a Dynamic Key for a Cyclochiral Lock

The discrimination of d-galactosamine (G), representative of the amino-sugar class of compounds, has been probed through nano-ESI-FT-ICR mass spectrometry by isolating the relevant [C·H·G]+ proton-bound complexes with the enantiomers of the cyclochiral resorcin[4]arene C and allowing them to react toward three primary amines (B = EtNH2, iPrNH2, and (R)- and (S)-sBuNH2). The system under investigation presents several features that help to unveil the behavior of unprotected G in such a supramolecular architecture: (i) the hydrophobic derivatization of the C convex side forces the polar guest G to be coordinated by the cyclochiral concave region; (ii) protonated d-galactosamine exists as an anomeric mixture, dynamically interconverting throughout the experimental time-window; and (iii) different basicities of B allow the experiment to subtly tune the reactivity of the [C·H·G]+ complexes. Three [C·H·G]+ aggregate-types were found to exist, differing in both their origin and reactivity. The most reactive adducts ([C·H·G]ESI+), generated in the electrospray environment, undergo a G-to-B ligand exchange in competition with a partial isomerization to the unreactive [C·H·G]GAS+-type complexes. Finally, the poorly reactive [C·H·G]SOL+ aggregates are formed in solution over an hours-long time scale. A cyclochirality effect on the reactivity was found to depend on the considered [C·H·G]+ aggregate-type.

Binding Motifs in the Naked Complexes of Target Amino Acids with an Excerpt of Antitumor Active Biomolecule: An Ion Vibrational Spectroscopy Assay

The structures of proton-bound complexes of 5,7-dimethoxy-4H-chromen-4-one (1) and basic amino acids (AAs), namely, histidine (His) and lysine (Lys), have been examined by means of mass spectrometry coupled with IR ion spectroscopy and quantum chemical calculations. This selection of systems is based on the fact that 1 represents a portion of glabrescione B, a natural small molecule of promising antitumor activity, while His and Lys are protein residues lining the cavity of the alleged receptor binding site. These species are thus a model of the bioactive adduct, although clearly the isolated state of the present study bears little resemblance to the complex biological environment. A common feature of [1+AA+H]⁺ complexes is the presence of a protonated AA bound to neutral 1, in spite of the fact that the gas-phase basicity of 1 is comparable to those of Lys and His. The carbonyl group of 1 acts as a powerful hydrogen-bond acceptor. Within [1+AA+H]⁺ the side-chain substituents (imidazole group for His and terminal amino group for Lys) present comparable basic properties to those of the α-amino group, taking part to a cooperative hydrogen-bond network. Structural assignment, relying on the comparative analysis of the infrared multiple photon dissociation (IRMPD) spectrum and calculated IR spectra for the candidate geometries, derives from an examination over two frequency ranges: 900-1800 and 2900-3700 cm^-1 . Information gained from the latter one proved especially valuable, for example, pointing to the contribution of species characterized by an unperturbed carboxylic OH or imidazole NH stretching mode.

Molecular Properties of Bare and Microhydrated Vitamin B5-Calcium Complexes

Pantothenic acid, also called vitamin B5, is an essential nutrient involved in several metabolic pathways. It shows a characteristic preference for interacting with Ca(II) ions, which are abundant in the extracellular media and act as secondary mediators in the activation of numerous biological functions. The bare deprotonated form of pantothenic acid, [panto-H]⁻, its complex with Ca(II) ion, [Ca(panto-H)]⁺, and singly charged micro-hydrated calcium pantothenate [Ca(panto-H)(H₂O)]⁺ adduct have been obtained in the gas phase by electrospray ionization and assayed by mass spectrometry and IR multiple photon dissociation spectroscopy in the fingerprint spectral range. Quantum chemical calculations at the B3LYP(-D3) and MP2 levels of theory were performed to simulate geometries, thermochemical data, and linear absorption spectra of low-lying isomers, allowing us to assign the experimental absorptions to particular structural motifs. Pantothenate was found to exist in the gas phase as a single isomeric form showing deprotonation on the carboxylic moiety. On the contrary, free and monohydrated calcium complexes of deprotonated pantothenic acid both present at least two isomers participating in the gas-phase population, sharing the deprotonation of pantothenate on the carboxylic group and either a fourfold or fivefold coordination with calcium, thus justifying the strong affinity of pantothenate for the metal.

Metabolic profiling of different wild and cultivated Allium species based on high-resolution mass spectrometry, high-performance liquid chromatography-photodiode array detector, and color analysis

Many plants of the genus Allium are widely cultivated and consumed for their nutraceutical and health-enhancing bioactive components effective in many metabolic and infectious diseases. In particular, Allium sativum L. (garlic), the most economically important Allium species, is known to present volatile, comparatively polar sulfur-containing compounds responsible for both the typical garlic aroma and antimicrobial property. More recently, the (moderately) polar portion of garlic metabolome, rich of polyphenols and amino acids, is gaining increasing interest as a source of antioxidants and primary nutrients. In this study, we have explored the chemical diversity of eight different hydroalcoholic extracts obtained by microwave-assisted extraction of white and red crop A. sativum and wild Allium triquetrum, Allium roseum, and Allium ampeloprasum, all originating from the Mediterranean Basin. The aim is to appraise their potential dietetic and healing value through an in-depth chemical characterization and contribute to preserve and exploit natural resources. The multimethodological method applied here is based on an untargeted metabolic profiling by means of high-resolution electrospray ionization Fourier-transform ion cyclotron resonance (ESI FT-ICR) mass spectrometry. More than 850 by ESI(+) and 450 by ESI(-) putative metabolites have been annotated covering all main classes of primary and secondary metabolites, including amino acids, alkaloids, organic and fatty acids, nucleotides, vitamins, organosulfur compounds, and flavonoids. The pigment and polyphenol components have been separated and quantified by a targeted chromatographic high-performance liquid chromatography-photodiode array detector (HPLC-PDA) and CIEL*a*b* colorimetric assay, showing characteristic yellow and red components in each extract, related to a different milieu of anthocyanins and flavonoids as assigned by high-resolution mass spectrometry (MS).

Can an Elusive Platinum(III) Oxidation State be Exposed in an Isolated Complex?

Platinum(IV) complexes are extensively studied for their activity against cancer cells as potential substitutes for the widely used platinum(II) drugs. Pt^IV complexes are kinetically inert and need to be reduced to Pt^II species to play their pharmacological action, thus acting as prodrugs. The mechanism of the reduction step inside the cell is however still largely unknown. Gas-phase activation of deprotonated platinum(IV) prodrugs was found to generate products in which platinum has a formal +3 oxidation state. IR multiple photon dissociation spectroscopy is thus used to obtain structural information helping to define the nature of both the platinum atom and the ligands. In particular, comparison of calculations at DFT, MP2 and CCSD levels with experimental results demonstrates that the localization of the radical is about equally shared between the d_xz orbital of platinum and the p_z of nitrogen on the amino group, the latter acting as a non-innocent ligand.

IRMPD Spectra of Protonated Hydroxybenzaldehydes: Evidence of Torsional Barriers in Carboxonium Ions

Protonation at the formyl oxygen atom of benzaldehydes leading to the formation of carboxonium ions yields two distinct isomers, depending on the relative orientation of the proton either cis or trans with respect to the hydrogen atom on the adjacent carbon. In this context, the IR multiple photon dissociation (IRMPD) spectra of protonated ortho, meta, and para-hydroxybenzaldehydes (OH-BZH⁺ ), delivered into the gas phase by electrospray ionization of hydro-alcoholic solutions, are reported in the 3200-3700 cm^-1 spectral range. This range is characteristic of O-H stretching modes and thus able to differentiate cis and trans carboxonium isomers. Comparison between IRMPD spectra and DFT calculations at the B3LYP/6-311++G(2df2p) level suggests that for both p-OH-BZH⁺ and m-OH-BZH⁺ only cis conformers are present in the ion population analyzed. For o-OH-BZH⁺ , IRMPD spectroscopy points to a mixture comprising one trans and more than one cis conformers. The energy barrier for cis-trans isomerization calculated for each OH-BZH⁺ isomer is a measure of the degree of π-electron delocalization. Furthermore, IRMPD spectra of p-OH-BZH⁺ , m-OH-BZH⁺ and protonated phenol (this last used as reference) were recorded also in the fingerprint range. Both the observed C-O and O-H stretching vibrations appear to be a measure of π-electron delocalization in the ions.

Insights into Cisplatin Binding to Uracil and Thiouracils from IRMPD Spectroscopy and Tandem Mass Spectrometry

The monofunctional primary complexes cis-[PtCl(NH₃)₂(L)]⁺, formed by the reaction of cisplatin, a major chemotherapeutic agent, with four nucleobases L, i.e., uracil (U), 2-thiouracil (2SU), 4-thiouracil (4SU), and 2,4-dithiouracil (24dSU), have been studied by a combination of infrared multiple photon dissociation (IRMPD) action spectroscopy in both the fingerprint (900-1900 cm^-1) and the N-H/O-H stretching (3000-3800 cm^-1) ranges, energy-resolved collision-induced dissociation (CID) mass spectrometry, and density functional calculations at the B3LYP/LACVP/6-311G** level. On the basis of the comparison across the experimental features and the linear IR spectra of conceivable structures, the cisplatin residue is found to promote a monodentate interaction preferentially with the O4(S4) atoms of the canonical forms of U, 4SU, and 24dSU and to the S2 atom of 2SU, yielding the most stable structures. Additional absorptions reveal the presence of minor, alternative tautomers in the sampled ion populations of 2SU and 24dSU, underlying the ability of cisplatin to increase the prospect of (therapeutically beneficial) nucleic acid strand disorder. Implication of these evidence may provide insights into drug mechanism and design.

Applications of Infrared Multiple Photon Dissociation (IRMPD) to the Detection of Posttranslational Modifications

Infrared multiple photon dissociation (IRMPD) spectroscopy allows for the derivation of the vibrational fingerprint of molecular ions under tandem mass spectrometry (MS/MS) conditions. It provides insight into the nature and localization of posttranslational modifications (PTMs) affecting single amino acids and peptides. IRMPD spectroscopy, which takes advantage of the high sensitivity and resolution of MS/MS, relies on a wavelength specific fragmentation process occurring on resonance with an IR active vibrational mode of the sampled species and is well suited to reveal the presence of a PTM and its impact in the molecular environment. IRMPD spectroscopy is clearly not a proteomics tool. It is rather a valuable source of information for fixed wavelength IRMPD exploited in dissociation protocols of peptides and proteins. Indeed, from the large variety of model PTM containing amino acids and peptides which have been characterized by IRMPD spectroscopy, specific signatures of PTMs such as phosphorylation or sulfonation can be derived. High throughput workflows relying on the selective fragmentation of modified peptides within a complex mixture have thus been proposed. Sequential fragmentations can be observed upon IR activation, which do not only give rise to rich fragmentation patterns but also overcome low mass cutoff limitations in ion trap mass analyzers. Laser-based vibrational spectroscopy of mass-selected ions holding various PTMs is an increasingly expanding field both in the variety of chemical issues coped with and in the technological advancements and implementations.

Vibrational signatures of curcumin's chelation in copper(II) complexes: An appraisal by IRMPD spectroscopy

Curcumin (Cur) is a natural polyphenol with a wide spectrum of biological activities and appealing therapeutic potential. Herein, it has been delivered by electrospray ionization as gaseous protonated species, [Cur + H]⁺, and as a Cu(ii) complex, [Cu(Cur - H)]⁺, a promising antioxidant and radical scavenger. The gas phase structures were assayed by infrared multiple photon dissociation (IRMPD) spectroscopy in both the fingerprint (800-2000 cm^-1) and hydrogen stretching (3100-3750 cm^-1) ranges. Comparison between the experimental features and linear IR spectra of the lowest energy structures computed at the B3LYP/6-311+G(d,p) level reveals that bare [Cu(Cur - H)]⁺ exists in a fully planar and symmetric arrangement, where the metal interacts with the two oxygens of the syn-enolate functionality of deprotonated Cur and both OCH₃ groups are engaged in H-bonding with the ortho OH. The effect of protonation on the energetic and geometric determinants of Cur has been explored as well, revealing that bare [Cur + H]⁺ may exist as a mixture of two close-lying isomers associated with the most stable binding motifs. The additional proton is bound to either the diketo or the keto-enol configuration of Cur, in a bent or nearly planar arrangement, respectively.

Binding motifs of cisplatin interaction with simple biomolecules and aminoacid targets probed by IR ion spectroscopy

The primary intermediates resulting from the interaction of cisplatin, cis-(PtCl2(NH3)2], most widespread antitumor drug, with biomolecular targets are characterized. Electrospray ionization is used to deliver ions formed in solution into the gas phase where they are structurally interrogated by vibrational “action” spectroscopy in conjunction with quantum chemical calculations. The aquation products, cis-[PtX(NH3)2(H2O)]+ (X = Cl, OH), lying along the path responsible for biological activity, are shown to display distinctive features responding to ligation pattern and optimized geometry. The IR spectra of trans-[PtX(NH3)2(H2O)]+ are different, testifying that cis and trans complexes are stable, non interconverting species both in solution and in the gas phase. Ligand substitution by simple nucleophiles (L = pyridine, 4(5)-methylimidazole, thioanisole, trimethylphosphate, acetamide, dimethylacetamide, urea and thiourea) yields cis-[PtCl(NH3)2(L)]+ complexes displaying remarkable regioselectivity whenever L presents multiple candidate platination sites. The incipient formation of cisplatin-derived complexes with the recognized biological amino acid targets L-histidine (His) and L-methionine (Met) has been investigated revealing the primary platination event to be mainly directed at the Nπ atom of the imidazole side chain of His and to the thiomethyl sulfur of Met. The isomer and conformer population of the ensuing cis-[PtCl(NH3)2(Met/His)]+ complexes, sampled in the gas phase, can be ascertained by photofragmentation kinetics on isomer/conformer specific resonances.

Real time evolution of unprotected protonated galactosamine probed by IRMPD spectroscopy

Conformational characterization of single α- and β-epimers of galactosamine in solution still remains an intriguing task because of their flexibility and ability to interconvert. This difficulty was circumvented by recording several "snapshots" of the epimerization process by means of fast ESI vaporization of a galactosamine·HCl sample solution at different times. Consequently, the so generated gaseous mixtures were spectroscopically investigated and the specific conformational features of both α- and β-epimers were assigned, despite the overlapping of several IR signals. Interestingly, from a comparison with time-resolved 1H-NMR data obtained for the same solutions, the catalyzing effect of the applied ESI technique in the anomerization process clearly emerges. Finally, the experimental data were supported using both the Density Functional Theory (DFT) and Block-Localized Wavefunction (BLW) approaches: the latter method was applied here for the first time for the investigation of charged species.

A multi-methodological approach in the study of Italian PDO "Cornetto di Pontecorvo" red sweet pepper

A multi-methodological approach was applied to study red sweet peppers (Capsicum annuum L.) ecotype "Cornetto di Pontecorvo" grown in a greenhouse or in open field. This approach includes morphological analysis, chemical composition determination, and biological activity evaluation of different extracts from pepper fruits. Untargeted analyses, namely NMR spectroscopy and mass spectrometry, allowed the comprehensive pepper metabolite profile of pepper pulp, peel and seeds hydroalcoholic and organic extracts to be determined, showing the presence of sugars, organic acids, amino acids and other secondary metabolites. Targeted analyses, such as HPLC-PDA, HPLC-TLC and spectrophotometric analyses allowed polyphenols, tannins, flavonoids and pigments content to be determined. Samples quality and freshness were verified by the low content of biogenic amines and mycotoxins, as determined using HPLC-FLD and HPLC-MS, respectively. Preliminary biological results demonstrated the ability of the organic extracts to inhibit α-amylase, a key enzyme in the control of glucose metabolism.

Complexation of halide ions to tyrosine: role of non-covalent interactions evidenced by IRMPD spectroscopy

The binding motifs in the halide adducts with tyrosine ([Tyr + X]^-, X = Cl, Br, I) have been investigated and compared with the analogues with 3-nitrotyrosine (nitroTyr), a biomarker of protein nitration, in a solvent-free environment by mass-selected infrared multiple photon dissociation (IRMPD) spectroscopy over two IR frequency ranges, namely 950-1950 and 2800-3700 cm^-1. Extensive quantum chemical calculations at B3LYP, B3LYP-D3 and MP2 levels of theory have been performed using the 6-311++G(d,p) basis set to determine the geometry, relative energy and vibrational properties of likely isomers and interpret the measured spectra. A diagnostic carbonyl stretching band at ∼1720 cm^-1 from the intact carboxylic group characterizes the IRMPD spectra of both [Tyr + X]^- and [nitroTyr + X]^-, revealing that the canonical isomers (maintaining intact amino and carboxylic functions) are the prevalent structures. The spectroscopic evidence reveals the presence of multiple non-covalent forms. The halide complexes of tyrosine conform to a mixture of plane and phenol isomers. The contribution of phenol-bound isomers is sensitive to anion size, increasing from chloride to iodide, consistent with the decreasing basicity of the halide, with relative amounts depending on the relative energies of the respective structures. The stability of the most favorable phenol isomer with respect to the reference plane geometry is in fact 1.3, -2.1, -6.8 kJ mol^-1, for X = Cl, Br, I, respectively. The change in π-acidity by ring nitration also stabilizes anion-π interactions yielding ring isomers for [nitroTyr + X]^-, where the anion is placed above the face of the aromatic ring.

Undervalued N3 Coordination Revealed in the Cisplatin Complex with 2'-Deoxyadenosine-5'-monophosphate by a Combined IRMPD and Theoretical Study

The complex obtained by the reaction of cisplatin and 2'-deoxyadenosine-5'-monophosphate (5'-dAMP) in water has been isolated and detected by electrospray ionization mass spectrometry. The so-formed cis-[PtCl(NH₃)₂(5'-dAMP)]⁺ complex has been studied in detail by infrared multiple photon dissociation (IRMPD) spectroscopy in two spectral ranges, namely, 700-1900 and 2800-3800 cm^-1, backed by quantum-chemical calculations at the B3LYP/LACV3P/6-311G** level of theory. In agreement with the computational results, the vibrational spectroscopic characterization of cis-[PtCl(NH₃)₂(5'-dAMP)]⁺ shows that the sampled ionic population comprises two major isomers, differentiated in the X-H stretching region by their distinct fragmentation patterns. One of these species presents coordination of the platinum moiety at the N3 position of adenine, whereas in the second one, platinum is bound at the N1 position of adenine. IRMPD kinetics have allowed an estimation of their relative proportions. Surprisingly, the most abundant component of cis-[PtCl(NH₃)₂(5'-dAMP)]⁺ is the N3 isomer, although it is slightly less stable than the other potential isomers in the gas phase. In contrast, the lowest-energy species, namely, the one showing cisplatin binding to the N7 position of adenine, seems to be the one less represented in the sampled ionic population. These findings suggest that the reaction of cisplatin with 5'-dAMP is governed by the kinetics of the process occurring in solution rather than by the thermodynamic factors.

Cisplatin and transplatin interaction with methionine: bonding motifs assayed by vibrational spectroscopy in the isolated ionic complexes

IRMPD spectroscopy discloses N- versus S-platination.

Cisplatin Primary Complex with l-Histidine Target Revealed by IR Multiple Photon Dissociation (IRMPD) Spectroscopy

The primary complex obtained from cisplatin and l-histidine in water has been detected and isolated by electrospray ionization. The so-obtained cis-[PtCl(NH₃ )₂ (histidine)]⁺ complex has been characterized in detail by high-resolution mass spectrometry (MS), tandem MS, IR multiple photon dissociation (IRMPD) spectroscopy, and by quantum chemical calculations. The structural features revealed by IRMPD spectroscopy indicate that platinum binds to the imidazole group, which presents tautomeric forms. Thus, depending on the position of the amino acid pendant on the imidazole ring, isomeric complexes are formed that are remarkably different with respect to the ease with which they undergo fragmentation when activated either by energetic collisions or by multiple IR photon absorption. It is shown here how IRMPD kinetics can allow their relative proportions to be estimated.

Amino Acid Oxidation: A Combined Study of Cysteine Oxo Forms by IRMPD Spectroscopy and Simulations

The redox activity of cysteine sulfur allows numerous post-translational protein modifications involved in the oxidative regulation of metabolism, in metal binding, and in signal transduction. A combined approach based on infrared multiple photon dissociation spectroscopy at the Centre Laser Infrarouge d'Orsay (CLIO) free electron laser facility, calculations of IR frequencies, and finite temperature ab initio molecular dynamics simulations has been employed to characterize the gas-phase structures of deprotonated cysteine sulfenic, sulfinic, and sulfonic acids, [cysSO_x ]^- (x=1, 2, 3, representing the number of S-bound oxygen atoms), which are key intermediates in the redox-switching chemistry of proteins. The ions show different structural motifs owing to preferential binding of the proton to either the carboxylate or sulfur-containing group. Due to the decreasing basicity of the sulfenic, sulfinic, and sulfonic terminals, the proton bound to SO^- in [cysSO]^- migrates to the carboxylate in [cysSO₃ ]^- , whereas it turns out to be shared in [cysSO₂ ]^- . Evidence is gathered that a mixture of close-lying low-energy conformers is sampled for each cysteine oxo form in a Paul ion trap at room temperature.

IR spectrum of the protonated neurotransmitter 2-phenylethylamine: dispersion and anharmonicity of the NH3(+)-π interaction

The structure and dynamics of the highly flexible side chain of (protonated) phenylethylamino neurotransmitters are essential for their function. The geometric, vibrational, and energetic properties of the protonated neutrotransmitter 2-phenylethylamine (H(+)PEA) are characterized in the N-H stretch range by infrared photodissociation (IRPD) spectroscopy of cold ions using rare gas tagging (Rg = Ne and Ar) and anharmonic calculations at the B3LYP-D3/(aug-)cc-pVTZ level including dispersion corrections. A single folded gauche conformer (G) protonated at the basic amino group and stabilized by an intramolecular NH(+)-π interaction is observed. The dispersion-corrected density functional theory calculations reveal the important effects of dispersion on the cation-π interaction and the large vibrational anharmonicity of the NH3(+) group involved in the NH(+)-π hydrogen bond. They allow for assigning overtone and combination bands and explain anomalous intensities observed in previous IR multiple-photon dissociation spectra. Comparison with neutral PEA reveals the large effects of protonation on the geometric and electronic structure.

Serine O-sulfation probed by IRMPD spectroscopy

The sulfation of amino acids is a frequent post-translational modification. It is highly labile, though, and characterizing it by mass spectrometry, an otherwise powerful and widely exploited tool in analytical proteomics, is a challenge. The presently reported study is aimed at revealing the O-sulfation of l-serine and elucidating the effects of protonation and deprotonation on the structure and stability of the ensuing ionic species, [sSer + H](+) and [sSer - H](-). These ions are obtained as gaseous, isolated species by electrospray ionization, trapped in a Paul ion-trap, and sampled by IR multiple photon dissociation (IRMPD) spectroscopy in either the 750-1900 cm(-1) fingerprint range, or the 2900 and 3700 cm(-1) range encompassing the N-H and O-H stretching modes. The recorded IRMPD spectra present diagnostic signatures of the sulfate modification which are missing in the spectra of the native serine ions, [Ser + H](+) and [Ser - H](-). The experimental IRMPD features have been interpreted by comparison with the linear IR spectra of the lowest energy structures that are likely candidates for the sampled ions, calculated at the M06-2X/6-311+G(d,p) level of theory. Evidence is gathered that the most stable conformations of [sSer + H](+) are stabilized by hydrogen bonding interactions between the protonated amino group and both the carbonyl and sulfate oxygens. [sSer - H](-) ions possess a negatively charged sulfate group involved in either a S=O···HN or a S=O···HO hydrogen bond. The experimental IRMPD spectra are consistent with the presence of multiple low-lying structures in a thermally equilibrated population of several species particularly in the case of [sSer - H](-) ions, where the high structural flexibility combined with the presence of a negative charge favors the co-existence of several different H-bonding motifs.

Elusive Sulfurous Acid: Gas-Phase Basicity and IR Signature of the Protonated Species

The ion corresponding to protonated sulfurous acid, H3SO3(+), has been successfully delivered into the gas phase by electrospray ionization of the solution of a suitable precursor and an in-source fragmentation process. The neutral acid is a highly elusive molecule. However, its gas-phase basicity has been ascertained by means of a kinetic study of proton-transfer reactivity. The structure of the H3SO3(+) sampled ion has been probed by IRMPD spectroscopy in two complementary IR frequency ranges in conjunction with density functional theory calculations and found to conform to a trihydroxosulfonium ion. The characteristic IR signatures may aid in deciphering the presence of this species in extraterrestrial atmospheres.

Nitrosyl–heme and anion–arene complexes: structure, reactivity and spectroscopy

Two topics are selected and illustrated to exemplify (i) a biological and (ii) an organic ionic intermediate. The reactivity behavior of NO adducts with ferric and ferrous hemes has shown remarkable similarities when examined in the gas phase, demonstrating that the largely different NO affinity displayed in solution and in biological media is due to the different coordination environment. In fact, ferrous hemes present a vacant or highly labile axial coordination site, prone to readily bind NO. The vibrational signatures of the NO ligand have also been probed in vacuo for the first time in the nitrosyl complexes deriving from ferrous and ferric hemes under strictly comparable five-coordination at the metal center. Negatively charged σ-adducts, from the association of anions with 1,3,5-trinitrobenzene, an exemplary π-electron-deficient arene, have been probed by IRMPD spectroscopy and found to display variable binding motifs from a strongly covalent σ-adduct (Meisenheimer complex) to a weakly covalent σ-complex, depending on the anion basicity.

Interaction of cisplatin with 5'-dGMP: a combined IRMPD and theoretical study

IR multiple photon dissociation (IRMPD) spectroscopy of cis-[Pt(NH3)2(5'-dGMP-H)](+) and cis-[PtCl(NH3)2(5'-dGMP)](+) ions (where 5'-dGMP is 2'-deoxyguanosine-5'-monophosphate), generated in the gas phase by electrospray ionization, was performed in two spectral regions, namely, 700-1900 cm(-1) and 2800-3800 cm(-1). For structural assignment, experimental IRMPD spectra were compared to IR spectra computed at the B3LYP/LACV3P/6-311G** level of theory. In agreement with computational results, the vibrational spectroscopic characterization of the cis-[Pt(NH3)2(5'-dGMP-H)](+) ion points to macrochelate species resulting from the simultaneous interaction of the metal with both the N7 atom of the guanine residue and an O atom of the phosphate group, structures that bear features in common with those characterized in solution by NMR spectroscopy. Concerning the cis-[PtCl(NH3)2(5'-dGMP)](+) ion, our study points to a monodentate complex involving exclusively the N7 position of guanine, as observed in solution. Also this species exhibits a compact form due to the formation of two hydrogen bonds involving the same ammonia ligand. For both complexes, IRMPD experiments show that a strong intramolecular hydrogen bond is established between one ammonia hydrogen and the carbonyl group of guanine. The strength of this particular interaction can be qualitatively estimated by looking at the redshift of the CO vibration with respect to an unperturbed C═O stretching mode in the fingerprint region. This point is also highlighted in the X-H (X = N, O) stretch region, by the shift of the N-H stretch frequency as a function of the number of hydrogen bonds involving the ammonia ligand.

Cation-π interactions in protonated phenylalkylamines

Phenylalkylamines of the general formula C6H5(CH2)nNH2 (n = 1-4) have been delivered to the gas phase as protonated species using electrospray ionization. The ions thus formed have been assayed by IRMPD spectroscopy in two different spectroscopic domains, namely, the 600-1800 and the 3000-3500 cm(-1) regions using either an IR free electron laser or a tabletop OPO/OPA laser source. The interpretation of the experimental spectra is aided by density functional theory calculations of candidate species and vibrational frequency analyses. Protonated benzylamine presents a relatively straightforward instance of a single stable conformer, providing a trial case for the adopted approach. Turning to the higher homologues, C6H5(CH2)nNH3(+) (n = 2-4), more conformations become accessible. For each C6H5(CH2)nNH3(+) ion (n = 2-4), the most stable geometry is characterized by cation-π interactions between the positively charged ammonium group and the aromatic π-electronic system, permitted by the folding of the polymethylene chain. The IRMPD spectra of the sampled ions confirm the presence of the folded structures by comparison with the calculated IR spectra of the various possible conformers. An inspection of the NH stretching region is helpful in this regard.

Kinetic control in the CID-induced elimination of H3PO4 from phosphorylated serine probed using IRMPD spectroscopy

InfraRed Multiple Photon Dissociation (IRMPD) spectroscopy was used to assay the structural features of the fragment ions resulting from the elimination of H3PO4 in the Collision-Induced Dissociation (CID) of protonated serine. The results are interpreted with the aid of density functional theory calculations. Experiment and theory point to an aziridine-ring structure, implying participation of the vicinal amino group in the formation of this species. This finding constitutes a benchmark for investigating the same process in the CID of phosphorylated peptides.

Cyanide-arene Meisenheimer complex generated in electrospray ionization mass spectrometry using acetonitrile as a solvent

The C - C bond formation activated under negative electrospray ionization of an acetonitrile solution of 1,3,5-trinitrobenzene is reported. The solvent function is to provide a source of cyanide ion, a highly problematic reagent, which is found to attack the electron-deficient aromatic ring to form a covalently bound anionic complex (Meisenheimer complex). The structure of the complex is elucidated by means of collision induced dissociation mass spectrometry and IR multiple photon dissociation spectroscopy in the 'fingerprint' region.

Tandem mass spectrometry of nitric oxide and hydrogen sulfide releasing aspirins: a hint into activity behavior

Aspirin (acetylsalicylic acid, ASA) is the most popular non-steroidal anti-inflammatory drug. However, due to its action on cyclooxygenase and its acid nature, aspirin is associated with adverse gastrointestinal effects. In an effort to minimize these side effects, NO-donor and H2S-donor ASA co-drugs have been designed and tested. Their mass spectrometric behavior is now analyzed and reported. Positive ions were obtained by electrospray ionization involving protonation or alkali metal attachment. Their dissociation processes have been studied by collision induced dissociation in a triple quadrupole instrument. High mass accuracy measurements have been recorded on a Fourier transform ion cyclotron resonance mass spectrometer. The protonated molecules dissociate by an exclusive or largely prevailing path leading to acetyloxy-substituted benzoyl cation, namely an ASA unit. The process is reminiscent of the enzymatic hydrolysis, releasing intact ASA to a large extent. Only at higher collision energy does the formal ketene loss disrupt the ASA moiety. The gas phase chemistry of protonated ASA-releasing drugs develops along elementary dissociation steps analogous to the reactive processes in complex biological environments. This notion may provide a tool for preliminary testing of new compounds.