[Aladan3]TIPP: a fluorescent delta-opioid antagonist with high delta-receptor binding affinity and delta selectivity

Unnatural Amino Acids for Biological Spectroscopy and Microscopy

Due to advances in methods for site-specific incorporation of unnatural amino acids (UAAs) into proteins, a large number of UAAs with tailored chemical and/or physical properties have been developed and used in a wide array of biological applications. In particular, UAAs with specific spectroscopic characteristics can be used as external reporters to produce additional signals, hence increasing the information content obtainable in protein spectroscopic and/or imaging measurements. In this Review, we summarize the progress in the past two decades in the development of such UAAs and their applications in biological spectroscopy and microscopy, with a focus on UAAs that can be used as site-specific vibrational, fluorescence, electron paramagnetic resonance (EPR), or nuclear magnetic resonance (NMR) probes. Wherever applicable, we also discuss future directions.

A Cysteine-Reactive Small Photo-Crosslinker Possessing Caged-Fluorescence Properties: Binding-Site Determination of a Combinatorially-Selected Peptide by Fluorescence Imaging/Tandem Mass Spectrometry

To determine the binding-site of a combinatorially-selected peptide possessing a fluoroprobe, a novel cysteine reactive small photo-crosslinker that can be excited by a conventional long-wavelength ultraviolet handlamp (365 nm) was synthesized via Suzuki coupling with three steps. The crosslinker is rationally designed, not only as a bioisostere of the fluoroprobe, but as a caged-fluorophore, and the photo-crosslinked target protein became fluorescent with a large Stokes-shift. By introducing the crosslinker to a designated sulfhydryl (SH) group of a combinatorially-selected peptide, the protein-binding site of the targeted peptide was deduced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)/fluorescence imaging followed by matrix-assisted laser desorption ionization-time of flight tandem mass spectrometry (MALDI-TOF-MS/MS) analysis.

Selection of Color-Changing and Intensity-Increasing Fluorogenic Probe as Protein-Specific Indicator Obtained via the 10BASE(d)-T

To obtain a molecular probe for specific protein detection, we have synthesized fluorogenic probe library of vast diversity on bacteriophage T7 via the gp10 based-thioetherificaion (10BASE(d)-T). A remarkable color-changing and turning-on probe was selected from the library, and its physicochemical properties upon target-specific binding were obtained. Combination analyses of fluorescence emission titration, isothermal titration calorimetry (ITC), and quantitative saturation-transfer difference (STD) NMR measurements, followed by in silico docking simulation, rationalized the most plausible geometry of the ligand-protein interaction.

Pharmacological traits of delta opioid receptors: pitfalls or opportunities?

RATIONALE

Delta opioid receptors (DORs) have been considered as a potential target to relieve pain as well as treat depression and anxiety disorders and are known to modulate other physiological responses, including ethanol and food consumption. A small number of DOR-selective drugs are in clinical trials, but no DOR-selective drugs have been approved by the Federal Drug Administration and some candidates have failed in phase II clinical trials, highlighting current difficulties producing effective delta opioid-based therapies. Recent studies have provided new insights into the pharmacology of the DOR, which is often complex and at times paradoxical.

OBJECTIVE

This review will discuss the existing literature focusing on four aspects: (1) Two DOR subtypes have been postulated based on differences in pharmacological effects of existing DOR-selective ligands. (2) DORs are expressed ubiquitously throughout the body and central nervous system and are, thus, positioned to play a role in a multitude of diseases. (3) DOR expression is often dynamic, with many reports of increased expression during exposure to chronic stimuli, such as stress, inflammation, neuropathy, morphine, or changes in endogenous opioid tone. (4) A large structural variety in DOR ligands implies potential different mechanisms of activating the receptor.

CONCLUSION

The reviewed features of DOR pharmacology illustrate the potential benefit of designing tailored or biased DOR ligands.

Fluorescent environment-sensitive dyes as reporters of biomolecular interactions

Monitoring biomolecular interactions is a fundamental issue in biosensing, with numerous applications ranging from biological research to clinical diagnostics. Fluorescent dyes capable of changing their color and brightness in response to changes of their environment properties, the so-called environment-sensitive dyes, have recently emerged as reporters of these interactions. The most well established of these are dyes that undergo excited-state charge transfer showing red shift of their single emission band with increase in the solvent polarity. The other promising class are dyes of the 3-hydroxychromone family that undergo excited-state intramolecular proton transfer and show solvent-sensitive dual emission. Examples of existing solvatochromic dyes and their biosensing applications are given, with particular focus on the 3-hydroxychromones. It is shown that solvatochromic dyes are powerful tools for monitoring conformation changes of proteins and their interactions with nucleic acids, proteins, and lipid membranes.

Two-color fluorescent l-amino acid mimic of tryptophan for probing peptide-nucleic acid complexes

Non-natural amino acids are important tools for site-selective probing of peptide properties and interactions. Here, for the first time a fluorescent l-amino acid, exhibiting excited-state intramolecular proton transfer (ESIPT) and hydration-sensitive dual emission, was synthesized. It is an analogue of l-tryptophan bearing a slightly larger 2-(2-furyl)-3-hydroxychromone aromatic moiety instead of indole. This new amino acid was incorporated through solid-phase synthesis into NC(11-55), the zinc finger domain of the HIV-1 nucleocapsid protein, that exhibits potent nucleic acid chaperone properties. It was substituted for the Trp37 and Ala30 residues, located in the distal finger motif and the linker between the fingers of NC(11-55), respectively. Though the highly conserved Trp37 residue plays a key role in NC(11-55) structure and activity, its substitution for the new fluorescent analogue preserved the folding, the nucleic acid binding and chaperone activity of the peptide, indicating that the new amino acid can conservatively substitute Trp residues. In the presence of oligonucleotides, the Trp37-substituted peptide, but not the Ala30 variant, showed strong changes of the dual emission corresponding to local dehydration. The results are in line with NMR data, suggesting that the fluorescent amino acid interacts similarly to Trp37 with the nucleobases and is thus screened from water. Due to the exceptional sensitivity of its ESIPT fluorophore to hydration in highly polar environment, the new amino acid appears as a promising tool for substituting Trp residues and site-selectively investigating peptide-nucleic acid complexes.

Environmentally sensitive fluorescent sensors based on synthetic peptides

Biosensors allow the direct detection of molecular analytes, by associating a biological receptor with a transducer able to convert the analyte-receptor recognition event into a measurable signal. We review recent work aimed at developing synthetic fluorescent molecular sensors for a variety of analytes, based on peptidic receptors labeled with environmentally sensitive fluorophores. Fluorescent indicators based on synthetic peptides are highly interesting alternatives to protein-based sensors, since they can be synthesized chemically, are stable, and can be easily modified in a site-specific manner for fluorophore coupling and for immobilization on solid supports.

Cyclic opioid peptide agonists and antagonists obtained via ring-closing metathesis

The opioid peptide H-Tyr-c[D-Cys-Phe-Phe-Cys]NH(2) cyclized via a methylene dithiother is a potent and selective mu opioid agonist (Przydial M.J. et al., J Peptide Res, 66, 2005, 255). Dicarba analogues of this peptide with Tyr, 2',6'-dimethyltyrosine (Dmt), 3-[2,6-dimethyl-4-hydroxyphenyl)propanoic acid (Dhp) or (2S)-2-methyl-3-(2,6-dimethyl-4-hydroxyphenyl)propanoic acid [(2S)-Mdp] in the 1-position were prepared. The peptides were synthesized on solid-phase by substituting d-allylglycine and (2S)-2-amino-5-hexenoic acid in position 2 and 5, respectively, followed by ring-closing metathesis. Mixtures of cis and trans isomers of the resulting olefinic peptides were obtained, and catalytic hydrogenation yielded the saturated -CH(2)-CH(2)- bridged peptides. All six Tyr(1)- and Dmt(1)-dicarba analogues retained high mu and delta opioid agonist potency and showed only slight or no preference for mu over delta receptors. As expected, the six Dhp(1)- and (2S)-Mdp(1)-dicarba analogues turned out to be mu opioid antagonists but, surprisingly, displayed a range of different efficacies (agonism, partial agonism or antagonism) at the delta receptor. The obtained results indicate that the mu versus delta receptor selectivity and the efficacy at the delta receptor of these cyclic peptides depend on distinct conformational characteristics of the 15-membered peptide ring structure, which may affect the spatial positioning of the exocyclic residue and of the Phe(3) and Phe(4) side chains.

Sensing peptide-oligonucleotide interactions by a two-color fluorescence label: application to the HIV-1 nucleocapsid protein

We present a new methodology for site-specific sensing of peptide–oligonucleotide (ODN) interactions using a solvatochromic fluorescent label based on 3-hydroxychromone (3HC). This label was covalently attached to the N-terminus of a peptide corresponding to the zinc finger domain of the HIV-1 nucleocapsid protein (NC). On interaction with target ODNs, the labeled peptide shows strong changes in the ratio of its two emission bands, indicating an enhanced screening of the 3HC fluorophore from the bulk water by the ODN bases. Remarkably, this two-color response depends on the ODN sequence and correlates with the 3D structure of the corresponding complexes, suggesting that the 3HC label monitors the peptide–ODN interactions site-specifically. By measuring the two-color ratio, we were also able to determine the peptide–ODN-binding parameters and distinguish multiple binding sites in ODNs, which is rather difficult using other fluorescence methods. Moreover, this method was found to be more sensitive than the commonly used steady-state fluorescence anisotropy, especially in the case of small ODNs. The described methodology could become a new universal tool for investigating peptide–ODN interactions.

4-[ω-[4-Arylpiperazin-1-yl]alkoxy]phenyl)imidazo[1,2-a]pyridine Derivatives: Fluorescent High-Affinity Dopamine D3Receptor Ligands as Potential Probes for Receptor Visualization

Sixteen long-chain arylpiperazines bearing the fluorescent moiety 2-phenylimidazo[1,2-a]pyridine were synthesized as fluorescent dopamine D3 receptors ligands (385 nM < Ki < 0.72 nM). The most potent D3 compounds 15a and 19a (Ki = 1.6 and 0.72 nM, respectively) showed good Stokes shift and high quantum yield in ethanol (Phi = 0.74 and 0.66, respectively). In the first attempt, 15a was unable to visualize D3 receptors expressed in CHO cells by epifluorescence microscopy.

Stereoselective syntheses of fluorescent non-natural aromatic amino acids based on asymmetric Michael additions

Four fluorescent non-natural aromatic amino acids have been synthesized based on a key stereoselective Michael addition reaction. S-1-phenylethylamine was employed as both the source of amine and the stereoselectivity controller. The overall yields were moderate (30-50%). Fluorescent properties of some of the fluorophores were also investigated. It was found that compounds with a dimethylamino group bonded to the aromatic ring display intramolecular charge transfer fluorescence.

Fluorogenic probes for monitoring peptide binding to class II MHC proteins in living cells

A crucial step in the immune response is the binding of antigenic peptides to major histocompatibility complex (MHC) proteins. Class II MHC proteins present their bound peptides to CD4(+) T cells, thereby helping to activate both the humoral and the cellular arms of the adaptive immune response. Peptide loading onto class II MHC proteins is regulated temporally, spatially and developmentally in antigen-presenting cells. To help visualize these processes, we have developed a series of novel fluorogenic probes that incorporate the environment-sensitive amino acid analogs 6-N,N-dimethylamino-2-3-naphthalimidoalanine and 4-N,N-dimethylaminophthalimidoalanine. Upon binding to class II MHC proteins these fluorophores show large changes in emission spectra, quantum yield and fluorescence lifetime. Peptides incorporating these fluorophores bind specifically to class II MHC proteins on antigen-presenting cells and can be used to follow peptide binding in vivo. Using these probes we have tracked a developmentally regulated cell-surface peptide-binding activity in primary human monocyte-derived dendritic cells.

Dicarba analogues of the cyclic enkephalin peptides H-Tyr-c[D-Cys-Gly-Phe-D(or L)-Cys]NH(2) retain high opioid activity

Dicarba analogues of the cyclic opioid peptides H-Tyr-c[d-Cys-Gly-Phe-d(or l)-Cys]NH2 were synthesized on solid phase by substituting allylglycines for the cysteines and cyclization by ring-closing metathesis between the side chains of the allylglycine residues. Mixtures of cis and trans isomers of the resulting olefinic peptides were obtained, and catalytic hydrogenation yielded the saturated -CH2-CH2- bridged peptides. The dicarba analogues retained high mu and delta agonist potencies. Remarkably, the trans isomer of H-Tyr-c[d-Allylgly-Gly-Phe-l-Allylgly]NH2 was a mu agonist/delta agonist with subnanomolar potency at both receptors.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

6-N,N-dimethylamino-2,3-naphthalimide: a new environment-sensitive fluorescent probe in delta- and mu-selective opioid peptides

A new environment-sensitive fluorophore, 6-N,N-(dimethylamino)-2,3-naphthalimide (6DMN) was introduced in the delta-selective opioid peptide agonist H-Dmt-Tic-Glu-NH(2) and in the mu-selective opioid peptide agonist endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH(2)). Environment-sensitive fluorophores are a special class of chromophores that generally exhibit a low quantum yield in aqueous solution but become highly fluorescent in nonpolar solvents or when bound to hydrophobic sites in proteins or membranes. New fluorescent delta-selective irreversible antagonists (H-Dmt-Tic-Glu-NH-(CH(2))(5)-CO-Dap(6DMN)-NH(2) (1) and H-Dmt-Tic-Glu-Dap(6DMN)-NH(2) (2)) were identified as potential fluorescent probes showing good properties for use in studies of distribution and internalization of delta receptors by confocal laser scanning microscopy.