New insights on mu/delta selectivity of opioid peptides: conformational analysis of deltorphin analogues

Tailoring the nuclear Overhauser effect for the study of small and medium-sized molecules by solvent viscosity manipulation

The nuclear Overhauser effect (NOE) is a consequence of cross-relaxation between nuclear spins mediated by dipolar coupling. Its sensitivity to internuclear distances has made it an increasingly important tool for the determination of through-space atom proximity relationships within molecules of sizes ranging from the smallest systems to large biopolymers. With the support of sophisticated FT-NMR techniques, the NOE plays an essential role in structure elucidation, conformational and dynamic investigations in liquid-state NMR. The efficiency of magnetization transfer by the NOE depends on the molecular rotational correlation time, whose value depends on solution viscosity. The magnitude of the NOE between ¹H nuclei varies from +50% when molecular tumbling is fast to -100% when it is slow, the latter case corresponding to the spin diffusion limit. In an intermediate tumbling regime, the NOE may be vanishingly small. Increasing the viscosity of the solution increases the motional correlation time, and as a result, otherwise unobservable NOEs may be revealed and brought close to the spin diffusion limit. The goal of this review is to report the resolution of structural problems that benefited from the manipulation of the negative NOE by means of viscous solvents, including examples of molecular structure determination, conformation elucidation and mixture analysis (the ViscY method).

The influence of the stereochemistry and C-end chemical modification of dermorphin derivatives on the peptide-phospholipid interactions

In this work the conformation of dermorphin, Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH₂, an opioid peptide and its analogues with different stereochemistry of alanine and different C-terminus is studied in aqueous and membrane environments. Using two-dimensional NMR techniques we demonstrate that in D₂O/H₂O peptides with D-alanine have extended conformation, while for the L-isomers more compact conformation is preferred. The analysis of ROESY HR MAS spectra of the peptides interacting with the DMPC bilayer indicates that both stereoisomers have still more extended conformation compared to aqueous phase, as shown by much weaker intermolecular interactions. The influence of Ala residue stereochemistry is also reflected in the interactions of the studied peptides with model membranes, as shown by the ³¹P NMR static spectra, in which the shapes of the phosphorus NMR signals originating from D-isomers correspond to spherically shaped vesicles in the presence of external magnetic field, in comparison to a more elongated ones observed for L-isomers, while TEM photographs shows that upon addition of D-isomers larger lipid vesicles are formed, in contrast to smaller ones for L-isomers. The location of aromatic fragments of dermorphins in the membrane is determined based on static ²H NMR and ¹H¹H RFDR MAS experiments. All aromatic rings were found to be inserted in the hydrophobic part of the bilayer, with the exception of the Tyr5 rings of D-Ala dermorphins. The influence of the C-terminal modification was found to be almost imperceptible.

A Comparative Study on Interactions of Antimicrobial Peptides L- and D-phenylseptin with 1,2-dimyristoyl-sn-glycero-3-phosphocholine

L-phenylseptin (L-Phes) and D-phenylseptin (D-Phes) are amphibian antimicrobial peptides isolated from the skin secretion of Hypsiboas punctatus. In the N-termini, L-Phes and D-Phes contain three consecutive phenylalanine residues, l-Phe-l-Phe-l-Phe and l-Phe-d-Phe-l-Phe, respectively. They are known to exhibit antimicrobial activity against Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Xanthomonas axonopodis pv. Glycines. However, their mechanism of action and the role of the D-amino acid residue have not been elucidated yet. In this study, the interactions of both peptides with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) were investigated by means of quartz crystal microbalance, circular dichroism, vibrational circular dichroism, 31P solid-state NMR, and molecular dynamics simulation. Both peptides have similar binding constants to the DMPC lipid bilayers, in the order of 106 M−1, and form an α-helix structure in the DMPC lipid bilayers. Both the peptides induce similar changes in the dynamics of DMPC lipids. Thus, in spite of the difference in the conformations caused by the chirality at the N-terminus, the peptides showed similar behavior in the membrane-bound state, experimentally and computationally.

The role of d-allo-isoleucine in the deposition of the anti-Leishmania peptide bombinin H4 as revealed by 31P solid-state NMR, VCD spectroscopy, and MD simulation

Bombinin H4 is an antimicrobial peptide that was isolated from the toad Bombina variegata. Bombinin H family peptides are active against gram-positive, gram-negative bacteria, and fungi as well as the parasite Leishmania. Among them, bombinin H4 (H4), which contains d-allo-isoleucine (d-allo-Ile) as the second residue in its sequence, is the most active, and its l-isomer is bombinin H2 (H2). H4 has a significantly lower LC50 than H2 against Leishmania. However, the atomic-level mechanism of the membrane interaction and higher activity of H4 has not been clarified. In this work, we investigated the behavior of the conformations and interactions of H2 and H4 with the Leishmania membrane using ³¹P solid-state nuclear magnetic resonance (NMR), vibrational circular dichroism (VCD) spectroscopy, and molecular dynamics (MD) simulations. The generation of isotropic ³¹P NMR signals depending on the peptide concentration indicated the abilities of H2 and H4 to exert antimicrobial activity via membrane disruption. The VCD experiment and density functional theory calculation confirmed the different stability and conformations of the N-termini of H2 and H4. MD simulations revealed that the N-terminus of H4 is more stable than that of H2 in the membrane, in line with the VCD experiment data. VCD and MD analyses demonstrated that the first l-Ile and second d-allo-Ile of H4 tend to take a cis conformation. These residues function as an anchor and facilitate the easy winding of the helical conformation of H4 in the membrane. It may assist to quickly reach to the threshold concentration of H4 on the Leishmania membrane. This article is part of a Special Issue entitled: d-Amino acids: biology in the mirror, edited by Dr. Loredano Pollegioni, Dr. Jean-Pierre Mothet and Dr. Molla Gianluca.

Novel protocol for the chemical synthesis of crustacean hyperglycemic hormone analogues--an efficient experimental tool for studying their functions

The crustacean Hyperglycemic Hormone (cHH) is present in many decapods in different isoforms, whose specific biological functions are still poorly understood. Here we report on the first chemical synthesis of three distinct isoforms of the cHH of Astacus leptodactylus carried out by solid phase peptide synthesis coupled to native chemical ligation. The synthetic 72 amino acid long peptide amides, containing L- or D-Phe³ and (Glp¹, D-Phe³) were tested for their biological activity by means of homologous in vivo bioassays. The hyperglycemic activity of the D-isoforms was significantly higher than that of the L-isoform, while the presence of the N-terminal Glp residue had no influence on the peptide activity. The results show that the presence of D-Phe³ modifies the cHH functionality, contributing to the diversification of the hormone pool.

Conformational search of peptides and proteins: Monte Carlo minimization with an adaptive bias method applied to the heptapeptide deltorphin

The energy function of a protein consists of a tremendous number of minima. Locating the global energy minimum (GEM) structure, which corresponds approximately to the native structure, is a severe problem in global optimization. Recently we have proposed a conformational search technique based on the Monte Carlo minimization (MCM) method of Li and Scheraga, where trial dihedral angles are not selected at random within the range [-180 degrees,180 degrees ] (as with MCM) but with biased probabilities depending on the increased structure-energy correlations as the GEM is approached during the search. This method, called the Monte Carlo minimization with an adaptive bias (MCMAB), was applied initially to the pentapeptide Leu-enkephalin. Here we study its properties further by applying it to the larger peptide with bulky side chains, deltorphin (H-Tyr-D-Met-Phe-His-Leu-Met-Asp-NH(2)). We find that on average the number of energy minimizations required by MCMAB to locate the GEM for the first time is smaller by a factor of approximately three than the number required by MCM-in accord with results obtained for Leu-enkephalin.

In vitro and in vivo opioid activity of [DPro(6)]dermorphin, a new dermorphin analogue

To study the effects of inducing stereo-chemical modifications in the structure of dermorphin (DM) so as to improve its mu-opioid receptor affinity and its resistance to C-terminal enzymatic degradation, in the Institute of Molecular Genetics of Moscow, we synthesized a new DM analogue ([DPro(6)]DM) and analyzed the changes induced in the biological activities of DM by substituting the Pro(6) residue with DPro(6). We compared the activity of the new DM analogue and DM in in vitro assays and in in vivo tests of analgesia, thermoregulation, heart rate recordings, and gastrointestinal motility in rats. In the in vitro tests, guinea pig ileum (GPI) and mouse vas deferens (MVD), although the opioid activities of [DPro(6)]DM indicated that the peptide was always less potent than DM, its lower IC(50) ratios (mu/delta) showed that it had higher mu-opioid receptor selectivity. In the in vivo analgesic test, [DPro(6)]DM, when injected intraperitoneally (i.p.) (0.5-5 and 10mg/kg) in rats, had the same antinociceptive efficacy as DM and when injected intranasally (i.n.) (0.005 and 0.02 mg/kg) it induced a more stable and long-lasting analgesia than DM (the AUC was about 91% higher for [DPro(6)]DM than for DM). Moreover, these data confirm that the intranasal route is advantageous for peripheral drug administration. In the heart rate study, [DPro(6)]DM and DM (0.5mg/kg, i.p.), induced a similar, weak bradycardia. The only difference was that [DPro(6)]DM induced a longer-lasting effect than DM. Conversely, in body temperature regulation [DPro(6)]DM induced weaker inhibitory activity than DM (56% of the DM-induced response); it did so only in a cold environment and at the maximal used dose (0.5mg/kg, i.p.) without inducing vasomotor effects. In the gastrointestinal study, [DPro(6)]DM and DM (0.005, 0.05, and 0.5mg/kg, i.p.) significantly slowed upper gastrointestinal transit of a charcoal meal and inhibited colonic propulsion. Comparison of the ED(50) values of [DPro(6)]DM (0.03 mg/kg) and DM (0.009 mg/kg) showed that the DM analogue was about three times less potent than DM in slowing gastrointestinal and colonic transit. In conclusion, all these data overall suggest that structural maneuvering in the Pro(6)-residue of the DM molecule changes its affinity for mu-opioid receptor subtypes and confirms the usefulness of experimental studies involving structural modifications in obtaining new therapeutic agents.

Synthesis, conformation and biological activity of dermorphin and deltorphin I analogues containingN-alkylglycine in place of residues in position 1, 3, 5 and 6

Syntheses are described of new dermorphin and [D-Ala2]deltorphin I analogues in which the phenylalanine, the tyrosine or the valine residues have been substituted by the corresponding N-alkylglycine residues. Structural investigations by CD measurements in different solvents and preliminary pharmacological experiments were carried out on the resulting peptide-peptoid hybrids. The contribution from aromatic side chain residues is prominent in the CD spectra of dermorphin analogues and the assignment of a prevailing secondary structure could be questionable. In the CD spectra of deltorphin analogues the aromatic contribution is lower and the dichroic curves indicate the predominance of random conformer populations. The disappearance of the aromatic contribution in the [Ntyr1,D-Ala2]-deltorphin spectrum could be explained in terms of high conformational freedom of the N-terminal residue. The kinetics of degradation of the synthetic peptoids digestion by rat and human plasma enzymes were compared with that of [Leu5]-enkephalin. The binding to opioid receptors was tested on crude membrane preparations from CHO cells stably transfected with the mu- and delta-opioid receptors. The biological potency of peptoids was compared with that of dermorphin in GPI preparations and with that of deltorphin I in MVD preparations. All the substitutions produced a dramatic decrease in the affinity of the peptide-peptoid hybrids for both the mu- and delta-opioid receptors. Nval5 and/or Nval6 containing hybrids behaved as mu-opioid receptor agonists and elicit a dose-dependent analgesia (tail-flick test) when injected i.c.v. in rats.

The mu-selective heptapeptide opioid dermorphin has two conformations around Phe3 psi with no head-to-tail interaction. A quantitative 2-D NMR and molecular modeling analysis

The mu opioid heptapeptide Dermorphin (DRM) is under 70 % of trans forms for the Tyr(5)-Pro(6) peptide bond in solution (CDCl(3)/DMSO-d(6) 1/1 vol/vol). Variations of NOE integrals at 5 temperatures show apparent correlation times of 0.8 to 0.9 ns (at 280 K) in that mixed solvent. Four NOE between non-adjacent residues reveal a large population of folded structures. However, in trans DRM, 4 adjacent NOE Phe(3)/Gly(4) can only be explained by an equilibrium between folded (psi(3) > 0) and extended (psi(3) > 0) conformations. Simulated annealing modeling gave about 60% (psi(3) > 0) and 40% (psi(3) > 0) of these conformer populations. Trans DRM study and previous studies on the heptapeptide opioids, dermenkephalin (DREK) and deltorphin-I (delta selective), and DREK(1-4)-DRM(5-7) hybrid (mu selective), show in folded structures more backbone bending of the first 4 residues in the mu opioids than in the delta peptides. Also, the main difference between mu- and delta-opioid peptides is a large fraction of extended conformations in mu heptapeptides. Either bending of the N-terminus, or extension of the C-terminal part in mu-opioid heptapeptides prevent the head-to-tail interactions which allow delta-opioid peptides to bind selectively to the delta-opioid receptor.

Efficiency of the multicanonical simulation method as applied to peptides of increasing size: the heptapeptide deltorphin

The advantage of the multicanonical (MUCA) simulation method of Berg and coworkers over the conventional Metropolis method is in its ability to move a system effectively across energy barriers thereby providing results for a wide range of temperatures. However, a MUCA simulation is based on weights (related to the density of states) that should be determined prior to a production run and their calculation is not straightforward. To overcome this difficulty a procedure has been developed by Berg that calculates the MUCA weights automatically. In a previous article (Yaşar et al. J Comput Chem 2000, 14, 1251-1261) we extended this procedure to continuous systems and applied it successfully to the small pentapeptide Leu-enkephalin. To investigate the performance of the automated MUCA procedure for larger peptides, we apply it here to deltorphin, a linear heptapeptide with bulky side chains (H-Tyr(1)-D-Met(2)-Phe(3)-His(4)-Leu(5)-Met(6)-Asp(7)-NH(2)). As for Leu-enkephalin, deltorphin is modeled in vacuum by the potential energy function ECEPP. MUCA is found to perform well. A weak second peak is seen for the specific heat, which is given a special attention. By minimizing the energy of structures along the trajectory it is found that MUCA provides a good conformational coverage of the low energy region of the molecule. These latter results are compared with conformational coverage obtained by the Monte Carlo minimization method of Li and Scheraga.

The delta-selective opioid peptide dermenkephalin and the mu-selective hybrid peptide dermenkephalin-[1-4]-dermophin-[5-7] display strikingly different conformations despite identical tetrapeptide N-termini. A quantitative 2-D NMR and molecular modeling analysis

The selective recognition of the aminoterminal binding pharmacophore Tyr-D-Xaa-Phe of the opioid heptapeptide dermorphin, Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2 (DRM)1, and of dermenkephalin, Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2 (DREK), by the mu-opioid receptor and delta-opioid receptor, respectively, depends upon the constitution / conformation of the C-terminal tripeptide. The hybrid peptide DREK-[1-4]-DRM-[5-7] is very potent at, and exquisitely selective for the mu-opioid receptor, and differs only from dermenkephalin by its C-terminal tripeptide. Comparison of the structural features of DREK-[1-4]-DRM-[5-7] and dermenkephalin by nmr analysis and molecular modeling revealed striking differences, as well in the trans (Tyr5 - Pro6) isomer (population 75%) than in the cis isomer.. Whereas the folded C-terminal tail of dermenkephalin influenced the tertiary structure of the N-terminal tetrapeptide and placed the Tyr1 and Phe3 aromatic rings in definite orientations that are best suited for the delta-receptor, there were only weak contacts, as shown by NOE data, between the aminoterminal and carboxyterminal parts of the hybrid peptide. This promoted increased flexibility of the whole backbone and relaxed orientations for the side-chains of Tyr1 and Phe3 that are compatible with the mu-receptor but unsuitable for the delta-receptor. The steric hindrance introduced by Pro6 in DREK-[1-4]-DRM-[5-7], plus the absence of large hydrophobic side-chains in positions 5 and 6 may prevent close contacts between the N-terminal and C-terminal domains and reorientation of the main pharmacophoric elements Tyr1 and Phe3.

Neurologically active plant compounds and peptide hormones: a chirality connection

The most dramatic, but seldom mentioned, difference between alkaloid and peptide opioids is the change of chirality of the alpha carbon of the tyramine moiety. We propose that the presence of Gly2 or D-Ala2 in the two most common message domains compensates this change by allowing the attainment of unusual conformations. A thorough conformational search of Tyr-D-Ala-Phe-NH-CH3 and of its isomer Tyr-L-Ala-Phe-NH-CH3 backs this view and establishes a solid link between alkaloid and peptide opioids. This finding supports the notion that morphine, like other neurologically active plant compounds, may bind to endogenous receptors in plants to regulate cell-to-cell signaling systems.

What peptides these deltorphins be

The deltorphins are a class of highly selective delta-opioid heptapeptides from the skin of the Amazonian frogs Phyllomedusa sauvagei and P. bicolor. The first of these fascinating peptides came to light in 1987 by cloning of the cDNA of from frog skins, while the other members of this family were identified either by cDNA or isolation of the peptides. The distinctive feature of deltorphins is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe, comparable to dermorphin, which is the prototype of a group of mu-selective opioids from the same source. The D-amino acid and the anionic residues, either Glu or Asp, as well as their unique amino acid compositions are responsible for the remarkable biostability, high delta-receptor affinity, bioactivity and peptide conformation. This review summarizes a decade of research from many laboratories that defined which residues and substituents in the deltorphins interact with the delta-receptor and characterized pharmacological and physiological activities in vitro and in vivo. It begins with a historical description of the topic and presents general schema for the synthesis of peptide analogues of deltorphins A, B and C as a means to document the methods employed in producing a myriad of analogues. Structure activity studies of the peptides and their pharmacological activities in vitro are detailed in abundantly tabulated data. A brief compendium of the current level of knowledge of the delta-receptor assists the reader to appreciate the rationale for the design of these analogues. Discussion of the conformation of these peptides addresses how structure leads to further hypotheses regarding ligand receptor interaction. The review ends with a broad discussion of the potential applications of these peptides in clinical and therapeutic settings.

Solution conformations of deltorphin-I obtained from combined use of quantitative 2D-NMR and energy calculations: a comparison with dermenkephalin

Deltorphin-I, Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2 and dermenkephalin, Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2, two highly related opioid peptides from frog skin, display very similar N-termini but strikingly different C-terminal tails. Nevertheless, both peptides are highly potent at, and exquisitely selective for the delta-opioid receptor. To identify common determinants concuring to the remarkably efficient targeting of deltorphin-I and dermenkephalin, combined use of quantitative two-dimensional nuclear magnetic resonance (53 dipolar interactions studied at four temperatures) and energy calculations using simulated annealing generated five groups of deltorphin-I conformers. These groups were pooled into two families whose overall conformation could be described either by a left-handed helix (Family I) or by a big loop (Family II), both stabilized by H-bonds. Proximity of D-Ala2-Phe3-Asp4 and Val5-Val6-Gly7 triads is an obvious structural similarity between almost all groups in both families of structures. Whereas differences between the two families originated mostly from a transition at psi Asp4 backbone dihedral angle, the backbone structures at segment 1-4 are similar and spatial arrangements of Tyr1 (t) and Phe3 (g-) are identical in one group of each family. Moreover, these two groups have a N-terminal tetrapeptide whose conformation most closely resembles that of a well-defined group of structures for dermenkephalin. Altogether, these results suggest that conformational attributes that are common to dermenkephalin and deltorphin-I, i.e., the backbone conformation of the N-terminal tetrapeptide and preferential orientations in the side-chain of Tyr1 (t) and Phe3 (g-) underlie their ability to bind with high selectivity to the delta-opioid receptor.

A novel D-amino acid-containing peptide, fulyal, coexists with fulicin gene-related peptides in Achatina atria

Fulicin (Phe-D-Asn-Glu-Phe-Val-NH2) is a neuropeptide from ganglia of the African giant snail (Achatina fulica). Previously, the sequences of nine fulicin gene-related peptides (FGRP-1 to -9) have been predicted from the cDNA encoding the ganglia fulicin precursor and the transcripts have been detectable in the heart. We synthesized twenty peptides related to fulicin and FGRPs containing either an L- or a D-amino acid at position 2 and used them to identify FGRPs in atrial extracts. We identified ten alpha-amidated peptides, including fulicin and confirmed their structures as follows: Tyr-Ala-Glu-Phe-Leu-NH2 (FGRP-9), [D-Ala2]FGRP-9 (fulyal), [L-Asn2]fulicin, fulicin, Ser-Tyr-Asp-Phe-Val-NH2 (FGRP-2), Thr-Tyr-Asp-Phe-Leu-NH2 (FGRP-3), Tyr-Asp-Phe-Ile-NH2 (FGRP-5), Ser-Pro-Tyr-Asp-Phe-Ile-NH2 (FGRP-6), Asn-Tyr-Asp-Phe-Val-NH2 (FGRP-7) and Ser-Pro-Tyr-Asp-Phe-Val-NH2 (FGRP-8). We analyzed the biological activities of synthetic FGRPs using the snail penis retractor muscle. The results revealed that fulyal remarkably potentiated the tetanic contraction at concentrations as low as 10(-12) M. FGRP-9 was about 10,000-fold less potent. Fulyal, like fulicin, seems to undergo preferential maturation to participate in the penis retractor muscle contraction as a neuropeptide containing a D-amino acid.

D-amino acids in animal peptides

D-amino acids have been detected in a variety of peptides synthesized by animal cells. These include opiate and antimicrobial peptides from amphibian skin, neuropeptides from snail ganglia, a hormone from crustaceans, and a constituent of a spider venom. cDNA cloning has shown that at those positions where a D-amino acid is found in the end-product, a normal codon for the corresponding L-amino acid is present. This implies that the D-residues are formed from L-amino acids by a posttranslational reaction. A prototype enzyme catalyzing such a reaction has recently been isolated from the venom of the funnel web spider.

Folded conformations of the delta-selective opioid dermenkephalin with head-to-tail interactions. A simulated annealing study through NMR restraints

Despite similar tripeptide N-termini, dermorphin (Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2) and dermenkephalin (Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2), naturally occuring opioid peptides from frog skin, exhibit high affinity but contrasting selectivity for the mu- and delta-opioid receptors, respectively. Structure-activity relationship studies have shown that the N-terminal tripeptide, Tyr-D-Xaa-Phe (where Xaa is either Ala or Met), is necessary for binding with both the mu- and delta-receptors while the nature and/or the conformation of the C-terminus His-Leu-Met-Asp-NH2 of dermenkephalin are responsible for addressing the peptide to the delta-receptor. In order to examine the conformational characteristics that are related to the selectivity of dermenkephalin towards the delta-receptor, 50 NOE restraints (10 between non-adjacent residues), and 7 dihedral angles, derived from a two-dimensional 1H-NMR study of dermenkephalin in dimethyl sulfoxide, were used in simulated annealing and energy minimization procedures. Twenty-four resulting conformers (60% of the generated structures) with no severe distance restraint violation were pooled into seven groups and three related families. These 24 conformers show close proximity between the two methionine residues, S-shaped structures, mean planes of N-terminal and C-terminal moieties almost at right angles to each other, a C-terminus region above the plane of the N-terminal region and g- as preferential orientation in the side chain of Phe. Aside these similarities, families of conformers differ by the preferential orientation in the side chain of Tyr (t or g-) and proximity between Tyr and Asp, or Tyr and the C-terminus. In contrast to previous models, practically no beta-turn structures exist for dermenkephalin, most of the NH hydrogen bonds participating to gamma-turns. The possible relationship between the conformational characteristics of dermenkephalin and the delta-opioid receptor selectivity is discussed.

Conformational restriction of Tyr and Phe side chains in opioid peptides: information about preferred and bioactive side-chain topology

The side chain of Tyr and Phe was fixed into the gauche(-) or gauche(+) conformation by using the Tic Htc structures, and into the trans conformation by using an aminobenzazepine-type (Aba) structure. When incorporated into dermorphin or deltorphin II, the Tic and Htc analogues all showed a large decrease in both mu and delta affinities and activities. Fixation of Phe(3) in the trans rotamer resulted in a large increase in delta affinity in the dermorphin analogue, whereas in the [Aba(3)-Gly(4)] deltorphin II analogue, good delta affinity is maintained despite the removal of the Glu side chain. Whereas several authors propose a gauche(-) preferred conformation for the Phe(3) side chain, these results suggest a trans conformation at the delta receptor. The use of these conformationally constrained residues for evaluating the preferred solution conformation in the flexible N-terminal tripeptide Tyr-D-Ala-Phe is illustrated. The (1)H-nmr parameters--chemical shift, temperature dependence, and nuclear Overhauser effects to the D-Ala(2) methyl protons in the different analogues--provide direct evidence to confirm the proposed sandwich conformation in the native peptides.

Structural studies of opioid peptides: a review of recent progress in x-ray diffraction studies

The solid state structures of many opioid peptide agonists have been elucidated by x-ray diffraction analysis. Recently, the first structure of an opioid peptide antagonist has been determined. Theoretically, linear peptides can have many different backbone conformations, yet early x-ray studies (1983-1987) on enkephalin and its analogues showed only two different backbone conformations: extended and single beta-bend. In 1989 enkephalin was observed in a third conformation, a double beta-bend. Since that time diffraction studies have been completed on the rationally designed linear opioid peptide agonists DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr) and DADLE (D-Ala2,D-Leu5-enkephalin) as well as on several cyclic enkephalin analogues including DPDPE (Tyr-[D-Pen-Gly-Phe-D-Pen]) and JOM-13 (Tyr-[D-Cys-Phe-D-Pen]). The most recent review of the x-ray studies on this class of compounds was written in 1988. This paper will update that review to include the results of studies completed since that time.

Comparative conformational analyses of mu-selective dermorphin and delta-selective deltorphin-II in aqueous solution by 1H-NMR spectroscopy

Two-dimensional 1H-NMR methods have been used to obtain complete proton resonance assignments and possible solution conformations of dermorphin (H-Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2) and deltorphin-II (H-Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2), naturally occurring mu- and delta-selective opioids, respectively, in order to examine the conformational characteristics that are closely related to the selectivities towards mu/delta-opioid receptors. With the use of the proton-proton distances derived from ROESY measurements in aqueous solution, 50 possible 3D structures are generated by means of distance geometry calculations. The conformers which satisfy the distance constraints and the torsion angles estimated from JNHC alpha H vicinal coupling constants within the allowable range are then subjected to molecular dynamics simulations for 10 ps after equilibration. Although dermorphin and deltorphin-II are both in equilibrium among many flexible conformers, some conformational differences are observed between these peptides: many conformers of dermorphin show a structure rounded at the N-terminal Tyr-D-Ala-Phe-Gly-Tyr and C-terminal Gly-Tyr-Pro-Ser-NH2 moieties, which are almost at right angles to each other, while those of deltorphin-II are characterized by a 'hook'-shaped backbone structure in which the nearly extended conformation of the Val-Val-Gly-NH2 sequence is located under the folded conformation of the N-terminal Tyr-D-Ala-Phe-Glu sequence. The possible relationship between these conformational characteristics and the mu/delta-opioid receptor selectivities is discussed.

Delta opioid receptor selective ligands; DPLPE-deltorphin chimeric peptide analogues

Further efforts to correlate the topography of the bioactive structures of DPDPE and the deltorphins, two delta-opioid receptor active peptide families, are reported. A number of DPLPE-deltorphin chimeric peptides have been synthesized in which the C-terminal dipeptide delta-address of the deltorphins (-Val-GlyNH2, -Nle-GlyNH2) have been linked to the highly delta-opioid selective cyclic peptides DPDPE or DPLPE. These studies demonstrate that a major structural feature determining high potency of hybrid analogues is the chirality of the amino acid residue in position 5. The radioligand binding assays have revealed a decrease in potency (compared to DPDPE) at delta-receptors when the C-terminal dipeptides were added to DPDPE. On the other hand, chimeric peptides of DPLPE with these same C-terminal dipeptides retained high delta-selectivity and affinity. Similar results were obtained using the mouse vas deferens (MVD) and guinea pig ileum (GPI) bioassays. The importance of the hydrophilicity of amino acids in positions 2 and 5 for delta-selectivity is consistent with the previous finding for DPLPE and DPDPE. On the other hand, the replacement of phenylalanine-4 with p-chlorophenylalanine-4 did not increase delta-selectivity as in DPDPE. These findings suggest that the delta-receptor interacts with hybridized enkephalins and deltorphins somewhat differently than with DPDPE.

Quantitative two-dimensional NMR study of dermenkephalin, a highly potent and selective delta-opioid peptide

Dermenkephalin, H-Tyr-(D)Met-Phe-His-Leu-Met-Asp-NH2, a highly potent and selective delta-opioid peptide isolated from frog skin, was studied in DMSO-d6 solution by two-dimensional nmr spectroscopy, including the determination of NH temperature coefficients, the evaluation of 3J coupling constants from phase-sensitive correlated spectroscopy (COSY) and the volumes of nuclear Overhauser effect (NOE) correlations. The two-dimensional NOE spectroscopy (NOESY) spectrum of dermenkephalin revealed sequential, medium-, and long-range effects. To put this information on a quantitative basis, special attention was devoted to J cross-peak suppression, quantification of the NOE volumes and analysis of the overlaps, normalization of the NOEs against diagonal peaks and H beta beta' geminal interactions. Although most of the dihedral angles deduced from the 3JN alpha coupling constants together with several Ni alpha i and alpha iNi+1 NOEs pointed to a partially extended peptide backbone, several NiNi+1 NOEs and beta iNi+1 interactions argued in favor of a folded structure. Moreover, several long-range correlations of strong intensities were found that supported a close spatial proximity between the side chains of D-Met2 and Met6, Tyr1 and His4, Tyr1 and Asp7, and His4 and the C-terminal amide group. In Phe, the g- rotamer in the side chain is deduced from the 3J alpha beta coupling constants and alpha beta and N beta NOE correlations. Whereas the amide proton dependency was not indicative of stable hydrogen bonds, the nonuniform values of the temperature coefficient may reflect an equilibrium mixture of folded and extended conformers. The overall data should provide realistic starting models for energy minimization and modelization studies.

Investigation of conformational equilibrium of polypeptides by internal coordinate stochastic dynamics. Met5-enkephalin

The equilibrium population of different conformational states of a polypeptide can in principle be obtained by a very long molecular dynamics simulation. The method of internal coordinate molecular dynamics earlier developed in this laboratory (A.K. Mazur and R.A. Abagyan J. Biomol. Struct. Dyn. 6,833 (1989)) allows one to use time steps much larger than usual for computing molecular trajectories. It is shown here that the sampling of the conformational space can be additionally enhanced by adding a random component to the set of forces applied to atoms. We describe the algorithms by which the random force is introduced and also a special method which excludes the fast rotation of polar hydrogens from equations of motion but keeps them movable. As a result the task stated in the title becomes realistic. Internal coordinate stochastic dynamics is applied for scanning the conformational space of the pentapeptide Met5-enkephalin which is a common test example widely used in theoretical studies. A large number of conformational transitions is observed during the 20 ns simulation starting from the global energy minimum thus allowing us to arrive at a nearly Boltzmann distribution of populations of conformational states. A few states are found which are distinguished by high apparent configurational entropy which turn out to correspond well to experimentally observed conformations of enkephalins.

Investigation of the structural parameters involved in the delta-opioid selectivity of several families of opioid peptides

Three series of highly delta-opioid selective peptides are now available, and each family is used as template to investigate the structural parameters involved in delta-receptor recognition and in the modulation of the selectivity of the parent peptide. The first series includes cyclic derivatives such as Tyr-D-Pen-Gly-Phe-D-Pen(DPDPE) and Tyr-D-Pen-Gly-Phe-Pen(DPLPE); the second are the synthetic linear constrained peptides [Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr(DSTBULET), Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr (OtBu)(BUBU) and especially Tyr-D-Cys(StBu)-Gly-Phe-Leu-Thr(OtBu) (BUBUC)] and the last one the natural peptides [Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2 (deltorphin or dermenkephalin) and Tyr-D-Ala-Phe-Asp-Val-Val-GlyNH2 ([D-Ala2] deltorphin I)]. In the present study, the possibly of transposing some of the decisive factors of delta-selectivity evidenced in the two other families, to the linear constrained peptides series was examined. With this aim in view, residues such as Phe3, pClPhe4 or Asp were introduced in the sequence of DSTBULET, BUBU or BUBUC. Direct comparison between the biochemical profiles of the [pClPhe4] analogs of the linear constrained peptides and their parent compounds shows that the addition of an electronegative atom on the Phe4 residue of enkephalin sequences is not an absolute parameter for delta-selectivity improvement. The hydrophobic delta-receptor subsite seems able to receive a range of molecular volumes and electronegativities. By contrast, this subsite cannot interact with a Phe3 aromatic ring introduced in this series of peptides. Moreover, the results obtained with linear peptides including additional negatively charged residues demonstrate that the proposed location of the delta-receptors in a cationic membrane environment is not adequate to explain the selectivity profile of a number of compounds.

Conformation of deltorphin-II in membrane environment studied by two-dimensional NMR spectroscopy and molecular dynamics calculations

Two-dimensional homonuclear Hartmann-Hahn spectroscopy and NOESY (nuclear Overhauser effect and exchange spectroscopy) 1H-NMR techniques have been used to obtain complete proton resonance assignments and to perform a conformational investigation of deltorphin-II (Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2), a naturally occurring delta-selective opioid peptide, in the membrane-mimetic micelles of perdeuterated dodecylphosphocholine. This was done in order to examine conformational characteristics that would be closely related to the selectivity towards the delta-opioid receptor. With the use of the proton-proton distances derived from NOESY measurements, 50 possible three-dimensional structures were generated by means of distance-geometry calculations, and 25 of them were subjected to the molecular-dynamics simulations of 10 ps, which were energetically constrained for the NOE interproton distances. Most of the possible conformers simulated showed a common feature such that the conformation of deltorphin-II is characterized by the S-shaped back-bone structure in which the turn conformation of the Val-Val-Gly-NH2 sequence is located under the helically folded conformation of the N-terminal Tyr-D-Ala-Phe-Glu sequence. The possible relationship between this conformational characteristic and the delta-opioid-receptor selectivity has been discussed.

Interaction of deltorphin with opioid receptors: molecular determinants for affinity and selectivity

Opioid receptor analyses of deltorphin A (H-Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2) analogues indicated the following: (a) increased negativity differentially affected affinities (Ki) and selectivity (Ki mu/Ki delta); (b) shifted sequence heptapeptides, [Asp5,Leu6,Met-NH2(7)] and [Asp4,His5,Leu6,Met-NH2(7)], reversed selectivity (delta-->mu); (c) substitutions at positions 4, 5, and 6 diminished selectivity, with changes in residue 5 being the most detrimental; (d) C-terminal deletions differentially effected Ki. These are the first data to demonstrate a reversal of delta selectivity in heptapeptides containing a negative charge and indicate that modifications in affinity occur through changes in both anionic and hydrophobic properties of residues at specific positions in the peptide. Deltorphin analogues might also be applied to differentiate between opioid receptor subsites.