Proposal for the biologically active conformation of opiates and enkephalin

Applications of biaryl cyclization in the synthesis of cyclic enkephalin analogs with a highly restricted flexibility

A series of 10 cyclic, biaryl analogs of enkephalin, with Tyr or Phe residues at positions 1 and 4, were synthesized according to the Miyaura borylation and Suzuki coupling methodology. Biaryl bridges formed by side chains of the two aromatic amino acid residues are of the meta-meta, meta-para, para-meta, and para-para configuration. Conformational properties of the peptides were studied by CD and NMR. CD studies allowed only to compare conformations of individual peptides while NMR investigations followed by XPLOR calculations provided detailed information on their conformation. Reliability of the XPLOR calculations was confirmed by quantum chemical ones performed for one of the analogs. No intramolecular hydrogen bonds were found in all the peptides. They are folded and adopt the type IV β-turn conformation. Due to a large steric strain, the aromatic carbon atoms forming the biaryl bond are distinctly pyramidalized. Seven of the peptides were tested in vitro for their affinity for the µ-opioid receptor.

A DFT and semiempirical model-based study of opioid receptor affinity and selectivity in a group of molecules with a morphine structural core

We report the results of a search for model-based relationships between mu, delta, and kappa opioid receptor binding affinity and molecular structure for a group of molecules having in common a morphine structural core. The wave functions and local reactivity indices were obtained at the ZINDO/1 and B3LYP/6-31G^∗∗ levels of theory for comparison. New developments in the expression for the drug-receptor interaction energy expression allowed several local atomic reactivity indices to be included, such as local electronic chemical potential, local hardness, and local electrophilicity. These indices, together with a new proposal for the ordering of the independent variables, were incorporated in the statistical study. We found and discussed several statistically significant relationships for mu, delta, and kappa opioid receptor binding affinity at both levels of theory. Some of the new local reactivity indices incorporated in the theory appear in several equations for the first time in the history of model-based equations. Interaction pharmacophores were generated for mu, delta, and kappa receptors. We discuss possible differences regulating binding and selectivity in opioid receptor subtypes. This study, contrarily to the statistically backed ones, is able to provide a microscopic insight of the mechanisms involved in the binding process.

Consensus 3D model of μ-opioid receptor ligand efficacy based on a quantitative Conformationally Sampled Pharmacophore

Despite being studied for over 30 years, a consensus structure-activity relationship (SAR) that encompasses the full range peptidic and nonpeptidic μ-opioid receptor ligands is still not available. To achieve a consensus SAR the Conformationally Sampled Pharmacophore (CSP) method was applied to develop a predictive model of the efficacy of μ-opioid receptor ligands. Emphasis was placed on predicting the efficacy of a wide range of agonists, partial agonists, and antagonists as well as understanding their mode of interaction with the receptor. Inclusion of all accessible conformations of each ligand, a central feature of the CSP method, enabled structural features between diverse μ-opioid receptor ligands that dictate efficacy to be identified. The models were validated against a diverse collection of peptidic and nonpeptidic ligands, including benzomorphans, fentanyl (4-anilinopiperidine), methadone (3,3-diphenylpropylamines), etonitazene (benzimidazole derivatives), funaltrexamine (C6-substituted 4,5-epoxymorphinan), and herkinorin. The model predicts (1) that interactions of ligands with the B site, as with the 19-alkyl substituents of oripavines, modulate the extent of agonism; (2) that agonists with long N-substituents, as with fentanyl and N-phenethylnormorphine, can bind in an orientation such that the N substitutent interacts with the B site that also allows the basic N-receptor Asp interaction essential for agonism; and (3) that the μ agonist herkinorin, that lacks a basic nitrogen, binds to the receptor in a manner similar to the traditional opioids via interactions mediated by water or a ion. Importantly, the proposed CSP model can be reconciled with previously published SAR models for the μ receptor.

Backbone alignment modeling of the structure-activity relationships of opioid ligands

Opioid studies are an important area of modern medicinal chemistry research. In this study we have provided innovative considerations to some long-standing problems in opioid studies, specifically the opioid pharmacophore and the potential binding modes of opioid ligands. Based on a new peptide backbone-alignment concept that we have developed along with this study, we discuss a wide variety of opioid ligands with respect to their structure-activity relationships.

Influence of intramolecular hydrogen bonding on the electronic structure of oxymorphone

Approximate ab initio molecular orbital methods are used to examine the structural and electronic properties of oxymorphone. The most stable conformation of the molecule is found to include an intramolecular hydrogen bond between the C-14 hydroxyl group and the nitrogen atom in agreement with available experimental data. The total molecular electron density is transformed to a set of localized molecular orbitals, one of which corresponds to the lone electron pair on nitrogen. The hydrogen bond is shown to produce substantial bending and stretching of the lone pair when compared to its shape when such hydrogen bonding is precluded.

Glycopeptide-membrane interactions: glycosyl enkephalin analogues adopt turn conformations by NMR and CD in amphipathic media

Four enkephalin analogues (Tyr-D-Thr-Gly-Phe-Leu-Ser-CONH(2), 1, and the related O-linked glycopeptides bearing the monosaccharide beta-glucose, 2, the disaccharide beta-maltose, 3, and the trisaccharide beta-maltotriose, 4) were synthesized, purified by HPLC, and biophysical studies were conducted to examine their interactions with membrane model systems. Glycopeptide 2 has been previously reported to penetrate the blood-brain barrier (BBB), and produce potent analgesia superior to morphine in mice (J. Med. Chem.2000, 43, 2586-90 and J. Pharm. Exp. Ther. 2001, 299, 967-972). The parent peptide and its three glycopeptide derivatives were studied in aqueous solution and in the presence of micelles using 2-D NMR, CD, and molecular mechanics (Monte Carlo studies). Consistent with previous conformational studies on cyclic opioid agonist glycopeptides, it was seen that glycosylation did not significantly perturb the peptide backbone in aqueous solution, but all four compounds strongly associated with 5-30 mM SDS or DPC micelles, and underwent profound membrane-induced conformational changes. Interaction was also observed with POPC:POPE:cholesterol lipid vesicles (LUV) in equilibrium dialysis experiments. Although the peptide backbones of 1-4 possessed random coil structures in water, in the presence of the lipid phase they each formed a nearly identical pair of structures, all with a stable beta-turn motif at the C-terminus. Use of spin labels (Mn(2+) and 5-DOXYL-stearic acid) allowed for the determination of the position and orientation of the compounds relative to the surface of the micelle.

Development of a common 3D pharmacophore for delta-opioid recognition from peptides and non-peptides using a novel computer program

A unified three-dimensional (3D) pharmacophore for recognition of the delta-opioid receptor by families of structurally diverse delta-opioid ligands, including peptides and non-peptides, has been determined. An additional structural feature required for delta-selectivity was also characterized using a subset of these ligands that are highly selective for the delta-opioid receptor. To obtain these pharmacophores, we have used a recently developed computer program that performs systematic and automated comparisons of molecules to determine whether any common 3D relationships exist among candidate recognition moieties in high-affinity analogs. All the low-energy conformations of each ligand are included in these comparisons. The program developed should be applicable in general to molecular superimposition problems in rational drug design and to develop both 3D recognition and activation pharmacophores for any receptor for which high- and low-affinity analogs and agonists and antagonists have been identified.

Comparison of cyclic delta-opioid peptides with non-peptide delta-agonist spiroindanyloxymorphone (SIOM) using the message-address concept: a molecular modeling study

Based upon the message-address concept, this molecular modeling study used the delta-selective agonist spiroindanyloxymorphone (SIOM) as a molecular template for a conformational search and analysis of delta-selective opioid peptides. It was assumed that the tyramine moiety plays the same role for delta-opioid receptor recognition in both peptide and non-peptide ligands. Using 20 reported low-energy conformations of Tyr-cyclo[D-Cys-D-Pen]-OH (JOM-13) for comparison, the geometrical relationship of the two aromatic rings present in SIOM was used for the identification of potential active conformations of JOM-13, from which two delta-receptor-binding models (I and II) were constructed. Models I and II differ from each other in the arrangement of the peptide backbones. To evaluate the two models, a conformational search of two other known delta-selective ligands, [D-Pen2,D-Pen5]enkephalin (DPDPE) and [D-Pen2,L-Pen5]enkephalin (DPLPE) was performed, using the geometrical relationship of the two aromatic rings defined in the two receptor-binding models as a molecular template. Among the conformations generated from the molecular simulation, low-energy conformers of DPDPE and DPLPE conforming to models I and II were identified. Unlike model I, conformers of DPDPE and DPLPE that fit model II contain a cis amide bond in the Gly3 residue.

Frog skin opioid peptides: a case for environmental mimicry

Naturally occurring environmental substances often mimic endogenous substances found in mammals and are capable of interacting with specific proteins, such as receptors, with a high degree of fidelity and selectivity. Narcotic alkaloids and amphibian skin secretions, introduced into human society through close association with plants and animals through folk medicine and religious divination practices, were incorporated into the armamentarium of the early pharmacopoeia. These skin secretions contain a myriad of potent bioactive substances, including alkaloids, biogenic amines, peptides, enzymes, mucus, and toxins (noxious compounds notwithstanding); each class exhibits a broad range of characteristic properties. One specific group of peptides, the opioids, containing the dermorphins (dermal morphinelike substances) and the deltorphins (delta-selective opioids), display remarkable analgesic properties and include an amino acid with the rare (in a mammalian context) D-enantiomer in lieu of the normal L-isomer. Synthesis of numerous stereospecific analogues and conformational analyses of these peptides provided essential insights into the tertiary composition and microenvironment of the receptor "pocket" and the optimal interactions between receptor and ligand that trigger a biological response; new advances in the synthesis and receptor-binding properties of the deltorphins are discussed in detail. These receptor-specific opioid peptides act as more than mimics of endogenous opioids: their high selectivity for either the mu or delta receptor makes them formidable environmentally derived agents in the search for new antagonists for treating opiate addiction and in the treatment of a wide variety of human disorders.

A supermolecule study of the effect of hydration on the conformational behaviour of leucine-enkephalin

A theoretical conformational study was performed on leu-enkephalin in its zwitterionic form, both in vacuo and in the presence of a number, n, of up to 13 water molecules saturating its first hydration shell. The intramolecular energy of enkephalin as well as the intermolecular enkephalin-water and water-water interaction energies were computed with the SIBFA procedure (Sum of Interactions Between Fragments Ab initio computed), which uses additive ab initio multipole systematics and analytical formulas grounded on ab initio SCF computations. Energy minimizations were performed with a polyvalent minimizer, Merlin, with which three distinct derivative and three distinct nonderivative minimizers can be activated in a sequential fashion. Eight different candidate conformations of enkephalin were used as starting points. These conformations are either those found in distinct X-ray structures, or those proposed on the basis of theoretical computations by other authors. In the absence of hydration, they converged towards distinct folded energy-minima, the best four ones being separated by an energy gap of 8.7 kcal/mol. In marked contrast, with up to n = 13, the energetical separation between the six best conformers narrowed down to congruent to 4 kcal/mol. They can be characterized by: (a) either a direct or a water-mediated ammonium-carboxylate interaction; b) either a close proximity (as in morphine) or a large separation between the aromatic rings of Tyr and Phe (intercenter separations of congruent to 4.5 A and congruent to 10.5 A, respectively), with each of the four mutual combinations of (a) and (b) being represented.

Conformational features responsible for the binding of cyclic analogues of enkephalin to opioid receptors. II. Models of mu- and delta-receptor-bound structures for analogues containing Phe4

Models of mu- and delta-receptor-bound backbone conformations of enkephalin cyclic analogues containing Phe4 were determined by comparing geometrical similarity among the previously found low-energy backbone structures of [D-Cys2,Cys5]-enkephalinamide, [D-Cys2,D-Cys5]-enkephalinamide, [D-Pen2,L-Pen5]-enkephalin and [D-Pen2,D-Pen5]-enkephalin. The present mu-receptor-bound conformation resembles a beta-I bend in the peptide backbone centred on the Gly3-Phe4 region. Two slightly different models were found for the delta-receptor-bound conformation; both of them are more extended than the mu-receptor-bound conformation and include a gamma-turn (or a gamma-like turn) on the Gly3 residue. Energetically favourable rotamers of Tyr and Phe side chains were also determined for the mu- and delta-conformations. The present models of mu- and delta-conformations share geometrical similarity with the low-energy structures of Leu-enkephalin and the Tyr-D-Lys-Gly-Phe-analogue.

A crystal molecular conformation of leucine-enkephalin related to the morphine molecule

Leucine-enkephalin (Try1-Gly2-Gly3-Phe4-Leu5) has been crystallized as a trihydrate from water solution. X-ray diffraction reveals a tightly folded molecular conformation with two fused beta III- (Gly2-Gly3) and beta I- (Gly3-Phe4) turns. The Tyr1 and Phe4 aromatic rings have a close orthogonal arrangement analogous to the tyramine and cyclohexenyl rings in morphine. This suggests that the conformation found in the trihydrate crystal structure could be required for recognition by mu-receptor sites.

Molecular-dynamics simulations of [Met5]- and [D-Ala2,Met5]-enkephalins. Biological implication of monomeric folded and dimeric unfolded conformations

To investigate the biologically active conformation of enkephalin, molecular-dynamics simulations were applied to [Met5]- and [D-Ala2,Met5]-enkephalins. The dynamic trajectory of monomeric extended [Met5]-enkephalin was analysed in terms of relative mobility between respective torsions of backbone chain. After 10 ps of the dynamics simulation, the conformational transition was converged into a stationary state among the beta-bend folded forms, where they are stabilized by several intramolecular hydrogen-bond formations. Similar conformational transition was also observed in the dynamics simulation of [D-Ala2,Met5]enkephalin, which is a more mu-receptor-specific peptide than [Met5]enkephalin. The geometrical correspondence between the monomeric enkephalin conformation in the stationary state and morphine molecule (a mu-specific rigid opiate) was surveyed by virtue of the triangular substructures generated by choosing three functional atoms in each molecule, and good resemblances were observed. On the other hand, the dynamics simulation of the antiparallel extended [Met5]enkephalin dimer showed a trajectory different from that of the monomeric one. Two intermolecular hydrogen bonds at Tyr1 (NH3+)...Met5(CO2-) end residues were held throughout the 100 ps simulation, the dimeric structure being consequently kept. The conformational transition of the backbone chains from the antiparallel extended form to the twisted one took place via an intermediate state. Many conformations revealed during the dynamics simulation showed that the relative orientations of each two Tyr1, Gly3, Phe4 and Met5 residues in the dimer are nearly related by a pseudo-C2-symmetry respectively, and both halves of the dimer structure could be further fitted to the monomeric folded enkephalin conformation. The monomeric and dimeric conformations of enkephalin at their stationary states are discussed in relation to the substrate-specificity for mu- and delta-opioid receptors.

Conformational modification of the PRP-hexapeptide by a direct covalent attachment of aromatic side chain groups

PRP-hexapeptide possessing the azo-bridge between Tyr1 and Phe5 residues, called azo-PRP-hexapeptide: (formula; see text), was synthesized and tested for immunoregulatory activity. High biological activity of the synthesized azo-PRP-hexapeptide suggests that the biologically active conformation of PRP-hexapeptide must be such that both aromatic rings (Tyr and Phe) are apparently close to each other.

Conformation-activity relationships of opiate analgesics

Extensive conformational calculations were performed on the potent opiate analgesics etorphine, PET, R30490 and etonitazene to determine all of their many low-energy conformations. The results were used to characterize four possible models for binding of a simple pharmacophore, comprising two phenyl rings plus a protonated nitrogen, to opiate analgesic receptors. These four models may define the necessary three-dimensional features leading to particular opiate actions. The model favoured for mu receptor activity can accommodate a protonated nitrogen, an aromatic ring (which may be substituted with an electronegative group) and a second lipophilic group. These structural features must be presented in a precise three-dimensional arrangement. It appears likely that a hydrophilic substituent in a certain region of the analgesic pharmacophore may also interact with the receptor as a secondary binding group.

Synthesis of four diastereomeric enkephalins incorporating cyclopropyl phenylalanine

Four diastereomeric D-Ala2, Leu5-enkephalins have been synthesized and their CD spectra and rat brain binding affinities determined. Only the peptides containing Z-cyclopropyl phenylalanine residues showed strong binding affinity. No correlation between CD spectra and bioactivity could be made.

NOE data at 500 MHz reveal the proximity of phenyl and tyrosine rings in enkephalin

Met5-enkephalin-a pentapeptide (Tyr-Gly-Gly-Phe-Met)-can exist in two possible folded arrangements with a rigid two-hydrogen-bonded network. In one arrangement, a Gly 2-Gly 3 beta-bend is formed and in the other a Gly 3-Phe 4 beta-bend. The two conformations are distinguished by the spatial relation of Tyr 1 and Phe 4: in the Gly 2-Gly 3 beta-bend, Tyr 1 and Phe 4 can be brought close to each other while in the Gly 3-Phe 4 beta-bend they are far apart (greater than 5 A). We have utilized one-dimensional (1D) nuclear Overhauser effect (NOE) measurements between the ring protons of Tyr 1 and Phe 4 to determine their proximity. The NOE data clearly show that a pair protons, one each from Tyr 1 and Phe 4, are as close as 3.3 A while other inter-proton distances are beyond 4.5 A. Therefore, we propose the presence of a Gly 2-Gly 3 beta-bend (in which Tyr 1 and Phe 4 are spatially close) for Met5-enkephalin in solution. The structure of Met5-enkephalin in solution is very similar to the single crystal structure of Leu5-enkephalin and tends to explain the biological activity data of several modified enkephalins.

Leucine enkephalin analogues containing a conformationally restrained N-terminal amino acid residue

Three analogues of leucine enkephalin, in which the terminal tyrosine-1 residue has been replaced by conformationally restrained aromatic amino acids, have been synthesized by classical solution methods. Their opiate agonist potencies on electrically stimulated guinea pig ileum and mouse vas deferens preparations were determined and compared with morphine, Met enkephalin, and Leu enkephalin. None of these analogues had analgesic properties when evaluated on the above tissue preparations or when evaluated by the hot-plate test in mice after subcutaneous and intracerebroventricular administration.

X-ray diffraction studies of enkephalins. Crystal structure of [(4'-bromo) Phe4,Leu5]enkephalin

In order to investigate the structure-activity relationship of [Leu5]- and [Met5]enkephalins, [(4'-bromo)Phe4, Leu5]-, [(4'-bromo)Phe4, Met5]- and [Met5] enkephalins were synthesized and crystallized. The crystal structure of [(4'-bromo) Phe4, Leu5]- enkephalin was determined by X-ray diffraction method using the heavy atom method and refined to R = 0.092 by the least-squares method. The molecule in this crystal took essentially the same type I' beta-turn conformation found in [Leu5]enkephalin [Smith & Griffin (1978) Science 199, 1214-1216). On the other hand, the preliminary three-dimensional Patterson analyses showed that the most probable conformations of [(4'-bromo)Phe4,Met5]- and [Met5]enkephalins are both the dimeric extended forms. Based on these insights, the biologically active conformation of enkephalin was discussed in relation to the mu- and delta-receptors.

Stabilization of beta-turn conformations in enkephalins. alpha-Aminoisobutyric acid analogs

Stereochemical constraints have been introduced into the enkephalin backbone by substituting alpha-aminoisobutyryl (Aib) residues at positions 2 and 3, instead of Gly. 1H n.m.r. studies of Tyr-Aib-Gly-Phe-Met-NH2, Tyr-Aib-Aib-Phe-Met-NH2 and Tyr-Gly-Aib-Phe-Met-NH2 demonstrate the occurrence of folded, intramolecularly hydrogen bonded structures in organic solvents. Similar conformations are also favoured in the corresponding t-butyloxycarbonyl protected tetrapeptides, which lack the Tyr residue. A beta-turn centred at positions 2 and 3 is proposed for the Aib2-Gly3 analog. In the Gly2-Aib3 analog, the beta-turn has Aib3-Phe4 as the corner residues. The Aib2-Aib3 analog adopts a consecutive beta-turn or 3(10) helical conformation. High in vivo biological activity is observed for the Aib2 and Aib2-Aib3 analogs, while the Aib3 peptide is significantly less active.

Operation of long-range substituent effects in rigid opiates: protonated and unprotonated oxymorphone

The structure of protonated oxymorphone (amine salt) was determined by an X-ray crystallographic study. Significant differences were found with the previously determined structure of unprotonated oxymorphone (free base). Upon protonation on nitrogen, an elongation of the N-C bound occurred, accompanied by subtle changes in bond lengths and angles distant from the site of protonation. These changes in geometry are interpreted as a reflection of long-range substituent effects.

270-MHz 1H nuclear-magnetic-resonance study of met-enkephalin in solvent mixtures. Conformational transition from dimethylsulphoxide to water

1H spectra at 270 MHz of zwitterionic Met-enkephalin pentapeptide (Tyr-Gly-Gly-Phe-Met) in (C2H3)2SO/water mixtures are reported and discussed in terms of solvent-induced conformational transitions. The analysis of the chemical shifts, line widths, coupling constants and rotamer populations around chi 1 and chi 2 suggests that the conformational properties of Met-enkephalin in the two solvents are quite different. In aqueous solution, the preferred structure, characterized by the absence of intramolecularly hydrogen-bonded NH groups and head-to-tail interactions, very likely is an equilibrium of unfolded conformations with approximately equal energy. In (C2H3)2SO, the preferred structure is folded, with the Met-5 NH intramolecularly bonded and the Gly-3 NH protected from the solvent, while the Gly-2 and Phe-4 amide protons are solvent exposed. A conformational transition of Met-enkephalin from the intramolecularly bonded to the unbonded one takes place at about 40 mol-% water in (C2H3)2SO, involving the Met-5 NH proton and the Tyr-Gly-Gly fragment. The Phe-4 and Met-5 phi angles do not change appreciably, which suggest that an inversion at the Gly-3 residue of the folded form, responsible for the conformational transition, does not affect the C-terminal moiety. At about 70 mol-% water in (C2H3)2SO a change in the solvent mixture properties affects the chi 1 rotamer populations and the ring dynamics of the aromatic side chains. The line broadening of the Tyr-1 delta and epsilon proton resonances indicates a specific interaction of the N-terminal ring with the solvent.

Theoretical conformational analysis of enkephalin analogues related to fluorescence and n.m.r. measurements in aqueous solution

The properties related to non-radiative energy transfer of a number of enkephalin analogues with tryptophan substituted for phenylalanine in position 4 and n.m.r. 3JNH-C alpha H coupling constants of corresponding [Phe4]-enkephalin analogues are being derived from semi-empirical conformational energy. The molecules considered contain a glycyl, a D-alanyl or an L-alanyl as second residue; two of the compounds are N-methylated at position 4 or 5. The [Trp4]-enkephalin analogues and the corresponding [Phe4]-enkephalin analogues display nearly parallel affinities in the opiate receptor binding assay (Schiller et al. (1). The comparison of computed and experimental properties shows that an ensemble of conformers is a satisfactory representation of the state of these molecules in water.

Conformational energy calculations on enkephalins and enkephalin analogs. Classification of conformations to different configurational types

Conformational energy calculations were carried out on the peptide enkaphalins (ENK) and selected analogs to find those conformers of low energy. The analogs studied include [D-Ala2]Enk-NH2, [D-Ala2]Enk, [D-Met2, Pro5]Enk-NH2, [D-Ala2, D-Phe5]Enk, [D-Ala2, D-Leu5]Enk, [D-Ala2, (N-Me)Phe4, Met5] Enk-NH2 and [D-Ala2, (N-Me)Met5]Enk-NH2. When the low-energy conformers for all the analogs are compared, different allowed backbone conformations are found which orient the functional side-chains such that three classifications of structures appear. Each classification shows a unique configuration of side-chain positions in space even though different backbone conformations are found within each classification.

To the problem of biologically active conformation of enkephalin

A semi-rigid structural analog of [Leu5] enkephalin, possessing the azo-bridge between Tyr1 and Phe4 residues, was synthesized, along with two other linear enkephalin analogs: [4'-amino Phe4] enkephalin and [4'/hydroxyphenyl/-azo Phe4] enkephalin. The results of the determination of the analgesic activity of the synthesized compounds suggest that the biologically active conformation of the enkephalin molecule should be such that both aromatic rings, Tyr1 and Phe4, are situated in close proximity.

Minimum structure opioids-dipeptide and tripeptide analogs of the enkephalins

Through a systematic reduction of peptide structure, a series of 25 tripeptide and 5 dipeptide amide and alcohol analogs of enkephalin were synthesized and assayed in vitro on the stimulated guinea pig ileum. Tyr-Pro-Phe-NH2, Tyr-D-Ala-Phe-NH2, Tyr-D-Ala-Phe-ol and Tyr-D-Phe-Phe-NH2 had 20-25% the potency of Met-enkephalin. Four aromatic alkylamides of the dipeptide Tyr-D-Ala were made with benzylamine, phenethylamine, phenylpropylamine and phenylbutylamine. All had full naloxone reversible enkephalin-like activity in the ileum assay. Tyr-D-Ala-phenylpropylamide has about 80% the potency of Met-enkephalin in vitro, and is equipotent with Tyr-D-Ala-Gly-Phe-Met-NH2 in producing analgesia in mice after intraventricular administration. Tyr-D-Phe-NH2 is the smallest peptide to show full intrinsic enkephalin-like activity in vitro, although its potency is very low.

A cyclic enkephalin analog with high in vitro opiate activity

A conformationally restricted analog of [Leu5]enkephalin was synthesized by cyclization of the COOH-terminal carboxyl group of leucine to the gamma-amino moiety of alpha, gamma-diaminobutyric acid (A2bu) substituted in position 2 of the peptide. Relative to [Leu5]enkephalin, the cyclic analog with D configuration in position 2, H-Tyr-cyclo(-N gamma-D-A2bu-Gly-Phe-Leu-), was 17.5 times more potent in the guinea pig ileum assay and twice as potent in the rat brain receptor binding assay, whereas its diastereomer H-Tyr-cyclo(-N gamma-L-A2bu-Gly-Phe-Leu-) showed low activity. The cyclic D isomer was also slightly more active than the open-chain reference compound [D-Nva2, Leu5]enkephalinamide in both assays, and it proved to be highly resistant to degradation by brain "enkephalinases." The steric constraints introduced in H-Tyr-cyclo(-N gamma-D-A2bu-Gly-Phe-Leu-) were shown to prevent the realization of most of the conformational features ascribed to linear enkephalin in solution or in the crystalline state and permitted an assessment of proposed models of the conformation of enkephalin when it is bound to the receptor.

Elucidation of the receptor-bound conformation of the enkephalins

The biologically relevant conformers of enkephalin predicted by solid state, solution state, and theoretical energy studies have been compared with the published structure-activity data on these compounds. No conformational technique proposes a model consistent with all the pharmacological data; the shortcomings of each approach are evaluated. An alternative approach, which correlates the structure-activity data of opiate compounds with that of the enkephalins, is described and shown to produce a model consistent with the available structure-activity data.