1H n.m.r. investigation of conformational features of cyclic enkephalinamide analogs

Structure-activity relationships of bifunctional cyclic disulfide peptides based on overlapping pharmacophores at opioid and cholecystokinin receptors

Prolonged opioid exposure increases the expression of cholecystokinin (CCK) and its receptors in the central nervous system (CNS), where CCK may attenuate the antinociceptive effects of opioids. The complex interactions between opioid and CCK may play a role in the development of opioid tolerance. We designed and synthesized cyclic disulfide peptides and determined their agonist properties at opioid receptors and antagonist properties at CCK receptors. Compound 1 (Tyr-c[d-Cys-Gly-Trp-Cys]-Asp-Phe-NH(2)) showed potent binding and agonist activities at delta and mu opioid receptors but weak binding to CCK receptors. The NMR structure of the lead compound displayed similar conformational features of opioid and CCK ligands.

Comparative conformational analysis of [D-Pen2,D-Pen5]enkephalin (DPDPE): a molecular mechanics study

A theoretical conformational analysis (molecular mechanics study) of the delta opioid receptor-selective enkephalin analog H-Tyr-D-Pen-Gly-Phe-D-Pen-OH (DPDPE) was performed, based on the use of the SYBYL software. The study led to the identification of several conformers that were significantly lower in energy than previously reported candidate conformers of DPDPE which, for comparative purposes, were also minimized by using the standard SYBYL force field. The results revealed a considerable degree of conformational flexibility of the DPDPE molecule, and suggested that incorporation of further conformational constraints into this enkephalin analog will be necessary in order to elucidate its receptor-bound conformation.

Conformational features responsible for binding of cyclic analogues of enkephalin to opioid receptors. I. Low-energy peptide backbone conformers of analogues containing Phe4

Low-energy peptide backbone conformers were found by means of energy calculation for several cyclic analogues of enkephalin in an attempt to assess models for receptor-bound conformations for opioid receptors of the mu- and delta-types. They included [D-Cys2, L-Cys5]- and [D-Cys2, D-Cys5]-enkephalinamides showing moderate preference for mu-receptors, the delta-selective compounds [D-Pen2, L-Pen5] and [D-Pen2, D-Pen5]-enkephalins and Tyr-D-Lys-Gly-Phe- analogue possessing very high affinity to receptors of the mu-type. The low-energy conformers obtained for these analogues were in good agreement with the results of calculations by other authors and with experimental evidence. All of the analogues contain a Phe residue in position 4 of the peptide chain which facilitates the eventual search for geometrical similarity between the low-energy backbone conformers of different analogues in question.

Conformational search in enkephalin analogues containing a disulfide bond

A systematic conformational search has been performed for the 14-membered ring in model compounds for disulfide-containing enkephalin analogues. The model compounds examined are [formula: see text], and the corresponding compounds with L-amino acids at the C-terminus. About 100 starting conformations were generated for each compound with the RNGCFM program and energy minimized with the AMBER program. Between 21 and 38 conformers within 3 kcal/mole of the apparent global minimum were found for each compound. There appeared to be fewer possible conformations of the disulfide-containing side chain than of the main chain. [formula: see text], whose parent compound is selective for opioid delta receptors, was found to prefer conformers with a positive dihedral angle of the disulfide bond, which is consistent with the previous proposal that delta-receptor selectivity may be associated with this conformational preference. Additional calculations were performed on the complete structure of [formula: see text] (DPDPE) with various possible conformations of the tyrosine and phenylalanine side chains. Conformational free energies and entropies were computed for these conformers from the molecular vibrations obtained from a normal mode analysis. As was found previously, conformers with low energies tended to have lower entropies, which resulted in a narrowing of the free energy differences between conformers. A conformer is identified that has the lowest energy hitherto found for DPDPE. It is suggested that DPDPE may be a useful compound for evaluating conformational search strategies because of its relatively small size and the number of conformers that have already been identified. Conformational energy calculations are also reported for naltrindole using the MM2(87) program. Naltrindole, which incorporates two aromatic 6-membered rings in a rigid structure, is a highly selective and potent opioid delta-receptor antagonist and may be an important clue regarding the biologically active conformer of DPDPE. Various conformers of DPDPE have been superimposed quantitatively onto the structure of naltrindole using the SUPER program and those conformers of DPDPE that are the best fit to naltrindole are reported.

Conformational analysis of enkephalin analogs containing a disulfide bond. Models for delta- and mu-receptor opioid agonists

Conformational analysis of the cyclic opioids H-Tyr-D-Pen-Gly-Phe-D-Pen-OH (DPDPE) and H-Tyr-D-Cys-Gly-Phe-D-Cys-OH (DCDCE) have been performed using the AMBER program. DPDPE is considerably more selective for delta-receptors than DCDCE. Using the RNGCFM program, a large number of ways were found to close the 14-membered disulfide-containing ring structure. However, intramolecular hydrogen bonds were only possible in gamma-turn and inverse gamma-turn conformations centered on the glycine residue which were associated with opposite chiralities of the disulfide bond. With the cyclic part of the molecules in either a gamma-turn or inverse gamma-turn, a systematic conformational analysis was performed on the tyrosine and phenylalanine sidechains. This showed that conformers with the tyrosine and phenylalanine phenyl rings in the vicinity of the disulfide bond were preferred due to attractive van der Waals forces. For DPDPE, however, this was only possible with a positive dihedral angle for the disulfide bond due to the presence of the beta-carbon methyls of Pen2. In contrast, these preferred conformers were possible with both chiralities of the disulfide bond in DCDCE. Conformational entropies and free energies were computed from the translational, rotational, and vibrational energy levels available to each conformer. The conformational entropies were found to vary significantly and to result in a re-ordering of the lowest energy minima. Based on these conformational differences in DPDPE and DCDCE and their differing pharmacological selectivities, tentative conformational preferences for delta- and mu-receptor opioid peptides are proposed.

Role of the conformational element in peptide-receptor interactions. Studies with cyclic opioid peptide analogs

Biological activity profiles of three different families of cyclic opioid peptide analogs are presented. It is illustrated that conformational constraints introduced through peptide cyclizations can have drastic effects on receptor affinity, selectivity and 'efficacy' ('intrinsic activity'). Conformational studies of cyclic opioid peptides by various physico-chemical techniques have been initiated and have already produced insight into the conformational requirements of the various opioid receptor types. On the basis of the results obtained, conformational restriction of opioid peptides may represent a first promising step towards the goal of developing peptide mimetics.

Role of steric interactions in the delta opioid receptor selectivity of [D-Pen2, D-Pen5]enkephalin

In order to assess the individual effects of each of the 3-methyl groups in residue 2 of [D-Pen2, D-Pen5]enkephalin on binding affinity to mu and delta opioid receptors, (2S,3S)methylcysteine ((3S)Me-D-Cys) and (2S,3R)methylcysteine ((3R)Me-D-Cys) were synthesized and incorporated into the analogs, [(3S)Me-D-Cys2, D-Pen5] enkephalin and [(3R)Me-D-Cys2, D-Pen5]enkephalin. Of these analogs, [(3S)Me-D-Cys2, D-Pen5]enkephalin appears from 1H n.m.r. spectra to assume a conformation similar to those of [D-Pen2, D-Pen5]enkephalin and the less delta receptor-selective, but more potent, [D-Cys2, D-Pen5]enkephalin. Assessment of binding affinity to mu and delta receptors revealed that [(3S)Me-D-Cys2, D-Pen5]enkephalin exhibits delta receptor affinity intermediate between [D-Pen2, D-Pen5]enkephalin and [D-Cys2, D-Pen5]enkephalin while its mu receptor affinity is similar to that of [D-Cys2, D-Pen5]enkephalin. These results suggest that, for [D-Pen2, D-Pen5]enkephalin, adverse steric interactions between the D-Pen2 pro-R methyl group and the mu receptor binding site lead to the low mu receptor binding affinity observed for this analog. By contrast, both the pro-R and pro-S D-Pen2 methyl groups lead to minor steric interactions which contribute to the somewhat lower delta receptor affinity of this compound.

Comparison of conformational properties of linear and cyclic delta selective opioid ligands DTLET (Tyr-D X Thr-Gly-Phe-Leu-Thr) and DPLPE (Tyr-c[D X Pen-Gly-Phe-Pen]) by 1H n.m.r. spectroscopy

The preferential conformations of the delta selective opioid peptides DPLPE (Tyr-c[D X Pen-Gly-Phe-Pen]) and DTLET (Tyr-D X Thr-Gly-Phe-Leu-Thr) were studied by 400 MHz 1H n.m.r. spectroscopy in DMSO-d6 solution. In neutral conditions, the weak NH temperature coefficients of the C-terminal residue (Pen5 or Thr6), associated with interproton NH-NH and alpha-NH NOE's (ROESY experiments), indicated large analogies between the backbone folding tendency of both the linear and cyclic peptides. Various gamma and/or beta turns may account for these experimental data. A similar orientation of the N-terminal tyrosine related to the folded backbones is observed for the two agonists, with a probable gamma turn around the amino acid in position 2. Finally, a short distance, about 10 A, between Tyr and Phe side chains and identical structural roles for threonyl and penicillamino residues are proposed for both peptides. These results suggest the occurrence of similar conformers in solution for the constrained peptide DPLPE and the flexible hexapeptide DTLET. Therefore, it may be hypothesized that the enhanced delta selectivity of DPLPE is related to a very large conformational expense of energy needed to interact with the mu opioid receptor, a feature not encountered in the case of DTLET. These findings might allow peptides to be designed retaining a high affinity for delta opioid receptors associated with a very low cross-reactivity with mu binding sites.

1H-n.m.r. investigation of conformational features of cyclic, penicillamine-containing enkephalin analogs

Conformational features of a series of cyclic, penicillamine-containing enkephalin analogs, all of which display selectivity for the delta opioid receptor, were studied by 1H n.m.r. in aqueous solution. Comparison of chemical shifts, coupling constants, and temperature dependence of amide proton chemical shifts suggests different conformational features among the analogs, some of which can be related to the different primary sequences of these peptides. The observation that some of the analogs display disparate individual conformational features while exhibiting similar opioid potency and receptor selectivity suggests that such analogs may share a similar overall topography or at the least maintain the same relative orientations of key portions of the molecule.