ROLE OF HYDROPHOBIC SUBSTITUENTS IN THE INTERACTION OF OPIOID TYR-TICDIPEPTIDE ANALOGS WITH DODECYLPHOSPHOCHOLINE MICELLES - MOLECULAR PARTITIONING IN MODEL MEMBRANE SYSTEMS

The conformational properties of three Tyr-Tic-NH-R dipeptide analogs [where R = (CH2)(2)-Ph, (CH2)(3)-Ph or (CH2)(2)-cHx; Ph = phenyl; cHx = cyclohexyl and Tic = tetrahydroisoquinoline-3-carboxylic acid] have been investigated in purely aqueous solution and in the presence of fu lly deuterated dodecylphosphocholine micelles. H-Tyr-Tic-NH-(CH2)(2)-P h is an opioid delta-agonist, whereas H-Tyr-Tic-NH-(CH2)(3)-Ph is a fa irly potent delta-antagonist. H-Tyr-Tic-NH-(CH2)(2)-cHx is a less pote nt delta-antagonist. H-1-NMR spectra revealed that conformers containi ng cis and trans configurations of the Tyr-Tic peptide bond were prese nt in all compounds in H2O and the H2O/lipid solvent. Analyses of the NMR data for the compounds in H2O indicate that in all three dipeptide s the C-terminal substituent is flexible and the Tyr-side-chain adopts a trans orientation in most of the conformations. This promotes a com pact Tyr-Tic structure. NOE patterns observed for the compounds in the micelle solution indicate that Tyr has an even greater tendency to as sume a trans side chain configuration in the biphasic-solvent system. This feature was more pronounced in the trans conformers than in the c is conformers. Specific lipid-peptide interactions were indicated by N OESY spectra acquired for micelle samples incorporating 20% (by mass) protonated lipid. According to the obtained NOE data, Tyr and Tic form an aromatic cluster which preferentially inserts into the lipid inter ior of the micelle for the trans conformers of all three dipetides and for the cis conformer of H-Tyr-Tic-NH-(CH2)(2)-Ph. For the cis isomer s, partitioning of the C-terminal substituents into the lipid phase ex hibited more diverse behaviour. The cis conformers of H-Tyr-Tic-NH-(CH 2)(3)-Ph and H-Tyr-Tic-NH-(CH2)(2)-cHx preferentially anchor to the mi celle via their C-terminal substituent, while the corresponding region in H-Tyr-Tic-NH-(CH2)(2)-Ph remains flexible and immersed in the aque ous phase. The inconsistent mode of peptide-micelle interaction observ ed for cis conformers of the three compounds studied is explained in t erms of differences in their dipeptide-substituent hydrophobicities. T he more apolar the substituent, the greater its tendency to preferenti ally insert into the lipid core of the micelle. Amide-proton temperatu re coefficients measured for the three peptides revealed differences a mongst the cis and trans isomers. The amide proton in the trans confor mer of each compound is highly exposed to the aqueous phase in both so lvent systems studied, whereas the cis NH proton of each peptide is on ly partially exposed. These results demonstrate that a subtle structur al modification of an active peptide analog can result in dramatic cha nges of its biological activity and its mode of partitioning at a memb rane surface.