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 fully deuterated
dodecylphosphocholine micelles. H-Tyr-Tic-NH-(CH2)2-Ph is an
opioid delta-agonist, whereas H-Tyr-Tic-NH-(CH2)3-Ph is a fairly potent delta-antagonist. H-Tyr-Tic-NH-(CH2)2-cHx is a less potent delta-antagonist. 1H-NMR spectra revealed that conformers containing cis and trans configurations of the
Tyr-Tic peptide bond were present 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
dipeptides the C-terminal substituent is flexible and the Tyr-side-chain adopts a trans orientation in most of the conformations. This promotes a compact
Tyr-Tic structure. NOE patterns observed for the compounds in the
micelle solution indicate that Tyr has an even greater tendency to assume a trans side chain configuration in the biphasic-
solvent system. This feature was more pronounced in the trans conformers than in the cis conformers. Specific
lipid-
peptide interactions were indicated by NOESY 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 interior of the
micelle for the trans conformers of all three
dipeptides and for the cis conformer of H-Tyr-Tic-NH-(CH2)2-Ph. For the cis isomers, partitioning of the C-terminal substituents into the
lipid phase exhibited more diverse behaviour. The cis conformers of H-Tyr-Tic-NH-(CH2)3-Ph and H-Tyr-Tic-NH-(CH2)2-cHx preferentially anchor to the
micelle via their C-terminal substituent, while the corresponding region in H-Tyr-Tic-NH-(CH2)2-Ph remains flexible and immersed in the aqueous phase. The inconsistent mode of
peptide-
micelle interaction observed for cis conformers of the three compounds studied is explained in terms of differences in their
dipeptide-substituent hydrophobicities. The more apolar the substituent, the greater its tendency to preferentially insert into the
lipid core of the
micelle.
Amide-
proton temperature coefficients measured for the three
peptides revealed differences amongst the cis and trans isomers. The
amide proton in the trans conformer of each compound is highly exposed to the aqueous phase in both
solvent systems studied, whereas the cis NH
proton of each
peptide is only partially exposed. These results demonstrate that a subtle structural modification of an active
peptide analog can result in dramatic changes of its
biological activity and its mode of partitioning at a membrane surface.