TIME-RESOLVED FLUORESCENCE STUDY OF CONFORMATIONAL DYNAMICS IN OPIOID-PEPTIDES

Rotational correlation times have been determined from fluorescence an isotropy decays of the tyrosyl residue in the opioid pentapeptides DPD PE (Tyr-D-Pen-Gly-Phe-D-Pen), DPDPE(SH)(2), and [Leu(5)]-enkephalin, r evealing internal peptide motions. Fluorescence decays were measured b y time-correlated single-photon counting. For all three peptides, the fluorescence emission is characterized by three-exponential intensity decays with amplitudes that are consistent with ground-state populatio ns of rotamers of the tyrosyl side chain. Rotational correlation times in water determined from single-exponential fits are 80-130 ps, in go od agreement with molecular dynamics simulations [Wang, Y.; Kuczera, K . J. Phys. Chem. 1996, 100, 2555-2563]. Internal peptide motions were studied by measurement of the rotational correlation times in solution s of 50% propylene glycol in water over the temperature range from 258 to 313 K. Two distinct temperature regions were observed. In the low- temperature regime the thermal viscosity coefficient for each peptide is approximately 0.07 K-1, the same as for free tyrosine. Hence, in th is temperature regime the rotational friction is imposed by the solven t alone, consistent with rigid-body rotational motion. At higher tempe ratures an additional source of reorientational motion is revealed by an apparent change in the thermal viscosity coefficient. The viscosity coefficient in the high-temperature regime is characteristic of the p eptide and not just the solvent, indicating the influence of internal dynamics. Double-exponential fits yielded further evidence of internal tyrosyl reorientational motions, which make increasingly large relati ve contributions at higher temperatures.