Conformation and mobility of tyrosine side chain in tetrapeptides. Specific effects of cis- and trans-proline in Tyr-Pro- and Pro-Tyr-segments.

We examined the properties of tyrosine in four free tetrapeptides: Ala-Ala-Tyr-Ala (AATA), Ala-Pro-Tyr-Ala (APTA), Ala-Tyr-Ala-Ala (ATAA) and Ala-Tyr-Pro-Ala (ATPA) by CD, n.m.r. and energy calculations. Experimental data (the aromatic 1Lb signal, rotamer populations around the C alpha-C beta bond (chi 1), rotations around C beta-C gamma(chi 2), chemical shifts of ortho- and meta-protons in the phenolic ring (in aqueous and Me2SO solutions), NH proton temperature coefficients and vicinal coupling constants 3JNH-C alpha H in the backbone (Me2SO solution) were compared with calculated minimum energy conformations. We find qualitative agreement between the results of the different techniques with respect to global tendencies of conformational behaviour: we present experimental evidence showing that the presence of proline in the sequence has a more pronounced effect on the side chain organization of the residues preceding it than on one succeeding it. This steric influence of proline on its immediate neighbor is even stronger in the cis isomer than in the more common trans isomer. The strong preference for Rotamer II (chi 1 = 180 degrees) over Rotamer I (chi 1 = -60 degrees) in ATPA (cis-form) concomitant with a noticeable deviation of chi 2 is a striking example.