MOTIONAL DYNAMICS OF RESIDUES IN A BETA-HAIRPIN PEPTIDE MEASURED BY C-13-NMR RELAXATION

Structurally characterizing partially folded peptides is problematic g iven the nature of their transient conformational states. C-13-NMR rel axation data can provide information on the geometry of bond rotations , motional restrictions, and correlated bond rotations of the backbone and side chains and, therefore, is one approach that is useful to ass ess the presence of folded structure within a conformational ensemble. A peptide 12mer, R(1)GITVNG(7)KTYGR(12), has been shown to partially fold in a relatively stable beta-hairpin conformation centered at NG. Here, five residues, G2, V5, G7, Y10, G11, were selectively C-13-enric hed, and C-13-NMR relaxation experiments were performed to obtain auto - and cross-correlation motional order parameters, correlation times, bond rotation angular variances, and bond rotational correlation coeff icients. Our results indicate that, of the three glycines, G7 within t he hairpin beta-turn displays the most correlated phi(t),psi(t) rotati ons with its axis of rotation bisecting the angle defined by the H-C-H bonds. These positively correlated bond rotations give rise to ''twis ting'' type motions of the HCH group. V5 and Y10 phi,psi bond rotation s are also positively correlated, with their CbetaCalphaH groups under going similar ''twisting'' type motions. Motions of near-terminal resi dues G2 and G11 are less restricted and less correlated and are best d escribed as wobbling-in-a-cone. V5 and Y10 side-chain motions, aside f rom being highly restricted, were found to be correlated with phi,psi bond rotations. At 303 K, where the hairpin is considered ''unfolded,' ' the peptide exists in a transient, collapsed state because backbone and side-chain motions of V5, G7, and Y10 remain relatively restricted , unlike their counterparts in GXG-based tripeptides. These results pr ovide unique information toward understanding conformational variabili ty in the unfolded state of proteins, which is necessary to solve the protein folding problem.