A NOVEL MODEL-FREE ANALYSIS OF C-13 NMR RELAXATION OF ALANINE-METHYL SIDE-CHAIN MOTIONS IN PEPTIDES

Alanine residues in two peptide sequences derived from the beta-sheet domain of platelet factor-4: IATLK (P5) and TAQLIA*TLK NGRKICLDLQA (P 20), were synthesized with C-13 enriched in C-alpha and C-beta positio ns. C-13 NMR relaxation measurements (proton coupled and decoupled) we re performed at two NMR frequencies (150 and 62.5 MHz) and over a wide range of temperatures (5 to 75 degrees C) to study motional dynamics of the alanine side-chain methyl group and alanine side-chain-backbone motional correlations, Cross-correlation spectral densities, J(HCH)(w (C)), for CbetaH3 in both peptides are positive, indicating methyl-gro up rotational anisotropy. Various rotational models and model-free app roaches have been used to analyze NMR relaxation data. The overall cor relation times show a stronger temperature dependence in P20 than in P 5, indicating the influence on alanine motions of folded conformationa l populations in P20. For analysis of alanine side-chain motions, an a lternative model-free approach parameterized with a novel mixing param eter, A(2), that depends on the ''geometry'' of C-alpha-C-beta and C-a lpha-H bond rotations is proposed, By plotting the standard order para meter S-2 versus A(2), motional models may be visually differentiated. Molecular dynamics calculations were performed to compare C-alpha-C-b eta and C-alpha-H motions. Significant anticorrelated phi(t) and psi(t ) backbone rotations can explain NMR relaxation data for P20. (C) 1996 Academic Press, Inc.