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: IA*TLK (P5) and TAQLIA*TLK NGRKICLDLQA (P20), were synthesized with C-13 enriched in C-alpha and C-beta positions. C-13 NMR relaxation measurements (proton coupled and decoupled) were 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-group rotational anisotropy. Various rotational models and model-free approaches have been used to analyze NMR relaxation data. The overall correlation times show a stronger temperature dependence in P20 than in P5, indicating the influence on alanine motions of folded conformational populations in P20. For analysis of alanine side-chain motions, an alternative model-free approach parameterized with a novel mixing parameter, A(2), that depends on the ''geometry'' of C-alpha-C-beta and C-alpha-H bond rotations is proposed, By plotting the standard order parameter S-2 versus A(2), motional models may be visually differentiated. Molecular dynamics calculations were performed to compare C-alpha-C-beta 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.