Internal motional amplitudes and correlated bond rotations in an alpha-helical peptide derived from C-13 and N-15 NMR relaxation

Peptide GFSKAELAKARAAKRGGY folds in an alpha -helical conformation that is stabilized by formation of a hydrophobic staple motif and an N-terminal cap ping box (Munoz V, Bianco FJ, Seriano L, 1995, Struct Biol 2:380-385). To i nvestigate backbone and side-chain internal motions within the helix and hy drophobic staple, residues F2, A5, L7, A8, and A10 were selectively C-13- a nd N-15-enriched and NMR relaxation experiments were performed in water and in water/trifluoroethanol (TFE) solution at four Larmor frequencies (62.5, 125, 150, and 200 MHz for C-13). Relaxation data were analyzed using the m odel free approach and an anisotropic diffusion model. In water, angular va riances of motional vectors range from 10 to 20 degrees and backbone phi,ps i bond rotations for helix residues A5, L7, A8, and A10 are correlated indi cating the presence of C-alpha-H, C-alpha-C-beta, and N-H rocking-type moti ons along the helix dipole axis. L7 side-chain CbetaH2 and CgammaH motions are also correlated and as motionally restricted as backbone CalphaH, sugge sting considerable steric hindrance with neighboring groups. In TEE which s tabilizes the fold, internal motional amplitudes are attenuated and rotatio nal correlations are increased. For the side chain of hydrophobic staple re sidue F2, wobbling-in-a-cone type motions dominate in water, whereas in TFE , the C-beta-C-gamma bond and phenyl ring fluctuate more simply about the C -alpha-C-beta bond. These data support the Daragan-Mayo model of correlated bond rotations (Daragan VA, Mayo KH, 1996, J Phys Chem 100:8378-8388) and contribute to a general understanding of internal motions in peptides and p roteins.