MANIFESTATION OF INTRAMOLECULAR MOTIONS ON PICOSECOND AND NANOSECOND TIME SCALES IN H-1-N-15 NMR RELAXATION - ANALYSIS OF DYNAMIC-MODELS OFONE-HELICAL AND 2-HELICAL SUBUNITS OF BACTERIOOPSIN

The influence of the internal dynamics of two polypeptides comprising transmembrane alpha-helix A or two alpha-helices A and B of bacterioop sin on experimentally accessible N-15 NMR relaxation rates was investi gated by molecular dynamics (MD) simulations, combined with more simpl e mechanic considerations. 'Model-free' order parameters and correlati on times of internal motions [Lipari, G, and Szabo, A. (1982) J: Am. C hem. Sec., 104, 4546-4559] were calculated for these models. It was fo und that both peptides exhibit two types of internal motions of the am ide bonds, on the pico- and nanosecond time scales, affecting N-15 NMR relaxation. The fast fluctuations are local and correspond to the lib rational motions of the individual N-H vectors in an effective potenti al of atoms of the surrounding matrix. In contrast, the motions on the nanosecond time scale imply concerted collective vibrations of a larg e number of atoms and could be represented as bending oscillation of a lpha-helices, strongly overdamped by the ambient solvent. A few other molecular mechanisms of slow internal motion were found, such as local distortions of the a-helices (e.g., a-aneurysm), delocalized distorti ons of the alpha-helical backbone, as well as oscillations of the tilt angle between the axes of the alpha-helices A and B. The results are compared with N-15 NMR relaxation data measured for the (1-36)bacterio opsin and (1-71)bacterioopsin polypeptides in chloroform-methanol (1:1 ) and in SDS micelles [Orekhov, V. Yu., Pervushin, K.V. and Arseniev, A.S. (1994) Eur: J. Biochem., 219, 887-896].