PEPTIDE DYNAMICS IN TRIGLYCINE - COUPLING OF INTERNAL BOND ROTATIONS AND OVERALL MOLECULAR TUMBLING

As a model system, triglycine (G1-G2-G3) permits a thorough investigat ion of peptide backbone motional dynamics by using C-13- and N-15-NMR relaxation. Previously, rotational model analyses of the nonterminal g lycine, G2, could not adequately explain C-13-NMR relaxation data Dara gan, V. A.; Mayo, K. H. Biochemistry 1993, 32, 11488). In this study, N-15-NMR relaxation measurements on N-15-enriched triglycine provide a dditional motional vectors for more complete rotational model analyses . The inadequancy in describing G2 internal motions with models of ani sotropic or restricted rotational diffusion is overcome by using a rot ational jump model which has been parameterized with a semiempirical c oefficient for backbone recoil rotation. Effectively, this recoil acti on couples internal bond rotations and overall molecular tumbling. Sto chastic dynamics computer simulations using this recoil coefficient al low calculation of triglycine (CH)-C-13 and (NH)-N-15 autocorrelation times and (CH2)-C-13 cross-correlation times. Good agreement between e xperiment and theory is found only when strong recoil coupling is take n into account.