STRUCTURAL AND DYNAMIC PROPERTIES OF A BETA-HAIRPIN-FORMING LINEAR PEPTIDE .2. C-13 NMR RELAXATION ANALYSIS

In the preceding paper in this issue (K. L. Constantine et al. J. Am. Chem. Sec. 1995, 117, 10841-10854), the structural and dynamic propert ies of the beta-hairpin forming linear peptide Y-Q-N-P-D-G-S-Q-A (one letter amino acid code; F. J. Blanco et al. J. Am. Chem. Sec. 1993, 11 5, 5887-5888) were characterized by molecular modeling using ensemble- averaged constraints. In this report, the dynamic behavior of the pept ide backbone is further investigated by 2D H-1-C-13 NMR methods at nat ural C-13 abundance. The dynamics of the backbone methine H-alpha-C-al pha sites were characterized by measurements of {H-1}-C-13 steady stat e NOEs, C-13 spin-lattice relaxation rates R(1)(C), C-13 spin-spin rel axation rates R(2)(C), relaxation rates of longitudinal two-spin order R(1zz)(H,C), and the spin-lattice relaxation rates of C-13-attached p rotons R(1)(H). Relaxation observables were fit using model-free spect ral density functions. The results of this analysis indicate relativel y low mobility on a picosecond-nanosecond time scale for residues 2, 3 , 4, and 5, intermediate flexibility for residue 7, and relatively hig h mobility on this time scale for residues 1, 8, and (especially) 9. R esidue 9 may also experience motions on a nanosecond-millisecond time scale. An unrestrained, water-solvated molecular dynamics simulation o f the peptide was also performed. This simulation included a 0.70 ns e quilibration period followed by 1.40 ns of production dynamics at 278 K. Order parameters derived from the C-13 relaxation data are compared to order parameters extracted from the molecular dynamics simulation and to order parameters derived from the ensemble-averaged modeling re sults. The combined data suggest that the peptide may mimic a protein folding intermediate, with significantly populated hydrogen bonds and ''loose'' interactions among hydrophobic and terminal charged groups.