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 properties 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, 115, 5887-5888) were characterized by molecular modeling using ensemble-averaged constraints. In this report, the dynamic behavior of the peptide backbone is further investigated by 2D H-1-C-13 NMR methods at natural C-13 abundance. The dynamics of the backbone methine H-alpha-C-alpha sites were characterized by measurements of {H-1}-C-13 steady state NOEs, C-13 spin-lattice relaxation rates R(1)(C), C-13 spin-spin relaxation 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 protons R(1)(H). Relaxation observables were fit using model-free spectral density functions. The results of this analysis indicate relatively low mobility on a picosecond-nanosecond time scale for residues 2, 3, 4, and 5, intermediate flexibility for residue 7, and relatively high mobility on this time scale for residues 1, 8, and (especially) 9. Residue 9 may also experience motions on a nanosecond-millisecond time scale. An unrestrained, water-solvated molecular dynamics simulation of the peptide was also performed. This simulation included a 0.70 ns equilibration 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 results. 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.