ORIGIN OF BETA-HAIRPIN STABILITY IN SOLUTION - STRUCTURAL AND THERMODYNAMIC ANALYSIS OF THE FOLDING OF MODEL PEPTIDE SUPPORTS HYDROPHOBIC STABILIZATION IN WATER

The origin of the stability of isolated beta-hairpins in aqueous solut ion is unclear with contrasting opinions as to the relative importance of interstrand hydrogen bonding, hydrophobic interactions, and confor mational preferences, the latter being associated largely with the tur n sequence. We have designed an unconstrained 16-residue peptide that we show folds autonomously in water to form a beta-hairpin that mimics the two-stranded anti-parallel beta-sheet DNA binding motif of the me t repressor dimer. The designed peptide, with a type I' turn (INGK), i s shown by CD and a range of NMR parameters to be appreciably folded ( approximate to 50% at 303 K) in aqueous solution with the predicted al ignment df the peptide backbone. We show that the folding transition a pproximates to a hue-state model. The hairpin has a marked temperature -dependent stability, reaching a maximum value at 303 K in water with both lower and higher temperatures destabilizing the folded structure. Van't Hoff analysis of H alpha chemical shifts, reveals that folding is endothermic and entropy-driven in aqueous solution with a large neg ative Delta C-p, all of which are reminiscent of proteins with hydroph obic cores: pointing to the hydrophobic effect as the dominant stabili zing interaction in water. We have examined the conformational propert ies of the C-terminal beta-strand (residues 9-16) in isolation and hav e shown that (3)J(alpha N) values and backbone intra-and inter-residue H alpha-NH NOE intensities deviate from those predicted for a random coil, indicating that the beta-strand has a natural predisposition to adopt an extended conformation in the absence of secondary structure i nteractions. A family beta-hairpin structures calculated from 200 (dis tance and torsion angle) restraints using molecular dynamics shows tha t the conformation of the hairpin mimics closely the DNA binding face of the met repressor dimer (backbone RMSD between corresponding beta-s trands of 1.0 +/- 0.2 Angstrom).