CONFORMATIONAL INVESTIGATION OF DESIGNED SHORT LINEAR PEPTIDES ABLE TO FOLD INTO BETA-HAIRPIN STRUCTURES IN AQUEOUS-SOLUTION

Background: Formation of secondary structure plays an important role i n the early stages of protein folding. The conformational analysis of designed peptides has proved to be very useful for identifying the int eractions responsible for the formation and stability of alpha-helices . However, very little is known about the factors leading to the forma tion of beta-hairpins. In order to get a good beta-hairpin-forming mod el peptide, two peptides were designed on the basis of beta-sheet prop ensities and individual statistical probabilities in the turn sites, t ogether with solubility criteria. The conformational properties of the two peptides were analyzed by two-dimensional NMR methods. Results: L ong-range cross-correlations observed in NOE and ROE spectra, together with other NMR evidence, show that peptide IYSNPDGTWT forms a highly populated beta-hairpin in aqueous solution with a type I beta-turn plu s a G1 beta-bulge conformation in the chain-bend region. The analogous peptide with a Pro5 substituted by Ser forms, in addition to the prev ious conformation, a second beta-hairpin hairpin with a standard type I beta-turn conformation, and the two forms are in fast dynamic equili brium with one another. The effect of pH demonstrates the existence of a stabilizing interaction between the Asn and Asp sidechains. The pop ulations of beta-hairpin conformations increase in the presence of tri fluoroethanol (a structure-enhancing solvent). On the other hand, some residual structure persists at a high denaturant concentration (8 M u rea). Conclusions: This work highlights the importance of the beta-tur n residue composition in determining the particular type of beta-hairp in adopted by a peptide, though a role of interstrand sidechain intera ctions in the stabilization of the formed beta-hairpin is not discarde d. The fact that trifluoroethanol can stabilize alpha-helices or beta- hairpins depending on the intrinsic properties of the peptide sequence is again shown. An additional example of the presence of residual str ucture under denaturing conditions is also presented.