ROLE OF BETA-TURN RESIDUES IN BETA-HAIRPIN FORMATION AND STABILITY INDESIGNED PEPTIDES

The sequence RGITVNGKTYGR has been reported as part of a de no-do desi gn peptide system. This peptide folds as a beta-hairpin structure with three residues per strand and two residue turns. Asn6 side-chain, the residue in position L1 of the beta-turn, appeared to be solvent expos ed, interacting only with in the turn but not with the rest of the pep tide. We have chosen this position as a good candidate to design mutat ions, based on the protein database statistical abundances, that shoul d mainly affect the turn stability and possibly the pairing: between s trands. We have found that all NMR parameters, in particular the confo rmational shift analysis of (CH)-H-alpha and the coupling constants, ( 3)J(HN alpha), correlate very well and show similar conformational fea tures in all the turn mutant peptides. The population estimates are in reasonable agreement among the different methods used, It appears tha t the peptide with Asn in position L1 is the most structured peptide, followed by the one with Asp6. The next structured peptide is the one with Gly6. The least populated peptides were those with Ala6 and Ser6. We have found a strong correlation between the hairpin population, as determined from the conformational shift of (CH)-H-alpha and the occu rrence of the different residues at position L1 of beta-hairpins with type I' beta-turn, in the protein in database. Our analysis demonstrat es that this peptide system is sensitive enough to register small ener gy changes in the hairpin structure; therefore, it constitutes an appr opriate model to quantify energy contributions, once the appropriate s heet/coil transition algorithm is developed. Comparison with the other studies indicate that the design of a specific hairpin structure must involve a sequence at the rum region favouring the desired turn type, and a sequence at the strands that avoids alternative interstrand sid e-chain pairings. (C) 1997 Academic Press Limited.