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Protein Science, Vol 6, Issue 1 147-155, Copyright © 1997 by Cold Spring Harbor Laboratory Press
ARTICLE |
A. J. DOIG, M. W. MACARTHUR, B. J. STAPLEY and J. M. THORNTON
Department of Biochemistry and Applied Molecular Biology, UMIST, P.O. Box 88, Manchester M60 1QD, United Kingdom
We have surveyed 393 N-termini of {alpha}-helices and 156 N-termini of 3(10)-helices in 85 high resolution, non-homologous protein crystal structures for N-cap side-chain rotamer preferences, hydrogen bonding patterns, and solvent accessibilities. We find very strong rotamer preferences that are unique to N-cap sites. The following rules are generally observed for N-capping in {alpha}-helices: Thr and Ser N-cap side chains adopt the gauche- rotamer, hydrogen bond to the N3 NH and have {psi} restricted to 164 +/- 8{deg}. Asp and Asn N-cap side chains either adopt the gauche- rotamer and hydrogen bond to the N3 NH with {psi} = 172 +/- 10{deg}, or adopt the trans rotamer and hydrogen bond to both the N2 and N3 NH groups with {psi} = 107 +/- 19{deg}. With all other N-caps, the side chain is found in the gauche+ rotamer so that the side chain does not interact unfavorably with the N-terminus by blocking solvation and {psi} is unrestricted. An i, i + 3 hydrogen bond from N3 NH to the N-cap backbone C=O in more likely to form at the N-terminus when an unfavorable N-cap is present. In the 3(10)-helix Asn and Asp remain favorable N-caps as they can hydrogen bond to the N2 NH while in the trans rotamer; in contrast, Ser and Thr are disfavored as their preferred hydrogen bonding partner (N3 NH) is inaccessible. This suggests that Ser is the optimum choice of N-cap when {alpha}-helix formation is to be encouraged while 3(10)-helix formation discouraged. The strong energetic and structural preferences found for N-caps, which differ greatly from positions within helix interiors, suggest that N-caps should be treated explicitly in any consideration of helical structure in peptides or proteins.
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