Determination of individual side-chain conformations, tertiary conformations, and molecular topography of tyrocidine A from scalar coupling constants and chemical shifts

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Abstract
The following data were reported for decapeptide tyrocidine A. H.alpha. and H.beta. chemical shifts and scalar coupling constants for most residues of tyrocidine A in methanol-d4 and dimethyl-d6 sulfoxide (Me2SO-d6) and the H.alpha. and H.beta. chemical shifts for other residues are covered. Scalar coupling constants 3J.alpha..beta. are reported for 9 side chains in methanol-d4 but only 7 side chains in Me2SO-d6, due to chemical shift degeneracy; the Gln9 and Tyr10 side chains in methanol-d4 were only approximately analyzed. A total spin-spin analysis of Pro5 in Me2SO d6 was made and, partly by comparison, also in methanol-d4. 3J.alpha..beta. values were converted to side-chain conformations for all residues in methanol-d4; comparisons, where possible, led to the conclusion that side chain conformations are similar in methanol-d4 and Me2SO d6. An absolute conformational analysis of Pro5 from 3J values and a method of assigning all pro-R,S protons are considered; Pro5 has a Ramachandran B, C2-Cexo-Cendo conformation. .chi.1,.chi.2 conformations of several aromatic residues were based on proton-chromophore distance measurement from anomalous chemical shifts and Johnson-Bovey diagrams. pro-R and pro-S assignments of H.beta.''s from anomalous chemical shifts, high-temperature dependence of anomalous chemical shifts, and backbone side-chain nuclear Overhauser effects are covered. Most tertiary conformations of the whole tyrocidine A molecule possess residues 4-8 and 10 in highly preferred (about 90%) .chi.1 conformations, but residues 1-3 and 9 have at least 2 .chi.1 rotamers. The 3 topographical regions of the molecule, i.e., hydrophobic region, a flat hydrophilic surface on the other side of the molecule, and a hydrophilic region consisting of 2 peptide backbone units and the side chains of Asn8, Gln9, and Tyr10 are described. Proposed side chain, .beta.-turn, and .beta.-pleated sheet conformations that readily account for all normal and anomalous chemical shifts are presented.