Journal of the American Chemical Society p. 3302 - 3306 (1982)
Update date:2022-08-05
Topics:
Barfield, M.
Al-Obeidi, F.A.
Hruby, V.J.
Walter, S.R.
An experimental and theoretical study is presented of the conformational and solvent dependencies of long-range H-H coupling constants 5J(H-Cα-C(O)-N-Cα-H) in compounds which model the peptide backbone.Molecular orbital results for Fermi contact coupling in cis- and trans-N-methylacetamides do not follow a conformational dependence of the homoallylic type; negative values are predicted for most out-of-plane orientations of the Cα-H bonds.In addition, the calculated values for 5CcisHH' and 5CtransHH' are of opposite signs in the planar conformation of cyclo-(Gly-Gly) and the boat conformation of cyclo-(Gly-Tyr).However, relative sign measurements show that these two coupling constants are of the same sign in cyclo-(Gly-Tyr), and that both are positive in cyclo-(Gly-Phgly).The inclusion of five water molecules in the MO calculations for cis-N-methylacetamide and ten water molecules in association with cyclo-(Gly-Gly) led to both positive 5JcisHH' and 5JtransHH'.As a consequence, any applicability of the empirical relationship of 5J(H-Cα-C(O-N-Cα-H) to φ and ψ angles in peptides does not have any theoretical basis in the molecular orbital theory for unhydrated amide bonds.
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