Determination of the sign of long-range H-1-C-13 scalar coupling constantsto non-protonated carbons: application to assignment

The sign of the scalar coupling between a carbonyl carbon and alpha-proton J(C',H-alpha), for the cyclic dipeptide cyclo(Leu-Gly) was determined relat ive to that between an amide proton and alpha-proton (3)J(H-N,H-alpha) by a n E.COSY-type HSQC experiment. The pulse sequence used in this experiment g ives a cross peak between the amide proton and carbonyl carbon which shows an E.COSY-type tilt caused by the alpha-proton. The sign information on J(C ',H-alpha) was used to distinguish between two-bond (intra-residue) and thr ee-bond (inter-residue) J(C',H-alpha)s, leading to the assignment of the ca rbonyl carbons of the cyclic dipeptide cyclo(Leu-Gly). Copyright (C) 1999 J ohn Wiley & Sons, Ltd.