A novel method for measuring long range proton-proton J coupling constantsin multi-methylene fragments with C-13 in natural abundance

A new heteronuclear two-dimensional NMR experiment, X.E.COSY, is described which makes possible the measurement of (3)J(HH) coupling constants between adjacent methylene groups in multi-methylene fragments. This new experimen t exploits E.COSY multiplet structures based on large heteronuclear one-bon d (1)J(XH) coupling constants for resolving basic patterns. It necessitates only one NMR-active heteronuclear spin per molecule, and therefore is feas ible with isotopes in natural abundance. X.E.COSY complements the XLOC puls e sequence, which works well for CH and CH3 groups but for methylene moieti es only allows measurement of the sum of J(HH) coupling constants between a proton and the two protons attached to the heteronuclear spin. The new seq uence has been tested on CH2CH and CH2CH2 moieties of a Gramicidin S sample , a C-2 symmetric cyclic pentapeptide dimer. X.E.COSY will be useful for st udying sidechain conformations in peptides and conformationally constrained peptides and peptide analogues containing methylene linkers or bridgers.