LEUCINE SIDE-CHAIN ROTAMERS IN A GLYCOPHORIN-A TRANSMEMBRANE PEPTIDE AS REVEALED BY 3-BOND CARBON-CARBON COUPLINGS AND C-13 CHEMICAL-SHIFTS

We have used a spin-echo difference NMR pulse sequence to measure thre e-bond J couplings between delta- and alpha-carbons of the leucine res idues in a micelle-associated helical peptide dimer that corresponds t o residues 62-101 of the transmembrane erythrocyte protein glycophorin A. The observed (3)J couplings correlate strongly with the C-13 chemi cal shift of the delta-methyl groups, and within experimental error bo th the shift distribution of the methyl carbons and the variations in (3)J can be accounted for by variations in side-chain rotamer populati ons. We infer that all leucine side chains in this peptide dimer are i n fast exchange among chi(2) rotamers and sample two of the three poss ible rotameric states, even when the side chain forms part of the dime r interface. The observed correlation of chemical shift with couplings can be traced to a gamma-gauche interaction of methyl and ex-carbons. This correlation may provide an alternate route to rotamer analysis i n some protein systems.