2-DIMENSIONAL ROTATIONAL-ECHO DOUBLE-RESONANCE OF VAL1-[1-C-13]GLY2-[N-15]ALA3-GRAMICIDIN-A IN MULTILAMELLAR DIMYRISTOYLPHOSPHATIDYLCHOLINEDISPERSIONS

The dipolar coupling between the Gly2 C-13-1 carbon and Ala3 N-15-amid e nitrogen was used to investigate the conformation and dynamics of th e Gly2-Ala3 C-13-N-15 peptide bond in Val1-[1-C-13]Gly2-[N-15]Ala3-gra micidin A incorporated into multilamellar dispersions of dimyristoylph osphatidylcholine. Measurement of the C-13-N-15 dipolar coupling const ant D of the labeled gramicidin in a powder and the effective dipolar coupling constant D(e) in a multilamellar dispersion was accomplished by two-dimensional rotational-echo double-resonance (2D REDOR) NMR, a magic-angle spinning experiment designed to measure weak dipolar coupl ing constants. The magnitudes of D and D(e) were measured by the mirro r-symmetric form of 2D REDOR, and the signs of D and D(e) were determi ned relative to the sign of the isotropic indirect spin-spin coupling constant J by the mirror-asymmetric form of 2D REDOR. From knowledge o f the magnitudes of D and D(e), four possible values were calculated f or the angle between the Gly2-Ala3 C-13-N-15 peptide bond and the gram icidin helical axis. Additional knowledge of the signs of D and D(e) p ermitted the set of possible values for the peptide bond angle to be r educed to a single angle and its supplement (64-degrees, 116-degrees). This information about the Gly2-Ala3 C-13-N-15 peptide bond angle eli minates the double-stranded, helical dimers and the left-handed, singl e-stranded, beta6.3 helical dimer but supports the right-handed, singl e-stranded, beta6.3 helical dimer as the structural model for gramicid in in multilamellar dispersions.