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

The dipolar coupling between the Gly2 C-13-1 carbon and Ala3 N-15-amide nitrogen was used to investigate the conformation and dynamics of the Gly2-Ala3 C-13-N-15 peptide bond in Val1-[1-C-13]Gly2-[N-15]Ala3-gramicidin A incorporated into multilamellar dispersions of dimyristoylphosphatidylcholine. Measurement of the C-13-N-15 dipolar coupling constant 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 coupling constants. The magnitudes of D and D(e) were measured by the mirror-symmetric form of 2D REDOR, and the signs of D and D(e) were determined relative to the sign of the isotropic indirect spin-spin coupling constant J by the mirror-asymmetric form of 2D REDOR. From knowledge of the magnitudes of D and D(e), four possible values were calculated for the angle between the Gly2-Ala3 C-13-N-15 peptide bond and the gramicidin helical axis. Additional knowledge of the signs of D and D(e) permitted the set of possible values for the peptide bond angle to be reduced to a single angle and its supplement (64-degrees, 116-degrees). This information about the Gly2-Ala3 C-13-N-15 peptide bond angle eliminates the double-stranded, helical dimers and the left-handed, single-stranded, beta6.3 helical dimer but supports the right-handed, single-stranded, beta6.3 helical dimer as the structural model for gramicidin in multilamellar dispersions.