Two-dimensional rotational-echo double resonance of Val1-[1-13C]Gly2-[15N]Ala3-gramicidin A in multilamellar dimyristoylphosphatidylcholine dispersions

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Abstract
The dipolar coupling between the Gly2 13C-1 carbon and Ala3 15N-amide nitrogen was used to investigate the conformation and dynamics of the Gly2-Ala3 13C-15N peptide bond in Val1-[1-13C]Gly2-[15N]Ala3-gramicidin A incorporated into multilamellar dispersions of dimyristoylphosphatidylcholine. Measurement of the 13C-15N dipolar coupling constant D of the labeled gramicidin in a powder and the effective dipolar coupling constant De 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 De were measured by the mirror-symmetric form of 2D REDOR, and the signs of D and De 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 De, four possible values were calculated for the angle between the Gly2-Ala3 13C-15N peptide bond and the gramicidin helical axis. Additional knowledge of the signs of D and De 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 13C-15N peptide bond angle eliminates the double-stranded, helical dimers and the left-handed, single-stranded, beta 6.3 helical dimer but supports the right-handed, single-stranded, beta 6.3 helical dimer as the structural model for gramicidin in multilamellar dispersions.