Steric interactions of valines 1, 5, and 7 in [valine 5, D-Alanine 8] gramicidin A channels

When the central valine residues 6, 7, and 8 of gramicidin A (gA) are shift ed by one position, the resulting [Val(5), D-Ala(8)]gA forms right-handed c hannels with a single-channel conductance and average duration somewhat les s than gA channels. The reduction in channel duration has been attributed t o steric conflict between the side chains of Val(1) and Val(5) in opposing monomers (Koeppe, R. E. II, D. V. Greathouse, A. Jude, G. Saberwal, L. L. P rovidence, and O. S. Andersen. 1994. J. Biol. Chem. 269:12567-12576). To in vestigate the orientations and motions of valines in [Val(5), D-Ala(8)]gA, we have incorporated H-2 labels at Val 1, 5, or 7 and recorded H-2-NMR spec tra of oriented and nonoriented samples in hydrated dimyristoylphosphatidyl choline. Spectra of nonoriented samples at 4 degrees C reveal powder patter ns that indicate rapid side chain "hopping" for Val(5), and an intermediate rate of hopping for Val(1) and Val(7) that is somewhat slower than in gA. Oriented samples of deuterated Val(1) and Val(7) show large changes in the methyl and C-beta-H-2 quadrupolar splittings (Delta v(q)) when Ala(5) in na tive gA is changed to Val(5). Three or more peaks for the Val(1) methyls wi th Delta v(q) values that vary with the echo delay, together with an interm ediate spectrum for nonoriented samples at 4 degrees C, suggest unusual sid e chain dynamics for Val(1) in [Val(5), D-Ala(8)]gA. These results are cons istent with a steric conflict that has been introduced between the two oppo sing monomers. In contrast, the acylation of gA has little influence on the side chain dynamics of Val(1), regardless of the identity of residue 5.