Stabilizing effect of D-alanine2 in gramicidin channels

We have investigated the effects of replacing Gly2 by D-Ala2 in gramicidin A (gA) analogues that have either L-Val, L-Ala, or Gly as the formyl-N-terminal residue. Circular dichroism, two-dimensional nuclear magnetic resonance, and hybrid channel experiments all show that [Ala1,D-Ala2]gA channels are structurally equivalent to the native [Val1,Gly2]gA channels, being formyl-NH-to-formyl-NH dimers of single-stranded, right-handed beta 6.3 helices. Replacing the Val1 of gA by Ala or Gly decreases the average channel duration. Replacing Gly2 by D-Ala in [Val1,Gly2]gA increases the average channel duration 4-fold and the single-channel conductance by approximately 15%; replacing Gly2 with D-Ala in [Ala1,Gly2]gA or [Gly1,Gly2]-gA leads in each case to a 10-fold increase in the average channel duration with only modest changes in the single-channel conductance, which depends on the identity of the position-one residue and the permeant ion. These results illustrate the importance of neighboring-residue side chain and backbone interactions for the modulation of channel properties.