The gramicidin ion channel: a model membrane protein.
The interactions between an aliphatic or phenyl side chain and an indole ring in a phospholipid environment were investigated by synthesizing and characterizing gramicidins in which Trp(9) was ring-labeled and D-Leu(10) was replaced by D-Val, D-Ala, or D-Phe. All three analogues form conducting channels, with conductances that are lower than that of gramicidin A (gA) channels. The channel lifetimes vary by less than 50% from that of gA channels. Circular dichroism spectra and size-exclusion chromatography show that the conformation of each analogue in dimyristoylphosphatidylcholine (DMPC) vesicles is similar to the right-handed beta(6.3)-helical conformation that is observed for gA. (2)H NMR spectra of oriented samples in DMPC show large changes for the Trp(9) ring when residue 10 is modified, suggesting a steric interaction between D-Leu(10) and Trp(9), in agreement with previous acylation studies (R. E. Koeppe II et al. (1995) Biochemistry 34, 9299-9307). The outer quadrupolar splitting for Trp(9) is unchanged with D-Phe(10), at approximately 153 kHz, but increases by approximately 25 kHz with D-Val(10) and decreases by approximately 10 kHz with D-Ala(10). With D-Ala(10) or D-Val(10), the outer resonance splits into two in a temperature-dependent manner. The NMR spectra indicate that the side chain torsion angles chi1 and chi2 for Trp(9) change when residue 10 is substituted. The changes in chi1 are small, in all cases less than 10 degrees, as is Deltachi2 when D-Ala(10) is introduced, but with D-Val(10) and D-Phe(10) Deltachi2 is at least 25 degrees. We conclude that D-Leu(10) helps to stabilize an optimal orientation of Trp(9) in gA channels in lipid bilayers and that changes in Trp orientation alter channel conductance and lifetime without affecting the basic channel fold.