Neighboring aliphatic/aromatic side chain interactions between residues 9 and 10 in gramicidin channels

The interactions between an aliphatic or phenyl side chain and an indole ri ng in a phospholipid environment were investigated by synthesizing and char acterizing gramicidins in which Trp(9) was ring-labeled and D-Leu(10) was r eplaced by D-Val, D-Ala, or D-Phe. All three analogues form conducting chan nels, with conductances that are lower than that of gramicidin A (gA) chann els. 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) vesi cles is similar to the right-handed beta(6.3)-helical conformation that is observed for gA. H-2 NMR spectra of oriented samples in DMPC show large cha nges for the Trp(9) ring when residue 10 is modified, suggesting a steric i nteraction between D-Leu(10) and Trp(9), in agreement with previous acylati on 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 simi lar to 153 kHz, but increases by similar to 25 kHz with D-Val(10) and decre ases by similar to 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 chi 1 and chi 2 for Trp (9) change when residue 10 is substituted. The changes in chi 1 are small, in all cases less than 10 degrees, as is Delta chi 2 when D-Ala(10) is intr oduced, but with D-Val(10) and D-Phe(10) Delta chi 2 is at least 25 degrees . We conclude that D-Leu(10) helps to stabilize an optimal orientation of T rp(9) in gA channels in lipid bilayers and that changes in Trp orientation alter channel conductance and lifetime without affecting the basic channel fold.