Closed state of gramicidin channel detected by X-ray in-plane scattering.

An analogue of gramicidin A (gA) was synthesized with the formyl group replaced by a BOC group. The analogue (BOC-gA) exhibited single channel conduction, but the channel is 5-order-of-magnitude destabilized relative to the gA channel. Hydrated mixtures of gramicidin and dilauroyl phosphatidylcholine in the molar ratio of 1:10 were prepared into uniformly aligned multiple bilayers, and X-ray scattering with the momentum transfer in the plane of the membrane was measured. Analysis with the help of computer simulations showed that 70% of BOC-gA are monomers. Thus for the first time it was shown that gramicidin monomers are stable inside the monolayers of a lipid membrane. Furthermore, the monomers have the same beta helical conformation as the dimeric channel. The result suggests the possibility that when a gramicidin channel is closed, it dissociates into two monomers floating in opposite monolayers.