Comparison of fluorescence energy transfer and quenching methods to establish the position and orientation of components within the transverse plane of the lipid bilayer. Application to the gramicidin A-bilayer interaction

Abstract

Fluorescence quenching and resonance energy transfer methods were used to investigate the position of fluorophores in the lateral and transverse planes of the lipid bilayer. A series of n-(9-anthroyloxy) fatty acids (n = 2, 6, 9, and 12) were used as energy-transfer acceptors so that apparent transfer distances from a membrane-bound donor (N-stearoyltryptophan) have a transverse and a lateral component. The energy-transfer method is not precise enough to discriminate between the positions of the fluorophores in the transverse plane of the bilayer. The n-(9-anthroyloxy) fatty acids are also susceptible to quenching by the indole moiety of tryptophan. The relative quenching efficiency can provide a semiquantitative measure of the position of quenching molecules in the lipid bilayer. The quenching techniques are applied to the determination of the orientation of gramicidin A in lipid bilayers. The tryptophan residues of gramicidin appear to be located near the membrane surface in agreement with the head-to-head dimeric structure proposed by others.