Tryptophan orientation in model lipid membranes

Tryptophans in membrane proteins display strong preference for the lipid membrane interface and are important for anchoring proteins at the proper longitudinal level. Linear dichroism spectroscopy on indoles in shear-deformed liposomes has been used to show that this positioning is accompanied by an intrinsically adopted orientation, also observed for tryptophans in membrane-bound peptides. Similarities in orientation of different indoles suggest that tryptophan will adopt this orientation independent of the protein it is part of. From the orientation of indole electronic transition moments La, Lb and Bb, a binding model is proposed where the indole long axis is ∼60–65° from the membrane normal and the indole plane is at an oblique angle. We propose that dipole–dipole interactions and steric constraints in the membrane hydrocarbon region determine positioning and orientation of tryptophans whereas hydrogen bonding and cation-π interactions with lipid head-groups, though contributing to the membrane affinity of indoles, are less important.