Importance of tryptophan dipoles for protein function: 5-fluorination of tryptophans in gramicidin A channels

In integral membrane proteins the amphipathic aromatic amino acid residues tryptophan and tyrosine tend to be localized at membrane/solution interface. The interfacial location of these residues is likely to be significant for membrane protein structure and function. Trp and Tyr have complex chemical characteristics, however, and it is difficult to deduce how these side chains determine protein structure and function. Specifically, Trp and Tyr not only are amphipathic but also dipolar, and electrostatic interactions that involve the side chain dipoles could be important for function. We evaluate the importance of the Trp dipole moment for ion channel function by replacing Trp residues in gramicidin A by the more polar 5-F-Trp and monitoring the ensuing changes in the conductance of membrane-spanning gramicidin channels. Trp-->5-F-Trp substitutions increase the conductance of the sequence-substituted channels, and we conclude that Trp side chains increase ion permeability through electrostatic interactions with the permeant cations.