Bilayers as Protein Solvents: Role of Bilayer Structure and Elastic Properties

The perspectives on membrane protein insertion, protein– bilayer interactions, and amino acid side hydrophobicity (J. Gen. Physiol. 129:351–377) deal with several key issues in membrane biology. The four informative papers (MacCallum et al., 2007; von Heijne, 2007; White, 2007; Wolfenden, 2007) obtain detailed quantitative data on the interaction of protein amino acid residues with different membrane models. Several types of membrane model systems are used and discussed, including nonpolar solvents, such as 1-octanol or cyclohexane, as well as one-component lipid bilayers, such as dioleoylphosphatidylcholine (DOPC) or palmitoyloleoylphosphatidylcholine (POPC). As tabulated by MacCallum et al. (2007) there are appreciable differences among these systems in terms of determining the free energies of transfer of specifi c amino acid residues from water to nonpolar phases. For example, both White (2007) and Wolfenden (2007) point out possible effects of different amounts of water in the reference phase in terms of measuring the “hydrophobicities” of amino acid residues. Since 1-octanol contains more water and is thus more polar than cyclohexane, one obtains different amino acid partition coeffi cients between water and these two solvents, particularly for certain residues such as tryptophan (Wolfenden, 2007).