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Influence of Trifluoroethanol on Membrane Interfacial Anchoring Interactions of Transmembrane -Helical Peptides

Suat Özdirekcan ^*, Thomas K. M. Nyholm ^*, Mobeen Raja ^*, Dirk T. S. Rijkers , Rob M. J. Liskamp  and J. Antoinette Killian ^*

^* Chemical Biology and Organic Chemistry, Bijvoet Center for Biomolecular Research, and Medicinal Chemistry and Chemical Biology, Utrecht Institute for Pharmaceutical Sciences, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands

Correspondence: Address reprint requests to Antoinette Killian, Chemical Biology and Organic Chemistry, Bijvoet Center for Biomolecular Research, Utrecht Institute for Pharmaceutical Sciences, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands. Tel.: 31-30-2533442; Fax: 31-30-2533969; E-mail: J.A.Killian{at}uu.nl.

Interfacial anchoring interactions between aromatic amino acid residues and the lipid-water interface are believed to be important determinants for membrane protein structure and function. Thus, it is possible that molecules that partition into the lipid-water interface can influence membrane protein activity simply by interfering with these anchoring interactions. Here we tested this hypothesis by investigating the effects of 2,2,2-trifluoroethanol (TFE) on the interaction of a Trp-flanked synthetic transmembrane peptide (acetyl-GW₂(LA)₈LW₂A-NH₂) with model membranes of dimyristoylphosphatidylcholine. Two striking observations were made. First, using ²H nuclear magnetic resonance on acyl chain deuterated lipids, we found that addition of 4 or 8 vol % of TFE completely abolishes the ability of the peptide to order and stretch the lipid acyl chains in these relatively thin bilayers. Second, we observed that addition of 8 vol % TFE reduces the tilt angle of the peptide from 5.3° to 2.5°, as measured by ²H NMR on Ala-d₄ labeled peptides. The "straightening" of the peptide was accompanied by an increased exposure of Trp to the aqueous phase, as shown by Trp-fluorescence quenching experiments using acrylamide. The observation of a reduced tilt angle was surprising because we also found that TFE partioning results in a significant thinning of the membrane, which would increase the extent of hydrophobic mismatch. In contrast to the Trp-flanked peptide, no effect of TFE was observed on the interaction of a Lys-flanked analog (acetyl-GK₂(LA)₈LK₂A-NH₂) with the lipid bilayer. These results emphasize the importance of interfacial anchoring interactions for membrane organization and provide new insights into how molecules such as TFE that can act as anesthetics may affect the behavior of membrane proteins that are enriched in aromatic amino acids at the lipid-water interface.