Evidence of Complete Hydrophobic Coating of Bombesin by Trifluoroethanol in Aqueous Solution: An NMR Spectroscopic and Molecular Dynamics Study

Bombesin is a tetradecapeptide that possesses a random coil structure in pure water. In the presence of 30 % (v/v) 2,2,2-trifluoroethanol (TFE), it adopts a partial helical conformation involving the C-terminal amino acids 6-14. This conformational change, known as the TFE effect, is studied here in terms of the solvation state of the peptide at different TFE concentrations by means of intermolecular homo- and heteronuclear NOE measurements. When an aqueous solution of bombesin is titrated with TFE, a continual decrease in the water/peptide interactions and a concomitant increase in the TFE/peptide interactions is observed, and at 30 % (v/v) TFE no homonuclear NOEs between water and the peptide can be detected. The conformational transition of the bombesin molecule is thus accompanied by a complete surface covering with TFE. A parallel molecular dynamics (MD) study of the peptide in aqueous solution with the single-point charge (SPC) water model and in a 30 % (v/v) TFE/water mixture with a recently developed TFE model has also been performed. The 10 ns simulations were in agreement with the experimental data. The calculations indicate stabilisation of the alpha-helix in the H(2)O/TFE mixture, in contrast to the situation in pure water, and clustering of the TFE molecules around the peptide.