Energetics of Lipid Binding in a Hydrophobic Protein Cavity

Hydrophobic bonding is central to many biochemical processes, such as protein folding and association. However, a complete description of the forces underlying hydrophobic interactions is lacking. The goal of this study was to evaluate the intrinsic energetic contributions of -CH3, >CH2, and -HC=CH- groups to protein lipid binding. To this end, Arrhenius parameters were measured for dissociation of gaseous deprotonated ions (at the -7 charge state) of complexes of bovine beta-lactoglobulin (Lg), a model lipid-binding protein, and a series of saturated, unsaturated, and branched fatty acids (FA). In the gas phase, the (Lg + FA)(7-) ions adopt one of two noninterconverting structures, which we refer to as the fast and slow dissociating components. The dissociation activation energies measured for the fast components of the (Lg + FA)(7-) ions were found to correlate linearly with the association free energies measured in aqueous solution, suggesting that the specific protein-lipid interactions are preserved in the gas phase. The average contributions that the -CH3, >CH2, and -HC=CH- groups make to the dissociation activation energies measured for the fast components of the (Lg + FA)(7-) ions were compared with enthalpies for the transfer of hydrocarbons from the gas phase to organic solvents. For >CH2 groups, the interior of the cavity was found to most closely resemble the relatively polar solvents acetone and N,N-dimethylformamide, which have dielectric constants (epsilon) of 21 and 39, respectively. For -CH3 groups, the solvent environment most closely resembles 1-butanol (epsilon = 17), although the energetic contribution is dependent on the location of the methyl group in the FA. In contrast, the solvation of -HC=CH- groups is similar to that afforded by the nonpolar solvent cyclohexane (epsilon = 2).