Measurement of Thermodynamic Parameters for Hydrophobic Mismatch 1:  Self-Association of a Transmembrane Helix^†

Membrane partitioning and self-association of transmembrane helices are crucial thermodynamic steps for membrane protein folding, although experimental difficulties have hampered quantitative estimations of related thermodynamic parameters, especially in lipid bilayer environments. This article reports for the first time, the complete set of thermodynamic parameters (ΔG, ΔH, ΔS, and ΔC_p) for the formation of the antiparallel dimer of the inert hydrophobic model transmembrane helix X-(AALALAA)₃-Y (X = 7-nitro-2-1, 3-benzoxadiazol-4-yl (NBD) and Y = NH₂ (I) or X = Ac and Y = NHCH₂CH₂-S-N-[4-[[4-(dimethylamino)phenyl]azo]phenyl]maleimide (DABMI) (II)) in dimonounsaturated phosphocholine lipid bilayers with different hydrophobic thicknesses (C14−C22) at 5−55 °C, as evaluated by fluorescence resonance energy transfer from I to II. Stronger dimerization was observed in thicker membranes and at lower temperatures (ΔG = −9 to −26 kJ mol^-1), driven by large negative ΔH values (−18 to −80 kJ mol^-1). Fourier transform infrared-polarized spectroscopy revealed that the peptide formed a stable transmembrane helix with an orientation angle of 15° in all bilayers without significant effects on lipid structures, suggesting that the depth to which the helix termini penetrate changes depending on the degree of hydrophobic mismatch. The enthalpy changes for helix−helix interactions can be well explained by the electrostatic interactions between helix macrodipoles in different dielectric environments. The new concept of dipole−dipole interaction as a basic driving force of helix dimerization will become a basis for understanding the structural and functional modifications in response to hydrophobic mismatch.