Myelin basic protein (MBP), an intrinsically disordered protein constituent of central nervous system myelin, possesses unique ligand-binding features. Using equilibrium headspace gas chromatography, we characterized the thermodynamics of noncovalent interactions between perfume molecules and MBP, both in its monomeric and hexameric forms. This technique can be extended to study protein binding to volatile ligands, with advantages including the ability to derive thermodynamic quantities for several solutes at infinite dilution without knowledge of their concentrations, in addition to data that span the pre- and post-aggregation regions of the protein. Parameters determined include the critical aggregation concentration of MBP, perfume molecule association constants with monomeric MBP, K₁₁, and aggregate MBP, K_n1, binding free energies, ΔG_b, aggregate-water partition coefficients, K_x, free energies of transfer of perfume from bulk water to MBP-pseudophase, ΔG_t, and intraaggregate activity coefficients at infinite dilution, γ_m^∞. 1:1 binding free energies did not change by more than 1 kJ/mol, while binding to the MBP pseudophase was favored by 6 kJ/mol. Although the magnitudes of K₁₁, K_n1, ΔG_b, K_x, and ΔG_tshow weak trends, γ_m^∞values show a strong and distinct trend in interaction, spanning four orders of magnitude amongst the perfume molecules. This parameter could potentially differentiate the presence of specific binding sites. Our work provides a model for interactions between small, relatively hydrophobic molecules with both monomeric and pseudophase protein aggregates, and perhaps applications in the formulation of safer consumer products.

Abul Hussam

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