Octyl-β-D-glucopyranoside partitioning into lipid bilayers: Thermodynamics of binding and structural changes of the bilayer

The interaction of the nonionic detergent octyl-β-D-glucopyranoside (OG) with lipid bilayers was studied with high-sensitivity isothermal titration calorimetry (ITC) and solid-state ²H-NMR spectroscopy. The transfer of OG from the aqueous phase to lipid bilayers composed of 1- palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) can be investigated by employing detergent at concentrations below the critical micellar concentration; it can be defined by a surface partition equilibrium with a partition coefficient of K = 120 ± 10 M^-1, a molar binding enthalpy of ΔH°(D) = 1.3 ± 0.15 kcal/mol, and a free energy of binding of AG°(D)= - 5.2 kcal/mol. The heat of transfer is temperature dependent, with a molar heat capacity of ΔC(p)= -75 cal K^-1 mol^-1. The large heat capacity and the near-zero ΔH are typical for a hydrophobic binding equilibrium. The partition constant K decreased to ~100 M^-1 for POPC membranes mixed with either negatively charged lipids or cholesterol, but was independent of membrane curvature. In contrast, a much larger variation was observed in the partition enthalpy. ΔH°(D) increased by about 50% for large vesicles and by 75% for membranes containing 50 mol% cholesterol. Structural changes in the lipid bilayer were investigated with solid-state ²H-NMR. POPC was selectively deuterated at the headgroup segments and at different positions of the fatty acyl chains, and the measurement of the quadrupolar splittings provided information on the conformation and the order of the bilayer membrane. Addition of OG had almost no influence on the lipid headgroup region, even at concentrations close to bilayer disruption. In contrast, the fluctuations of fatty acyl chain segments located in the inner part of the bilayer increased strongly with increasing OG concentration. The ²H-NMR results demonstrate that the headgroup region is the most stable structural element of the lipid membrane, remaining intact until the disordering of the chains reaches a critical limit. The perturbing effect of OG is thus different from that of another nonionic detergent, octaethyleneglycol mono- n-dodecylether (C₁₂E₈), which produces a general disordering at all levels of the lipid bilayer. The OG-POPC interaction was also investigated with POPC monolayers, using a Langmuir trough. In the absence of lipid, the measurement of the Gibbs adsorption isotherm for pure OG solutions yielded an OG surface area of As = 51 ± 3 Ų. On the other hand, the insertion area A₁ of OG in a POPC monolayer was determined by a monolayer expansion technique as A₁ = 58 ±10 Ų. The similar area requirements with As ≃ A₁ indicate an almost complete insertion of OG into the lipid monolayer. The OG partition constant for a POPC monolayer at 32 mN/m was Kp ≃ 320 M^-1 and thus was larger than that for a POPC bilayer.