A differential vapor-pressure equipment for investigations of biopolymer interactions

The design and performance of an equipment for the measurement of vapor pressures over liquid or solid samples is presented. The equilibrium pressure difference, ΔP, between a sample and a reference of known vapor pressure is recorded as a function of composition and/or temperature. Through the use of high-accuracy capacitance manometers and a leak-tight system of stainless steel pipes, below-sealed valves and metal-gasket fittings, ΔP can be measured with a resolution of about 0.5 μbar (0.05 Pa) in some applications. This sensitivity level, along with other features of the equipment, particularly a “gas-phase titration” routine for changing the cell composition, makes it effective for the investigations of several types of biopolymer interactions. These include isothermal studies of net affinities such as the adsorption of water to proteins or membranes, the preferential interaction of biopolymers with the components of a mixed solvent, the partitioning of solutes between a membrane and the aqueous bulk and the weak, specific binding of ligands to macromolecules. Furthermore, a temperature-scanning mode allows real-time elucidation of such interactions at thermally induced conformational changes in biopolymers. Selected examples of these applications are presented and discussed.