Interaction of purple membrane with solvents. I. Applicability of solubility parameter mapping

We carried out spectral studies on the interaction between purple membrane fragments (isolated from Halobacterium halobium) and a series of different solvents, classified quantitatively according to their solubility parameters delta d, delta p, delta h. These represent the contribution of dispersion forces, polar forces, and hydrogen bonding, respectively, to the cohesive energy density of the solvent. Purple membrane fragments, kept in the dark, were suspended in each of the solvents as well as in binary mixtures of solvents, and the spectrum of the resulting suspension was recorded in the wavelength region 250-700 nm. The interaction of each solvent with the membrane fragments can be represented by a point on either a ternary diagram, where each of the three axes represents one of the solubility parameters, or a binary diagram, where one of the two axes is a combination of two of the solubility parameters (delta v = square root of delta d2 + delta p2 or delta a = square root of delta p2 + delta h2). In the former type of solvent map the contribution of each of the parameters is distinct but only their relative contributions are expressed. In the latter the absolute values of delta i are considered. In each of these modes of presentation an inner closed region is observed. The solvents inside its borders interact with bacteriorhodopsin with a resultant spectral change. Mixtures of solvents fit the maps according to their calculated delta values. Thus, a mixture of an apolar solvent with a highly polar solvent interacts with bacteriorhodopsin, even though each of these solvents alone does not.