Effect of hydrogen ion concentration on rhodopsin-lipid interactions

This study concerns the role titratable chemical groups play in rhodopsin-lipid interactions. Tempo partitioning techniques are used to construct DMPC-rhodopsin partial binary phase diagrams. Such diagrams are constructed at pH 7.0, where rhodopsin has a net negative charge, and at pH 5.0, where rhodopsin is virtually isoelectric. It is found that at pH 5.0, increases in rhodopsin content induce a depression in temperature of the solidus curve. At pH 7.0, the solidus curve is not a strong function of temperature. Additional studies, involving nitroxide-labeled palmitic acid, methyl palmitate, and lecithins, indicate that the pH-dependent changes in the phase behavior of DMPC-rhodopsin systems are due to the titration of rhodopsin's carboxylic acid groups.