Raman study of calcium-induced fusion and molecular segregation of phosphatidylserine/dimyristoyl phosphatidylcholine-d54 membranes.

Raman spectroscopy has been used to study the effect of Ca2+ on the molecular properties of model membranes consisting of mistures of phosphatidylserine and dimyristoyl phosphatidylcholine-d54. The I2880/I2935 intensity ratio associated with the C-H stretching modes is used to monitor the phosphatidylserine molecules, while the linewidth at 2103 cm-1 associated with the C-2H stretching modes is used for the dimyristoyl phosphatidyl-choline-d54 molecules. Membranes containing phosphatidylserine and dimyristoyl phosphatidylcholine-d54 at a molar ratio of 1 : 2 show evidence of initial immiscibility, which is further enhanced by the addition of Ca2+. Membranes containing phosphatidylserine and dimyristoyl phosphatidylcholine-d54 at a molar ratio of 2 : 1 are completely miscible, and fuse with the addition of Ca2+. The phosphatidylserine molecules in the fused product of mixed vesicles have highly rigid acyl chains, and behave identically to those in an earlier Raman study of Ca2+-induced fusion of pure phosphatidylserine vesicles. The dimyristoyl phosphatidylcholine-d54 molecules are also solid-like, suggesting a higher degree of miscibility with phosphatidylserine in the presence of Ca2+ than dipalmitoyl or distearoyl phosphatidylcholine.