DEPENDENCE OF THE BILAYER PHASE-TRANSITION TEMPERATURES ON THE STRUCTURAL PARAMETERS OF PHOSPHATIDYLCHOLINES

Most saturated diacyl phosphatidylcholines C(X):C(Y)PC (saturated 1,2-diacyl-sn-glycero-3-phosphocholine with X carbons in the sn-1 acyl chain and Y carbons in the sn-2 acyl chain), in excess water, can self-assemble into lamellae which, upon heating, may undergo multiple thermotropic phase transitions at well-defined, discrete temperatures. The transition temperature corresponding to the main or the gel to liquid-crystalline phase transition (T(m)) is known for many bilayers of fully hydrated phosphatidylcholines. In this study, we have analyzed the T(m) values of 44 molecular species of phosphatidylcholines in terms of their structural and packing characteristics in the gel-state bilayer. Two general equations are thus derived: T(m) = 162.26 - 3651.71 (1/N) - 88.42 (DELTAC/N) for C(X):C(Y)PC with X greater-than-or-equal-to Y, and T(m) = 157.68 - 3525.44 (1/N) - 93.28 (DELTAC/N) for C(X):C(Y)PC with X < Y. Here, N is the minimal hydrophobic thickness of the dimeric C(X):C(Y)PC in the gel-state bilayer and DELTAC is the effective chain length difference between the sn-1 and sn-2 acyl chains for the monomeric C(X):C(Y)PC in the gel-state bilayer. The advantage of these two equations in predicting the T(m) values for phosphatidylcholines with DELTAC/CL values in the range of 0.07 to 0.40 is their simplicity. A figure containing a total of 173 calculated T(m) values is also presented.