The metastability of saturated phosphatidylcholines depends on the acyl chain length

We report the peak temperatures and enthalpies of the main (gel to liquid crystal) transitions and 's-transitions' of symmetric saturated PC's (CnPC) with n varying from 12 to 22, where s-transition is defined as the transition between metastable and stable phases. The line for s-transition peak temperature versus n (t = 1.98n -16.0 degrees C) crosses the corresponding pre- and main-transition curves between n = 12 and 14. We also find that the metastability pattern for lipids of n less than 14 is different to that for n greater than 14 (and is similar to that of the phosphatidylethanolamine analogs). For n greater than 14 the enthalpy of the s-transition decreases with increasing n and extrapolates to zero above n = 20, implying that the s-(orthorhombic) and gel (quasi-hexagonal) phases become identical. That simultaneous gel-liquid crystal transition and metastability is restricted to the range n = 12 to 20 may be correlated with the predominance in natural lipids of these acyl chain lengths.