Effect of FA chain length on normal- and reversed-phase HPLC of phospholipids

Forty-seven saturated synthetic diacyl PA, PC, PE, PG, and PS and five unsaturated diacyl phospholipids (PL) underwent normal- and reversed-phase RP) HPLC with isocratic isopropanol/hexane/water (5:4:1) and methanol/chloroform/acetonitrile/water (79.5:9:8:3.5) mobile phases, respectively. For normal-phase HPLC, capacity factors (k'(i)) decrease with chain length (n) of the two identical PL FA residues, whereas the opposite occurs with RP (C-18)-HPLC. Plots of ln k'(i) vs. n for individual PL classes are in general curved, violating the linear free-energy relationship. For PL of the same n but with different head groups, k'(i) with normal-phase HPLC varies as PE