Recently, we have shown by high resolution differential scanning calorimetry that the position of a cis double bond (⌬-bond) in a series of 1-stearoyl-2-octadecenoyl-phosphatidylcholines can affect the phase transition temperature (T m ) or enthalpy (⌬H) of the gelto-liquid crystalline phase transition of this series of lipids in the following manner. The value of T m (or ⌬H) is minimal when the ⌬-bond is positioned at C(11) in the sn-2 acyl chain; in addition, this value increases steadily as the ⌬-bond migrates toward either end of the acyl chain, resulting in a symmetrical, inverted bell-shaped profile (Wang, Z.-q., Lin, H.-n., Li, S., and Huang, C. (1995) J. Biol. Chem. 270, 2014 -2023). In this communication, we have further demonstrated the inverted bell-shaped profile of T m using 1-arachidoyl-2-eicosenoyl-phosphatidylcholines. In addition, we have extended the lipid series of 1-stearoyl-2-octadecenoyl-phosphatidylcholines to include 1-arachidoyl-2-octadecenoyl-phosphatidylcholines and 1-behenoyl-2-octadecenoyl-phosphatidylcholine, each series with a ⌬-bond at varying carbon position of 6, 7, 9, 11, 12, and 13. Calorimetric results obtained with these three series of lipids indicate that the inverted bell-shaped curve shifts toward higher temperatures in a nonuniform manner as the saturated sn-1 acyl chain length increases from 17 to 19 and then to 21 C-C bond lengths. Specifically, the T m (or ⌬H) values are nearly identical for these cis-monoenoic lipids when their ⌬-bonds are positioned at C(13). Based on the height of the rotational energy barrier obtained with molecular mechanics calculations, it is evident that the rotational flexibility of the single C-C bond adjacent to the ⌬-bond in 1-stearoyl-2-octadecenoyl-phosphatidylcholine increases as the ⌬-bond migrates from C(9) to C(13). The differential scanning calorimetry results obtained with the three series of lipids can thus be attributed to an increase in the rotational flexibility of the short chain segment succeeding the C(14) atom in the sn-2 octadecenoyl chain. In this communication, we also propose that in the gel-state bilayer of sn-1 saturated/ sn-2 cis-monounsaturated phosphatidylcholine the entire length of the shorter segment of the sn-2 acyl chain acts as a structural perturbing element; hence, it is mainly responsible for the large lower T m of the monoenoic lipid relative to the saturated counterpart. Finally, two general equations relating T m with the structural parameters of cis-monoenoic phosphatidylcholines are presented. These equations, formulated primarily on the assumption that the short segment of the sn-2 acyl chain acts as a perturbing element, are shown to have strong predictive power in estimating the T m values of the gel-to-liquid crystalline phase transitions for sn-1 saturated/sn-2 cis-monounsaturated phosphatidylcholines.Phosphatidylcholines isolated from the plasma membrane of eukaryotic cells are a structurally diverse group of phospholipids. The bewildering variety of membrane phosphatidylcholines originates fro...