Thermodynamic Analysis of Chain-Melting Transition Temperatures for Monounsaturated Phospholipid Membranes: Dependence on cis-Monoenoic Double Bond Position

Abstract

Unsaturated phospholipid is the membrane component that is essential to the dynamic environment needed for biomembrane function. The dependence of the chain-melting transition temperature, T t, of phospholipid bilayer membranes on the position, n u, of the cis double bond in the glycerophospholipid sn-2 chain can be described by an expression of the form T t = T t c(1 + h′ c| n u − n c|)/(1 + s′ c| n u − n c|), where n c is the chain position of the double bond corresponding to the minimum transition temperature, T t c, for constant diacyl lipid chain lengths. This implies that the incremental transition enthalpy (and entropy) contributed by the sn-2 chain is greater for whichever of the chain segments, above or below the double-bond position, is the longer. The critical position, n c, of the double bond is offset from the center of the sn-2 chain by an approximately constant amount, δn c ≈ 1.5 C-atom units. The dependence of the parameters T t c, h′ c, and s′ c on sn-1 and sn-2 chain lengths can be interpreted consistently when allowance is made for the chain packing mismatch between the sn-1 and sn-2 chains. The length of the sn-2 chain is reduced by ∼0.8 C-atom units by the cis double bond, in addition to a shortening by ∼1.3 C-atom units by the bent configuration at the C-2 position. Based on this analysis, a general thermodynamic expression is proposed for the dependence of the chain-melting transition temperature on the position of the cis double bond and on the sn-1 and sn-2 chain lengths. The above treatment is restricted mostly to double-bond positions close to the center of the sn-2 chain. For double bonds positioned closer to the carboxyl or terminal methyl ends of the sn-2 chain, the effects on transition enthalpy can be considerably larger. They may be interpreted by the same formalism, but with different characteristic parameters, h′ c and s′ c, such that the shorter of the chain segments makes a considerably smaller contribution to the calorimetric properties of the chain-melting transition.