On the use of deuterated phospholipids for infrared spectroscopic studies of monomolecular films: a thermodynamic analysis of single and binary component phospholipid monolayers

A thermodynamic study of monolayer mixing behavior was performed using binary lipid mixtures of DPPC + DOPG (7:1 mol:mol) as well as a mixture containing the phosphocholine acyl chain perdeuterated analog of DPPC, DPPC-d(62) (i.e. 7:1 mol:mol DPPC-d(62) + DOPG). An analysis was performed on these isotherms that calculated the Helmholtz excess free energy of mixing to determine potential thermodynamic differences in the fully protiated DPPC:DOPG monolayer versus the identical monolayer substituted with acyl chain perdeuterated DPPC (i.e. DPPC-d(62)). The main conclusions of this study are that: (1) the temperature-dependent thermodynamic phase behavior of DPPC-d(62) differs substantially from that of DPPC, and these differences appear magnified when these molecules are studied as monomolecular films at the A/W interface as opposed to bulk phase systems; and (2) the calculated excess free energy of mixing values, Delta F-xs(A), for the deuterium-containing 7:1 DPPC-d(62):DOPG binary monolayer film show a type of near-ideal mixing behavior above 20 degrees C that is consistent with the 'squeezing-out' of the DOPG component in the binary mixture containing the DPPC-d(62) component. This type of behavior does not occur in the graph of the excess free energy of mixing for the fully protiated 7:1 DPPC:DOPG binary monolayer film. The use of acyl chain perdeuterated DPPC (i.e. DPPC-d(62)) in binary monomolecular mixtures with other phospholipids produces a real and measurable difference on the thermodynamic properties of the monolayer when compared to the case of the fully protiated monomolecular film. In particular, the presence of DPPC-d(62) in a 7:1 mol:mol DPPC-d(62):DOPG binary monomolecular film may over-state the fluid nature of the monolayer at any given temperature, thereby leading to an over-estimate of the amount of material potentially 'squeezed out' of the monomolecular film. These results have implications for a commonly used IR spectroscopic method that relies on the incorporation of deuterium-labeled phospholipids into monolayers for vibrational spectroscopic analysis of individual components in a multicomponent monomolecular film. (C) 1998 Published by Elsevier Science Ireland Ltd. All rights reserved.