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-d62 (i.e. 7:1 mol:mol DPPC-d62 + 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-d62). The main conclusions of this study are that: (1) the temperature-dependent thermodynamic phase behavior of DPPC-d62 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, deltaFxs(A), for the deuterium-containing 7:1 DPPC-d62: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-d62 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-d62) 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-d62 in a 7:1 mol:mol DPPC-d62: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.
Baldyga
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DD
Department of Chemistry, University of Georgia, Athens 30602-2556, USA.
Dluhy
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RA
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry; D003903:Deuterium; D008432:Mathematical Computing; D010715:Phosphatidylglycerols / Q000737:chemistry; D010743:Phospholipids / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013816:Thermodynamics
D010715:Phosphatidylglycerols; D010743:Phospholipids; D015060:1,2-Dipalmitoylphosphatidylcholine; C051388:1,2-dioleoyl-sn-glycero-3-phosphoglycerol; D003903:Deuterium