Anchoring of hydrophobically modified poly(sodium acrylate)s into DDP vesicle bilayers : Hydrophobic match and mismatch

Differential scanning microcalorimetric thermograms have been recorded for aqueous solutions containing vesicles formed by sodium di-n-dodecyl phosphate, in the presence of different concentrations of poly(sodium acrylate-co-n-alkyl methacrylate), where n-alkyl= C^sub 9^H^sub 19^, C^sub 12^H^sub 25^, C^sub 18^H^sub 37^. The mole fraction of hydrophobic moieties in the copolymer is 0.04. The main phase transition temperature (T^sub m^) is hardly affected by the presence of poly(sodium acrylate)s bearing n-dodecyl chains, whereas the anchoring of polymers bearing n-nonyl or n-octadecyl groups reduces the main phase transition temperature significantly from ca. 34°C to ca. 32°C. In parallel, the enthalpy of transition per mole of DDP monomer ([delta]^sub m^ H^sub int^) is lowered upon adding polymer. Again, the polymer containing n-dodecyl moieties hardly affects [delta]^sub m^ H^sub int^. These patterns are explained by the notion that the extent of the disruptive effect of alkyl chains incorporated into the bilayer depends on the extent of the mismatch between the chain lengths of the intruding alkyl chains and the hydrophobic moieties composing the vesicle bilayer. Added hydrophobically modified polymers increase the cooperativity of the melting process, as shown by the increase of n^sub DDP^. We suggest that the anchoring poly(sodium acrylate-co-n-alkyl methacrylate) relieves the strain in the curved outer monolayer of a pure DDP bilayer by allowing the presence of larger "patches" characterized by low curvature.[PUBLICATION ABSTRACT]