Abstract
We report on the improved determination of the partition coefficient,Pe, of the small hydrophobic spin probe di-tert-butyl nitroxide (DTBN) between the aqueous and the fluid hydrophobic phase of dipalmitoylphosphatidyl choline (DPPC) multilayers, made in terms of the second integral evaluation of the isotropic ESR resonance lines. The calculation of the second integral was carried out taking into account both the width and amplitude of the resonance lines and a shape factor related to the degree of inhomogeneous line broadening. The expression ofPc obtained in this way has been tested by computer simulation using Voightian lineshapes for the ESR absorption resonance lines. The method has been applied to the determination of the degree of cooperativity of the gel → liquid-crystal phase transition of DPPC multilamellae, and the results were compared with those obtained using the traditional expression ofPe. Finally, the calculation of the cooperativity parameter of the main phase transition of DPPC multilayers results improved of about 70% by using the new expression ofPc.
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