Blood porphyrins in binary mixtures of N,N-dimethylformamide with 1-octanol and chloroform: The energetics of solvation, (solute + cosolvent) interactions and model calculations

• The energetics of solvation of photosensitizers in binary systems was studied.
• The (solute + non-electrolyte) pair interactions were examined.
• Affinity of porphyrins to protein-like species in a lipid-like phase was established.
• The method for predicting enthalpies of solute transfer in mixed solvents was created.

This study provides the first accurate analysis of the energetics of solvation of blood porphyrins in binary solvents which are considered as appropriate models for a smooth transition from a polar protein-like phase to an apolar lipid-like environment. Our results do indicate that hematoporphyrin dimethylether dimethylester (HDEDE) and deuteroporphyrin dimethylether (DDE), as well as the model of their ester side-chains ethyl acetate (EtOAc), reveal more exothermic solvation in chloroform (CHCl3) than in dimethylformamide (DMF) and, especially, in 1-octanol (OctOH). The energetics of pair interaction between dissolved species and cosolvent molecules both in a protein-like and a lipid-like environment are clearly associated with these solvation effects. The interaction between blood porphyrins and DMF in OctOH is accompanied by large negative enthalpy changes at both temperatures, whereas in chloroform, forming strong H-bonds with dissolved species, the interaction is strongly thermochemically repulsive. All solute molecules interact in the energetically unfavorable way with OctOH and CHCl3 in DMF, the effect being much stronger pronounced for chloroform. The most significant result of this work is that it is possible to connect this pair interaction in a highly diluted solution with the solute behavior in the entire range of the binary mixture. The approach proposed is independent of a solute and solvent structure, it provides a good prediction of the energetics of solvation in mixed solvents and can be extended for a lot of other biologically active solutes.

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