| dc.description.abstract | Porphyrins are molecules that are investigated for their importance in applications such as cancer therapy, photochemical processes, and chemical catalysis. One important porphyrin property is the ability to absorb visible and ultraviolet light when illuminated. This leads to a transfer of energy from the excited porphyrin molecule to nearby oxygen molecules. The resulting excited oxygen molecules are toxic to tumor cells and infectious bacteria. Interestingly, porphyrins are known to self-aggregate in water. This property affects porphyrin light-absorbance behavior and therefore impacts the applications of these molecules. Thus, it is important to identify those factors that modulate porphyrin self-aggregation. Here, we have investigated the effect of the polymer polyvinylpyrrolidone (PVP) on the spectrophotometric behavior of tetraanionic meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS). It is known that TPPS self-aggregation significantly increases from neutral to acidic pH. Thus, our experiments were performed at pH 3.0, 4.5 and 7.0. In all cases, our measurements showed that TPPS binds to PVP. Furthermore, contrary to the case of free porphyrin, our results demonstrated that the spectrophotometric properties of porphyrin-polymer supramolecular assemblies are independent of pH. To characterize strength and stoichiometry of the resulting porphyrin-polymer supramolecular assemblies, we performed isothermal-titration-calorimetry (ITC) measurements at neutral pH. This technique allowed us to identify the correct model describing the complicated associative equilibria occurring in solution. Furthermore, our ITC results revealed that the formation of porphyrin-polymer supramolecular assemblies is entropically favored with respect to porphyrin self-aggregation. | |
