Interactions of zinc octacarboxyphthalocyanine with selected amino acids and with albumin

• The photostability of zinc octacarboxyphthalocyanine was studied upon UV irradiation.
• Amino acids increase the zinc octacarboxyphthalocyanine photostability.
• DFT and TD-DFT were used to investigate the effect of complex formation on structure and UV–Vis spectra

Effect of selected amino acids (glycine, l-histidine, l-cysteine, l-serine, l-tryptophan) and albumin on the spectroscopic properties and photostability of zinc octacarboxyphthalocyanine (ZnPcOC) was explored in the phosphate buffer at a pH of 7.0. The photodegradation of ZnPcOC alone and in the presence of amino acids or albumin has been investigated in aqueous phase using UV-366 nm and daylight irradiation. Kinetic analysis showed that the interaction with amino acids or albumin enhances the photostability of ZnPcOC. To answer the question of how zinc phthalocyanine interacts with amino acids extensive DFT calculations were performed. Analysis of the optimized geometry features of ZnPcOC: amino acids complexes in the gas phase and in water environment as well as the BSSE corrected interaction energies indicates that the more likely is the formation of equatorial complexes in which H-bonds are formed between the COOH groups of the phthalocyanine and carboxyl or amino groups of amino acids. UV–Vis spectra calculated by employing time dependent density functional theory (TD-DFT) are also consistent with this conclusion.

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