Spectroscopic evidence of xanthine compounds fluorescence quenching effect on water-soluble porphyrins

• Association of water soluble porphyrins and chlorophyll with xanthine compounds have been studied.
• Xanthine compounds interact with porphyrins, quenching their emission.
• Fluorescence quenching effect is of static nature, accompanied by the additional specific binding interactions.
• The association and fluorescence quenching constants were calculated.

The formation of π-stacked complexes between water-soluble porphyrins: 4,4′,4″,4″′-(21H,23H-porphine-5,10,15,20-tetrayl)tetrakis-(benzoic acid) (H2TCPP), 5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphine (H2TPPS4), 5,10,15,20-tetrakis[4-(trimethylammonio)phenyl]-21H,23H-porphine tetra-p-tosylate (H2TTMePP), 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine tetra-p-tosylate (H2TMePyP), the Cu(II) complexes of H2TTMePP and H2TMePyP, as well as chlorophyll a with xanthine, theophylline (1,3-dimethylxanthine) and theobromine (3,7-dimethylxanthine) has been studied analysing their absorption and steady-state fluorescence spectra in aqueous (or acetone in case of chlorophyll a) solution. During titration by the compounds from xanthine group the bathochromic effect in the porphyrin absorption spectra as well as the hypochromicity of the porphyrin Soret maximum can be noticed. The fluorescence quenching effect observed during interactions in the systems examined suggests the process of static quenching. The association and fluorescence quenching constants are of the order of magnitude of 103 − 102 mol−1. The results obtained show that xanthine and its derivatives can quench the fluorescence of the porphyrins according to the number of methyl groups in the molecule of quencher.

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