Spectroscopic study of porphyrin-caffeine interactions

Abstract

The association between water-soluble porphyrins: 4,4',4″,4'''-(21 H,23 H-porphine-5,10,15,20-tetrayl)tetrakis-(benzoic acid) (H(2)TCPP), 5,10,15,20-tetrakis(4-sulfonatophenyl)-21 H,23 H-porphine (H(2)TPPS(4)), 5,10,15,20-tetrakis[4-(trimethylammonio)phenyl]-21 H,23 H-porphine tetra-p-tosylate (H(2)TTMePP), 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21 H,23 H-porphine tetra-p-tosylate (H(2)TMePyP), the Cu(II) complexes of H(2)TTMePP and H(2)TMePyP, as well as chlorophyll a with caffeine (1,3,7-trimethylxanthine) has been studied analysing their absorption and emission spectra in aqueous (or acetone in case of chlorophyll a) solution. During the titration by caffeine the porphyrins absorption spectra undergo the evolution - the bathochromic effect can be observed as well as the hypochromicity of the Soret maximum. The association constants were calculated using curve-fitting procedure (K(AC) of the order of magnitude of 10(3) mol(-1)). Whereas the emission spectra point at the presence of the fluorescence quenching effect testifying for the partial inactivation of the porphyrin molecule. The fluorescence quenching constants were calculated from Stern-Volmer plots. The results obtained show that caffeine can interact with water-soluble porphyrins and through formation of stacking complexes is able to quench their ability to emission.

Fig. 1

The molecular structures of (a) caffeine(1,3,7-trimethylxanthine), (b) chlorophyll a and (c) H₂TTMePP (5,10,15,20-tetrakis[4-(trimethylammonio)phenyl]-21 H,23 H-porphine)

Fig. 2

Evolution of H₂TTMePP absorption spectrum during titration by caffeine. The dependence of absorbance versus porphyrin concentration for the process presented. The concentrations of the porphyrin and caffeine in solution changed in the range 3.27 – 1.92 (× 10^-7 mol dm^-3) and 0–4.13 × 10^-4 mol dm^-3, respectively

Fig. 3

Evolution of H₂TTMePP emission spectrum during titration by caffeine. The dependence of fluorescence intensity versus porphyrin concentration for the process presented. All the concentrations as in Fig. 2

Fig. 4

Evolution of chlorophyll a absorption spectrum during titration by caffeine. The dependence of absorbance versus chlorophyll a concentration for the process presented. The concentrations of chlorophyll a and caffeine in solution changed in the range 7.34 – 4.31 (× 10^-7 mol dm^-3) and 0–4.13 × 10^-4 mol dm^-3, respectively

Fig. 5

Evolution of chlorophyll a emission spectrum during titration by caffeine. The dependence of fluorescence intensity versus chlorophyll a concentration for titration by caffeine, water and acetone. All the concentrations as in Fig. 4

Fig. 6

Stern-Volmer plots for all the porphyrin compounds examined during titration by caffeine