Sugar-dependent photodynamic effect of glycoconjugated porphyrins: a study on photocytotoxicity, photophysical properties and binding behavior to bovine serum albumin (BSA)

Abstract

The photocytotoxicity of four glycoconjugated porphyrins, namely 5,10,15,20-tetrakis[4-(beta-D-glucopyranosyloxy)phenyl]porphyrin (p-1a), 5,10,15,20-tetrakis[4-(beta-D-galactopyranosyloxy)phenyl]porphyrin (p-1b), 5,10,15,20-tetrakis[4-(beta-D-xylopyranosyloxy)phenyl]porphyrin (p-1c) and 5,10,15,20-tetrakis[4-(beta-D-arabinopyranosyloxy)phenyl]porphyrin (p-1d), was evaluated in HeLa cells in the concentration range from 1 to 7 microM using a light dose of 16 J x cm(-2) with a wavelength greater than 500 nm. The photocytotoxicity depends on the sugar moieties, and increases in the order of p-1d<p-1a<p-1b<p-1c. The order of the photocytotoxicity is at variance with that of the cellular uptake reported previously. On the other hand, the photophysical properties of the glycoconjugated porphyrins also depends on the sugar moieties in physiological media such as phosphate buffered saline (PBS) containing 10 wt.% bovine serum albumin (BSA). In particular, the oscillator strength in the range above 500 nm increases in the order of p-1d=p-1a<p-1c<p-1b, which is good agreement with the order of the photocytotoxicity in HeLa cells. The interaction between the glycoconjugated porphyrins and BSA was evaluated by means of electronic absorption, fluorometric and circular dichroic (CD) titrations. Fluorometric titration showed no differences in the apparent binding constants, K, between the glycoconjugated porphyrins p-1a, p-1b, p-1c and p-1d. On the other hand, the number of binding sites, n, depends on the sugar moieties of the glycoconjugated porphyrin, and increases in the order of p-1b<p-1a<p-1d<p-1c. CD titration was also characterized by the n value determined by fluorometric titration, suggesting the n value is a good descriptor for the interaction between glycoconjugated porphyrins and BSA. However, it was found that the n value was poorly related to the photophysical properties in physiological media and the photocytotoxicity. Even though the role of the sugar moieties on the photodynamic effect is not fully understood, the photophysical properties of the glycoconjugated porphyrins are strongly modulated by the physiological media resulting in the sugar-dependent photocytotoxicity.