Abstract :
A meso-substituted cationic porphyrin (TMPyP) showed a photocytotoxicity against Gram-positive and Gram-negative bacteria. In
order to determine the mechanism involved in the phototoxicity of this photosensitizer, electron paramagnetic resonance (EPR) experiments
with 2,2,6,6-tetramethyl-4-piperidone (TEMP), a specific probe for singlet oxygen, and the spin-trap 5,5-dimethyl-1-pyrroline-Noxide
(DMPO) were carried out with illuminated TMPyP. An EPR signal characteristic of TEMP-singlet oxygen (TEMPO) adduct
formation was observed, which could be ascribed to singlet oxygen (1O2) generated by TMPyP photosensitization. The signal for the
DMPO spin adduct of superoxide anion (DMPO-OOH) was observed in DMSO solution but not in aqueous conditions. However,
an EPR spectrum characteristic of the DMPO-hydroxyl radical spin adduct (DMPO-OH) was observed in aqueous conditions. The
obtained results testify a primary hydroxyl radical ( OH) generation probably from superoxide anion ðO 2 Þ via the Fenton reaction
and/or via Haber-Weiss reaction. Gram-positive and Gram-negative bacteria inactivation by TMPyP photosensitization predominantly
involved Type II reactions mediated by the formation of 1O2, as demonstrated by the effect of quenchers for 1O2 and scavengers for OH
(sodium azide, thiourea, and dimethylsulphoxide). Participation of other active oxygen species cannot however be neglected since Type I
reactions also had a significant effect, particularly for Gram-negative bacteria. For Gram-negative bacteria the photoinactivation rate
was lower in the presence of superoxide dismutase, a specific O 2 scavenger, and/or catalase, an enzyme which specifically eliminates
H2O2, but was unchanged for Gram-positive bacteria. The generation of 1O2, O 2 and OH by TMPyP photosensitization indicated that
TMPyP maintained a photodynamic activity in terms of Type I and Type II mechanisms.
2008 Elsevier Ltd. All rights reserved
