The spectral and catalytic studies of chlorophylls and pheophytins in mimetic biotransformation of α-pinene

Natural porphyrin pigments isolated from spinach in the form of a crude extract, chlorophyll a, chlorophyll b, pheophytin a and pheophytin b were studied as potential catalysts for the biomimetic transformation of α-pinene to trans-pinocarveol, pinocarvone and myrtenal. The reaction of monoterpene oxidation was performed in an organic solution under visible light irradiation. Significant changes in the distribution of the products were achieved using β-pinene as the substrate. The pheophytins showed the highest catalytic efficiency among the photocatalysts examined. The photographic visualization of α-pinene oxidation course at the beginning of the reaction, after 4 and 24 h exposure to light showed a distinct shift in the colour excited catalysts. This may indicate a change in their structure leading to their degradation. The absorbance and fluorescence spectra revealed that pheophytins are significantly stable photocatalysts in comparison to both chlorophylls. Pheophytin b displayed the highest stability in the biotransformation, whereas the highest rate of photodecomposition was proved for chlorophyll b under the reaction condition. Differences were noticed in the behaviour of the chlorophylls in the photodegradation process. From the changes in the intensity of the singlet oxygen band on infrared fluorescence spectra of pigments upon pinene addition, and from the reactivity experiments with radicals scavengers, it can be deduced that the biotransformation of pinene by chlorophylls and pheophytins proceeds by common oxygenation mechanism involving singlet oxygen and free radicals.

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► We investigate the plant pigments as biomimetic catalysts for oxidative transformation of pinenes under mild conditions.
► Pheophytins are efficient light-excited catalysts for α- and β-pinene biotransformation to pinocarveol, pinocarvone and myrtenal.
► Absorbance and fluorescence spectra revealed that pheophytins are significantly stable photocatalysts in comparison to chlorophylls.
► Biotransformation of pinene by chlorophylls and pheophytins proceeds by common mechanism involving singlet oxygen and free radicals.