Abstract
As a naturally occurring anthraquinone pigment, chrysophanol (MHAQ) has attracted considerable attention in recent years owing to its efficient photosensitivity under the solar spectrum. Considering the successful use of time-dependent density functional theory (TD-DFT) in investigating the photo-physicochemical behaviors of dyes and pigments, we performed a study by means of TD-DFT calculations, which provided us with various excited state properties of chrysophanol, including absorption spectrum, lowest triplet excited-state energy, vertical electron affinity and vertical ionization potential. On the basis of the calculated results, the photosensitive mechanisms of chrysophanol were discussed and some deeper insights were gained. First, we indicated that the experimentally observed chrysophanol’s photo-damage to DNA in oxygen-free media is more likely to arise from MHAQ•+ rather than from T1 state chrysophanol. Second, we revealed that it is the MHAQ•− that is responsible for the O2•− generation in solvents. Based on the photosensitive activities, chrysophanol may be potentially used as the photodynamic medicine for clinical therapy of the diseases occurring on the shallow surface and vascular capillary diseases.
Highlights
Chrysophanol (3-methyl-1,8-dihydroxyanthraquinone, MHAQ) belongs to a family of anthraquinone pigments that naturally exist in rheum, a Chinese herbal medicine growing abundantly in China
As the use of diffuse functions is essential for accurate determination of the energetics, for ion radicals and excited states [7], the vertical electron affinitie (VEA: the difference between the total electronic energy of parent molecule and anion radical) and vertical ionization potential (VIP: the difference between the total electronic energy of parent molecule and cation radical) were calculated by a combined DFT method B3LYP/631+G(d,p)//B3LYP/6-31G(d,p), which means that B3LYP/6-31+G(d,p) was employed to perform a singlet-point calculation on the basis of B3LYP/6-31G(d,p) optimized structures
The time-dependent density functional theory (TD-DFT)-calculated ET1 values of chrysophanol in different medium are listed in Table 2, from which we find there occurs a certain red-shift of absorption band in solvent comparing with that in vacuum
Summary
Chrysophanol (3-methyl-1,8-dihydroxyanthraquinone, MHAQ) belongs to a family of anthraquinone pigments that naturally exist in rheum, a Chinese herbal medicine growing abundantly in China. Besides their biological activities, these pigments are well known as photosensitizers [1,2]. As a family of photosensitizers, the anthraquinone derivatives possess several advantages, such as low costs and efficient photosensitivity under the solar spectrum. The absorption spectra of anthraquinones in the visible area are spread over 400—500 nm with logε = 4.02 (433 nm), 3.85 (443 nm) and 3.04 (433 nm) for chrysophanol, emodin and mixed anthraquinone derivatives extracted from rheum, respectively [3]. The light with wavelengths from 400 nm to 500 nm can only penetrate into the tissue no more than 1 mm, it may be a better choice to apply them for the photodynamic therapy of diseases occurring in shallow surfaces, such as some vascular diseases
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