Hypericin, absorption

The pharmacokinetics of hypericin and pseudohypericin piasma have been studied as weii (Brockmoiier et ai. 1997). Human subjects receiving piacebo, or 900, 1800, or 3600 mg of a standardized hypericum extract (LI 160), which contained 0, 2.81, 5.62, and 11.25 mg of totai hypericin and pseudohypericin, achieved maximum total plasma concentrations at 4 hours (0.028, 0.061, and 0.159 mg/L, respectively). The half-lives of absorption, distribution, and elimination were 0.6, 6.0, and 43.1 hours, respectively, using 750 pg of hypericin, and are slightly different for 1578 pg of pseudohypericin (1.3, 1.4, and 24.8 hours, respectively) (Kerb et ai. 1996). The systemic availability of the hypericum extract LI 160 is between 14 and 21%. Comparable results are found in another study using LI 160 (Staffeldt et ai. 1994). Long-term dosing of 3 x 300 mg per day showed that steady-state levels of hypericin are reached after 4 days. 

We now consider two objections that may be raised to the assignment of the excited-state processes in hypericin and hypocrellin as H-atom transfer. These are the absence of a deuterium isotope effect for the 10-ps component of hypericin and hypocrellin and the mirror image symmetry between the absorption and the emission spectra in hypericin and hypocrellin. 

This tunable source was used to investigate the transient absorption kinetics of hypericin in DMSO as a function of pump wavelength (Fig. 1.10). The startling result is that using pump wavelengths from 495 to 600 nm, the data can be fit globally by a sum of two exponentials, which except for two traces exhibits the 10-ps component characteristic of H-atom transfer. Fit results are compiled in Table 1.1. 

Figure 1.10. Transient absorption kinetics for hypericin as a function of pump wavelength. In each panel, the first value corresponds to the pump wavelength the second, to the probe wavelength. The significant feature of these data is that they can be uniformly fit to two time constants, of which one corresponds to the characteristic H-atom-transfer time of 10 ps. The results of a global fit to these data are compiled in Table 1.1.

TABLE 1.1 Absorption Transients for Hypericin in DMSO as a Function of Pump Wavelength"... 

Hypomycin B is unique in that it has only one intramolecular hydrogen bond as opposed to the two in hypocrellin A and the four in hypericin (Fig. 1.1). Picosecond transient absorption data for hypomycin B fail to reveal any stimulated... 

The photophysics of the polycyclic quinone hypericin and its analogs has been studied quite exhaustively by Petrich and co-workers [57]. The interest in this compound stems from its proven antiviral activity, induced by light. Ultrafast time-resolved transient absorption studies indicate that ESIPT is the primary non-radiative process in hypericin. 

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