The weak and reversible binding of the antifungal drug, griseofulvin (GF), to calf thymus DNA has been demonstrated by difference spectroscopy and the quenching of the fluorescence of G F by DNA observed. The value of K , was determined to be 800 M." by fluorescence quenching titration.Adenosine and guanosine also exhibit difference spectra with G F and quench G F fluorescence indicating that they may be the site of both binding and energy transfer. The in vitro photosensitization of DNA by griseofulvin is shown to occur. It is proposed that the clinically observed in vivo photosensitizing action of griseofulvin may result from binding followed by excitation energy transfer and that this may also be important in the antifungal activity of the drug.
Using ultraviolet absorption spectrometry, rubratoxin B can be quantitatively determined over the concentration range 1.0 × 10−3 to 3.8 × 10×5M in acetonitrile or p-dioxane. Fluorescence, while observable, is a practical method only with apparatus of exceptional sensitivity and low signal-to-noise ratio. The calculated mean lifetime of fluorescence is 0.211 nsec. The calculated fluorescence quantum yield is 0.058. No phosphorescence was observed for rubratoxin B. Reasonable precautions should be taken to avoid exposure of dissolved rubratoxin B to light but the solid form is relatively stable. While water hydrolyzes the anhydride ring on the toxin, the process is reversible so that subsequent analysis is possible. Rubratoxin B is relatively stable to dry oxidation, in the dark, at room temperature even on thin layer chromatographic plates where the large surface area gives maximal exposure to oxygen.
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