Abstract
In this study, several properties of actinomycin D have been examined: The effect of different solvents upon absorption, optical rotary dispersion and circular dichroism show that the optical activity does not coincide with the absorbance maximum around 440 mμ. Further an association between the polypeptide rings of actinomycin in aqueous solutions is quite possible. The increase in optical rotatory dispersion when the actinomycin‐DNA complex is formed shows that the molecular asymmetry of the actinomycin molecule is enhanced. This is in contrast to the observations made upon acridine‐DNA complexes where the optical activity decreases with increasing DNA concentration. This clearly shows that the optical activity of the actinomycin‐DNA complex does not result from interactions between dye molecules specifically oriented by the DNA, but by interactions between DNA and the dye. The fixation of actinomycin on helical DNA imposes a highly specific orientation of both the phenoxazine and the two lactone rings, the result of which is the increase of the optical activity of the dye. The optical rotatory dispersion of the weak actinomycin‐deoxyguanosine complex can clearly be distinguished from the much firmer actinomycin‐DNA complex. The optical rotatory dispersion curve of this complex is different both in amplitude and shape compared with that of the actinomycin‐DNA complex. This is due to the greater sterical freedom which imposes much less constraint to the conformation of the bound actinomycin molecule. Acid denaturation of the actinomycin‐DNA complex below pH 3 shows the appearance of an association of the type actinomycin‐deoxyguanosine. While it is difficult to separate these changes by absorbance measurements it can unequivocally be distinguished by optical rotatory dispersion. No complex formation could be detected between actinomycin and polydeoxyguanylic acid.
Published Version
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