✍ Scribed by Takashi Ito; Mitsuo Zama; Jumpei Amagasa
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Circular dichroism spectra of acridine orange bound to E. coli tRNA were studied at varying tRNA phosphate‐to‐dye (P/D) ratios for both unfractionated and purified materials in the absence of Mg^++^. From the rather discrete features exhibited in the circular dichroism spectra three types of interactions were observed: (1) A high P/D ratio such as 75.2 or 49.8 indicates the interaction between the nucleotide base and dye molecule. The spectra with a large positive peak at 515 mμ are, however, quite different from that of DNA–AO complex under similar conditions. (2) With an intermediate P/D ratio (26.5 to 9.6) dye molecules bound strongly to the polynucleotide chain. (3) With low P/D ratios (≤7.5) the interaction appears to be due to the stacked dye molecules in the single‐stranded part of tRNA. The spectra of the third group have an isobestic point at 477 mμ. Below a P/D ratio of 4 the spectrum shows one positive and two negative bands which may be the characteristics of circular dichroism of stacked dyes in polynucleotide chain.
Although no drastic change in the conformation of tRNA itself was detectable in the presence of Mg^++^ in the ultraviolet region, a dramatic change was observed in the circular dichroism of tRNA–acridine orange complex when Mg^++^ concentration was increased to 10^−3^M. It was inferred that certain conformational changes other than simple hydrogen bond formation occured in tRNA molecules at this high Mg^++^ concentration, so that the amount of bound dye in the stacking condition was increased through the transition.
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