Binding of modified guanine nucleotides to ribonuclease T1 as studied by absorption spectroscopy. 8-aminoguanosine 2'-phosphate, 8-methylaminoguanosine 2'-phosphate, and 8-dimethylaminoguanosine 2'-phosphate.

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The title nucleotides have been synthesized and their binding to ribonuclease T<SUB>1</SUB> studied by means of absorption spectroscopy. Measurements were made at pH 5.5 and 4.1, where a major portion of the base moiety of the nucleotides was in a neutral form and a cationic form protonated at N7, respectively. At both pH's, difference absorption spectra were induced upon binding of the nucleotides to the enzyme. A comparison of the difference spectra with that for guanosine 2′-phosphate has led to the conclusion that perturbation in the bound nucleotide is the principal cause for the difference spectra. The difference spectra at pH 5.5 and 4.1 were similar, suggesting that the protonation at N7 of the nucleotides can be ruled out as the cause for the difference spectra. The dissociation constants at pH 5.5 and 4.1 of the enzyme–8-aminoguanosine 2′-phosphate complex were of the same magnitude, indicating that both the neutral and the protonated forms bind tightly to the enzyme. However, the cationic form of 8-methylamino- or 8-dimethylaminoguanosine 2′-phosphate has been shown to interact with the enzyme much more loosely than the neutral form.
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