Sterische Voraussetzungen für die Carboxymethylierung von His 12 der Ribonuclease A durch 2´- oder 3´-Bromacetylester modifizierter Nucleoside

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Abstract
The 2''- and 3''-bromoacetylesters of different nucleosides were synthesized and tested in the carboxymethylation of His 12 of pancreatic RNase A. The 2''-esters of ribosides react more than 100 times faster compared with 3''-esters or 2''-arabinosides. 2''(3'')-esters of nucleosides which are not substrates (adenosine, N3-methyluridine or 6-methyluridine) do not react, obviously due to an inexact orientation of the base. The difference spectrum obtained after the reaction of 2''-acetamido-2''-deoxyuridine with RNase A is identical with the difference spectrum of 2''-cytidylic acid, thus indicating that the base occupies the same position as in the binding of substrate analogous inhibitors. Immobility of His 12 and its reaction with the CH2-Br group leads to a disagreement with the concept of a base catalysis of His 12 at the 2''-OH-group in the reaction with substrates. The pH dependence of the reaction is identical with the carboxymethylation of His 12 and 119 using bromoacetate. Therefore, binding to His 119 cannot be considered when interpreting the bell-shaped curve, which might be explained by the alternative, the existence of a triprotonated dimidazole system between His 119 and His 12. No activity can be detected for His-12-carboxymethylated RNase at any pH.