Abstract
1. Dehydration rate constants of the photohydrates of cytosine, cytidine and cytidine monophosphate are presented as a function of pH with a view to elucidating the reaction mechanisms. 2. Photohydrates were produced by ultraviolet irradiation at a high exposure rate at 0° and pH 8–9 in the absence of buffer. These conditions lead to a high degree of reversibility after nearly complete photolysis. 3. Buffers with p K's between 3 and 7 were found to accelerate dehydration of cytosine and cytidine hydrates in the pH range 2–7. 4. The ionization constants of the hydrates were determined by potentiometric titration. 5. A model involving general base catalysis is proposed to account for the observed rate-pH profiles. It agrees well with the experimental results. Rate constants for catalysis by various basic species are presented. 6. Reversal of the hydrate of cytidine monophosphate was found to be insensitive to added buffers. In this case the rate is greatly influenced by the phosphate group which acts as an effective internal catalyst. Our model accounts qualitatively for this effect.
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