Uridine photohydrate

According to this mechanism, the x>K of a singlet excited uracil is about 4, and thus excitation of uracil greatly increases its proton affinity and reduces its hydrogen bond-forming capacity. Similar variations of photohydration rate were observed for 1-ethyluracil, and 1-cyclohexyl uracil but the concommittant reaction of neutral excited molecules with water was faster, and was the predominant reaction for uridine photohydration up to pH 10.64 The photohydration rates of the two uridine phosphates was fastest at low pH values, and showed two inflection points, one at pH 1.5 and one at pH 6.161 It is clear from these data that the different ionic species of uracil, and its derivatives have greatly different rates of photohydration, and that these reactions exhibit the characteristics of an electrophilic attack of the excited species on neutral water molecules. 

This reaction has also been shown to occur in cytidine, cytidylic acid, uracil, uridine, and uridylic acid (found in RNA) but reportedly not in thymine, thymidine, or thymidylic acid/55 The photohydration has been found to be partially reversible, dehydration being nearly complete at extremes of temperature and pH. 

The other photoproduct which is formed is one resulting from the light-induced addition of water across the 5,6-double bond of the pyrimidine, shown in II for uracil. This product will be denoted in this paper by the term photohydrate. Little has been reported about the mechanism of this reaction, or about the nature of the responsible intermediate (4). It is the purpose of this paper to report some observations about the variation with pH and salt concentration of the rate of the photohydration process in four uracil derivatives—(1) uracil (R = H), (2) 1-ethyluracil (EU, R = ethyl), (3) 1-cyclohexyluracil (CU, R = cyclohexyl), and (4) uridine (R = ribosyl). 

The rate constants for this back reaction, as functions of pH, are shown in Figure 6 for the photohydrates of uracil, EU, CU, and uridine all at 32°C. The shape of these curves, exhibiting catalysis by both H30+... 

The values for the rate constants for photohydration of uracil, EU, CU, and uridine, calculated as described in the Experimental section, have been plotted as functions of the pH, and the plots are shown in Figure 7. The photohydration rates for all four compounds are sensitive to the pH the effect is most pronounced for uracil and least apparent for uridine. The photohydration of dimethyluracil is independent of the pH in the range of 2 to 71. 

It is evident from the non-zero rate for uracil photohydrate formation at high values of pH that the fast reaction of 1(UH+) with water is accompanied by a slower reaction which is presumably the reaction of 1U with water. This type of reaction is apparently quite fast for EU and CU, although still slower than the reaction of the protonated excited state. It is possible that both protonated and neutral excited uridine species react at the same rate. 

Quantum yields for photooxidation of NADPH and (NADP>2 at 365 nm were determined with the aid of the ferrioxalate actinometer, according to Parker Quantum yields at 254 nm were based on the use of the photohydration reaction of uridine, as previously described. ... 

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