The ultraviolet absorption spectra of the pyrimidine ribonucleosides and ribonucleotides.

It is well known that the high and characteristic absorption of nucleic acids in ultraviolet light is due to the absorption of their purine and pyrimidine components. Several workers have measured the absorption curves of the aminoand oxypurines (l-3) and Jordan has obtained similar data for adenosine and guanosine and for the corresponding nucleotides (3). In the latter instances substitution of the carbohydrate radical in position 9 of the purine ring does not change the absorption appreciably, but, as shown by Gulland and coworkers (4), substitution in the 7 position of either adenine or guanine causes definite changes in the absorption spectra. Although the absorption curves of the free pyrimidines, uracil, cytosine, and thymine (2, 5, S), and of thymine desoxyriboside (7) have been determined, similar data have not been published for the corresponding ribonucleosides, cytidine and uridine, or for the nucleotides, cytidylic acid and uridylic acid. With the growing interest in the metabolism of these compounds (8, 9), knowledge of their absorption characteristics would be useful for their determination in biological material. As it has been possible recently to prepare them in appreciable yield and of a high degree of purity (10, ll), we have accordingly measured their absorption in comparison with the free pyrimidines, cytosine and uracil. As all of these compounds may exist in different tautomeric forms, depending on the hydrogen ion concentration of the solvents employed, dilute acid and dilute alkali were used to provide conditions under which one or the other form would be present. For comparison 0.05 M phosphate buffer at pH 7.0 was also used.