The reaction of nitrous acid with deoxyribonucleic acid (DNA) isolated from different sources was investigated. Contrary to the results found with ribonucleic acid from tobacco mosaic virus, in which all the aminobases are deaminated with about the same velocity, in native DNA the aminobases react with very different reaction velocities. In this case adenine and cytosine react much slower than does the guanine, also the ratios of velocities of the bases vary with the H-ion concentration of the solution. These results confirm the existence of hydrogen bonds involving only the aminogroups of adenine and cytosine in native DNA, whereas in denaturated DNA the aminogroups of all the bases are exposed to nitrous acid deamination.
Irradiation of uridylic acid (Up) in buffer (at 254 mµ or 280 mµ) yields the hydrated molecule (UpH2O), which on subsequent alkaline treatment decomposes to Up (65%) and a new compound A (35%) with the pyrimidine-ring opened. Irradiation in ice (254 mµ) leads to the formation of
the uridylic acid dimer (Up/Up), which on re-irradiation reverts to Up (230 mµ, in neutral solution) or compound A, via Up and UpH2O) (254 mµ, in alkaline solution).
Similarly the hydrated cytidylic acid (CpH2O) is formed by irradiation of cytidylic acid (Cp) in buffer (254 mµ). In a subsequent dark reaction most of the CpH2O-molecules revert to Cp, a minor part being deaminated to UpH2O. Alkaline treatment of an irradiated Cp-solution yields Cp, Up and compound A, the latter two arising by decomposition of UpH2O. Neither Cp nor Up dimers were found after irradiation of Cp in ice (254 mµ).
All the photoproducts of Up and Cp, in which the pyrimidine-ring was still preserved, lost 1 - 3% of their phosphoric acid (Pi) under the experimental conditions.
The alkaline decomposition of UpH2O is used as a method for the titration of hydrated uracil residues in irradiated ribonucleic acid (RNA). Following irradiation (254 mµ) the RNA is degraded by alkali completely to mononucleotides and three new photoproducts: compound A, ribose phosphate, and another compound B, which behaves like Up dimer. Form the amount of compound A found, the number of hydrated uracils originally formed can be calculated. Furthermore the kinetics of the formation of the photoproducts are studied.
The biological implications of these studies are discussed.