228 ERNST FREESE 



dCMP or (leoxyliydioxynu'tliylcvtidylatc iilII.MPi sliuw no effect 

 (Freese et ai. 19611)1. It should be noted tliat the ])K value of hydrazine 

 is about 8.4; the j)!! apparently influences its chemical specificity 

 api)recial)ly. 



The chenucal effect of hi/dro.vylamine (HA) on the i)yriniidine bases 

 has been observed only recently. The strongest effect (measured by the 

 decrease of UV absorption at 260 or 276 mp.) of a 1 M solution at pH 

 7.5 is on C. (MPv. dCMP, and U, UR, UMP; HMC or dHMP .^liows less 

 effect, while methyl-C, T, and dTAlP react too little to give any signifi- 

 cant change of UV absorption (Freese et al., 1961a,b). The diffeience 

 between the methylated and unniethylated uracil and cytosinc may be 

 due to the sliglit electron-donating ability of the methyl grou]) or to 

 steric effects. 5-Bromouracil or its deoxyriboside reacts very rapidly 

 owing to the electronegative bromine (Freese et al., 1961b). Again the 

 |)H controls the chemical specificity (the pK of HA is 6.2) such that at 

 1)H 6.15 the reactivity of CMP/U.MP in RNA is 30 and at pH 9.15 the 

 ratio is 1/8 (Schuster, 1961). 



The reaction of HA with U or UMP causes the irreversible loss of 

 UV absorption in the 260-280 m/.i. range, breaks and removes the pyi-im- 

 idine ring, producing pliosphoribosyl urea and 5-isoxasolone (Schuster, 

 1961). The reaction with the cytosine com|X)unds causes the loss of the 

 260-280 m/x absori)tion; but exposure to low pH return the UV absorj)- 

 tion partially or completely. The compound obtained after exposure to 

 low pH is not C but the hydroxy lamino compound IV in Fig. 10 (Freese 

 et al., 1961b). The exact structure of the intermediate non-UV-absorbing 



I n ni n? 



Fic. 10. Reactions of liydroxylamine \vit)i cytosine. Reaction II -^ III in\()lves 

 the li))oration of an amino group. Reaction III ^ IV is slow at neutral l)ut rai)i(l at 

 low pH. 



compound obtained after reaction with DNA is not known but the 

 analysis of 1-methylcytosine by Brown and Schell (1961) indicates that 

 it is the dihydroxylamine compound given as formula III in Fig. 10. 



