Bisulfite with pyrimidines, reactions

Recent studies have demonstrated reactions of bisulfite with pyrimidines. Addition compounds are formed at the 5,6-double bond of the pyrimidine (5). Mutagenic effects have been observed with phage A (10) and with Escherichia coli (11). However, the concentrations of bisulfite used in these experiments were 1M or higher, and one wonders if such effects would be observed at the much lower concentrations that could arise by exposure of vegetation to SOo-polluted air. 

Sulfur dioxide (bisulfites) also reacts with thiamine yielding inactive compounds (see Section 5.6.6), forms adducts with riboflavin, nicotinamide, vitamin K, inhibits ascorbic acid oxidation (see Section 5.14.6.1.6) and reacts with ascorbic acid degradation products, reduces o-quinones produced in enzymatic browning reactions back to 1,2-diphenols (see Section 9.12.4), causes decolourisation of fruit anthocyanins (see Section 9.4.1.5.7) and reacts with synthetic azo dyes to form coloured or colourless products (see Section 11.4.1.3.2). Sulfur dioxide also reacts with pyrimidine bases in vitro, specifically with cytosine and 5-methylcytosine. Important reactions of sulfur dioxide are shown in Figure 11.4. 

As in the case of pyrimidine bases discussed previously, adenine and guanine are subject to nucleophilic displacement reactions at particular sites on their ring structures (Figure 1.50). Both compounds are reactive with nucleophiles at C-2, C-6, and C-8, with C-8 being the most common target for modification. However, the purines are much less reactive to nucleophiles than the pyrimidines. Hydrazine, hydroxylamine, and bisulfite—all important reactive species with cytosine, thymine, and uracil—are almost unreactive with guanine and adenine. 

Reactions of nucleophiles. A number of nucleophilic reagents add reversibly at the 6 position of pyrimidines. Thus, bisulfite adds to uridine (Eq. 5-10).528 Hydroxylamine (HONH2) adds in a similar fashion to give a compound with -HNOH in the 6 positions.528 Sodium borohydride (NaBH4), which can be viewed as a donor of a hydride ion (H ), reduces uridine to the 5,6-dihydro derivative. This presumably occurs by attack of the hydride ion at position 6 in a manner analogous to the reaction of bisulfite in Eq. 5-10. 

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