Aflatoxin B
1 (AFB) epoxide forms an unstable N7 guanine adduct in DNA. The adduct undergoesbase-catalyzed ring opening to give a highly persistent formamidopyrimidine (FAPY) adduct which existsas a mixture of forms. Acid hydrolysis of the FAPY adduct gives the FAPY base which exists in two separablebut interconvertible forms that have been assigned by various workers as functional, positional, orconformational isomers. Recently, this structural question became important when one of the two majorFAPY species in DNA was found to be potently mutagenic and the other a block to replication [Smela, M.E.; Hamm, M. L.; Henderson, P. T.; Harris, C. M.; Harris, T. M.; Essigmann, J. M.
Proc. Natl. Acad. Sci.U.S.A. 2002,
99, 6655-6660]. NMR studies carried out on the AFB-FAPY bases and deoxynucleoside3',5'-dibutyrates now establish that the separable FAPY bases and nucleosides are diastereomeric N
5formyl derivatives involving axial asymmetry around the congested pyrimidine C5-N
5 bond. Anomerizationof the protected
-deoxyriboside was not observed, but in the absence of acyl protection, both anomerizationand furanosyl
pyranosyl ring expansion occurred. In oligodeoxynucleotides, two equilibrating FAPYspecies, separable by HPLC, are assigned as anomers. The form normally present in duplex DNA is themutagenic species. It has previously been assigned as the
anomer by NMR (Mao, H.; Deng, Z. W.;Wang, F.; Harris, T. M.; Stone, M. P.
Biochemistry 1998,
37, 4374-4387). In single-stranded environmentsthe dominant species is the
anomer; it is a block to replication.