Amine conjugates of activated aflatoxin (AF) B(1) are formed in various systems, e.g., buffer amines and with protein lysine groups. Structures have been published in the literature, but the evidence is indirect in that (i) halogenated AFB(1) was usually used as the precursor and (ii) the assignment of the structure of the five-membered ring formed by cyclization is based on NMR chemical shifts. To better define these adducts and distinguish among several possibilities, we synthesized AFB(1) dialdehyde and reacted this with the surrogate methylamine at neutral pH, to simplify the system. The isolated product had the expected molecular ion (mass spectrometry) and showed pH-dependent UV spectra similar to those published for a lysine conjugate. Nuclear Overhauser enhanced spectroscopy (two-dimensional NMR, 800 MHz) of the sample (2H(2)O) showed proximity of the N-CH(3) protons only with a singlet at delta 4.10, assigned to the methylene of the added five-membered ring, but not to a delta 6.53 singlet assigned as the vinylic proton of that ring. All protons in the coumarin-furanone portion of the system were correlated to each other but not to those in the added five-membered ring. These experiments establish the structure as 2,3-dihydro-2-oxo-4-(1,2,3,4-tetrahydro-7-hydroxy-9-methoxy-3,4-dioxocyclopenta[c]benzopyran-6-yl)-1H-pyrrole-1-methane. The similarity of the reaction to that occurring in the reaction of AFB(1) dialdehyde with lysine and the agreement of the UV spectra suggest that this structure is applicable for the lysine analogue. The NMR results support the possible structure B of Sabbioni et al. [Sabbioni, G., Skipper, P. L., Buchi, G., and Tannenbaum, S. R. (1987) Carcinogenesis 8, 819-824] and the proposed structure 8 of Sabbioni [Sabbioni, G. (1990) Chem.-Biol. Interact. 75, 1-15] but not alternative proposals. Kinetic and mechanistic considerations of the reaction of lysine with AFB(1) dialdehyde are presented in the previous article in this issue [Guengerich, F. P., Arneson, K. O., Williams, K. M., Deng, Z., and Harris, T. M. (2002) Chem. Res. Toxicol. 15, 780-793].