Sequence-specific 1H and 13C NMR assignments have been made for residues that form the five-stranded parallel beta-sheet and the flavin mononucleotide (FMN) binding site of oxidized Anabaena 7120 flavodoxin. Interstrand nuclear Overhauser enhancements (NOEs) indicate that the beta-sheet arrangement is similar to that observed in the crystal structure of the 70% homologous long-chain flavodoxin from Anacystis nidulans [Smith et al. (1983) J. Mol. Biol. 165, 737-755]. A total of 62 NOEs were identified: 8 between protons of bound FMN, 29 between protons of the protein in the flavin binding site, and 25 between protons of bound FMN and protons of the protein. These constraints were used to determine the localized solution structure of the FMN binding site. The electronic environment and conformation of the protein-bound flavin isoalloxazine ring were investigated by determining 13C chemical shifts, one-bond 13C-13C and 15N-1H coupling constants, and three-bond 13C-1H coupling constants. The carbonyl edge of the flavin ring was found to be slightly polarized. The xylene ring was found to be nonplanar. Tyrosine 94, located adjacent to the flavin isoalloxazine ring, was shown to have a hindered aromatic ring flip rate.