Left, 13C NMR spectra of isotopologues of 1 in complex with the N-terminal domain of riboflavin synthase (wild type) at pH 6.9 (neutral) and pH 11.0 (alkaline). Right, labeling patterns of 1 isotopologue samples used; A, [7α-13C1]-1; B, [6α,6,7,7α-13C4]-1; C, 1 biosynthesized from [1-13C1]glucose; D, 1 biosynthesized from [2-13C1]glucose; E, [U-13C13]-1; the size of the dots indicates 13C enrichment.