The first committed biosynthetic step toward clavulanic acid, the clinically important beta-lactamase inhibitor, is catalyzed by the thiamin diphosphate (ThDP)-dependent enzyme N2-(2-carboxyethyl)arginine synthase (CEAS). This protein carries out a unique reaction among ThDP-dependent processes in which a C-N bond is formed, and an electrophilic acryloyl-thiazolium intermediate of ThDP is proposed to be involved, unlike the nucleophilic enamine species typically generated by this class of enzymes. Here we present evidence for the existence of the putative acryloyl adduct and report the unexpected observation of a long-wavelength chromophore (lambda = 433 nm), which we attribute to this enzyme-bound species. Chemical models were synthesized that both confirm its expected absorption (lambda = 310-320 nm) and exclude self-condensation and intramolecular imine formation with the cofactor as its cause. Circular dichroism experiments and others discount charge transfer as a likely explanation for the approximately 120 nm red shift of the chromophore ( approximately 25 kcal). Examples are well-known of charged molecules that exhibit significantly red-shifted UV-visible spectra compared to their neutral forms as, for example, polyene cations and dyes such as indigo and the cyanines. Rhodopsin is the classic biochemical example where the protein (opsin)-bound protonated Schiff base of retinal displays a remarkable range of red-shifted absorptions modulated by the protein environment. Similar tuning of the chromophoric behavior of the enzyme-bound CEAS acryloyl.ThDP species may be occurring.