3,4-Dihydro-3,4,6-trimethyl-2H,8H-pyrano-[3,2g]-1,3-benzoxazin-2,8-dione, a Potential Fluorogenic Reporter Group Reagent for Esterases—Synthesis and Interaction with Chymotrypsin

The synthesis of a novel “reporter group” reagent—3,4-dihydro-3,4,6-trimethyl-2H,8H-pyrano-[3,2g]-1,3-benzoxazin-2,8-dione (DTPBD)—is described. This compound has a cyclic carbamate functionality and thus, like the previously used 3,4-dihydro-3-methyl-6-nitro-2H-1,3-benzoxazin-2-one (DMNB), has the potential to label an esterase such as chymotrypsin. In so doing, DTPBD would incorporate into the protein a covalently linked 4-methylumbelliferone derivative, thus providing a sensitive reporter group that is both chromophoric and fluorescent. Experiments show that DTPBD reacts with chymotrypsin in the predicted manner, except that the labeling process is freely reversible (unlike the case with DMNB). 4-Methylumbelliferyl acetate (which is structurally closely related to DTPBD) is a good substrate for chymotrypsin. The rate of acylation of the enzyme is about an order of magnitude faster than with p-nitrophenyl acetate (which in turn is structurally related to DMNB), an unexpected observation in view of the relative leaving-group abilities of the groups concerned. The results suggest that these various modifiers and substrates show subtle but significant differences in the way they position themselves in chymotrypsinʹs binding site.