Kinetic and Spectroscopic Properties of (7-nitrobenz-2-oxa-1,3-diazole)aminoalkyl Methylphosphonofluoridates and Their Conjugates with Acetylcholinesterase Molecular Forms*

The synthesis, kinetic, and spectral characterization of (7-nitrobenz-2-oxa-1,3-diazole)aminoethyl and (7nitrobenz-2-oxa1,3-diazole)aminopentyl methylphosphonofluoridate are described. These homologous organophosphorous agents contain the environmentally sensitive 7-nitrobenz-2-oxa1,3-diazole chromophore. They inhibit acetylcholinesterase from Torpedo at rates exceeding lo’ M” min” to form long-lived conjugates with one chromophore/80-kilodalton subunit. The intensity, position, and line width of the absorption spectra of the conjugates and reactivation kinetics in the presence and absence of the bisquaternary oxime 1,l ’-trimethylene-bis(4-formylpyridinium bromide) dioxime indicate that these agents form conjugates in which the NBD-aminoalkyl moieties experience distinctive microscopic environments within the active center. NBD-aminoethyl methylphosphono-acetylcholinesterase undergoes oxime-induced as well as spontaneous reactivation at rates that are 3.6 and 35 times faster, respectively, than the corresponding rates measured for the NBD-aminopentyl conjugate. Hence, reactivation exhibits a marked dependence on structure of the methylphosphonate. Fluorescence emission at wavelengths greater than 520 nm is highly quenched and exhibits quantum efficiencies of less than 5%. Absorption maxima for the covalent NBD-aminoethyl methylphosphono-acetylcholinesterase appear at 475-480 nm while those for the corresponding NBD-aminopentyl methylphosphono-acetylcholinesterase appear at 485-490 nm. Bandwidths of the absorption maxima are substantially broader for the acetylcholinesterase adduct with NBD-aminoethyl methylphosphonofluoridate (3870 cm”) than for the enzyme adduct with NBD-aminopentyl methylphosphonofluoridate (2870 om”). The CD spectrum of NBD-aminopentyl methylphosphonoacetylcholinesterase shows optical activity coincident with the shape and position of the absorption spectrum. In contrast, in addition to optically active transitions at the absorption maxima, the CD spectrum of NBDaminoethyl methylphosphono-acetylcholinesterase shows intense optical activity at 430 nm, a wavelength region coincident with the region of spectral broadening. The spectral properties of a-chymotrypsin conju-