Resonance Raman and electronic absorption spectral studies of some beta-(2-furyl)acryloylglyceraldehyde-3-phosphate dehydrogenases.

Electronic absorption and resonance Raman spectra, together with rates of arsenolysis, are used to show that the acylated subunits of the rabbit furylacryloylglyceraldehyde-3-phosphate dehydrogenase (FA-rabbit enzyme), but not the FA-sturgeon enzyme, exist as a mixed population of at least two forms. One form of the FA-rabbit enzyme has a high rate of arsenolysis, and its near-UV absorption maximum (lambda max) is red shifted on binding NAD+, while the other has a lower rate of arsenolysis, and its absorption spectrum appears to be unaffected by excess NAD+. The FA-sturgeon enzyme exists as a single population that has a high rate of arsenolysis, and its lambda max is red shifted on binding NAD+. This red shift and the concomitant position of v C==C, the band due to the ethylenic double bond stretching vibration in the resonance Raman spectrum, indicate that in the presence of NAD+ the pi electrons of the FA chromophore are polarized, i.e., a permanent dipole has been set up along the long axis of the FA group. It is argued that this does not result in activation of the carbonyl group by setting up polarization (i.e., delta + C--O delta -) within that group alone. A comparison of the vC==C's and lambda max's for a series of FA derivatives of the type (formula, see text) where X = H, N, or O, shows a clear correlation. However, when X = S as, e.g., in thiol esters, this correlation breaks down. This is explained in terms of a possible through-space d pi-p pi overlap between the ethylenic bond pi orbitals and the empty sulfur 3d orbitals.