15min:

PHILIP J. MORGAN, DIANE M. MITCHELL AND DAVID W. PRATT, Department of Chemistry, University of Pittsburgh, PA 15260.

Recently, a controversy has developed over the proper assignment of the electronic spectrum of trans-p-coumaric acid ( tP CA), the chromophore in photoactive yellow protein. Ryan et al. claim that two closely spaced peaks near 33,200 cm^-1 are the S₁ S₀ origin bands of tP CA, whereas de Groot and Buma argue (based on REMPI results) that the spectrum should be contributed to the decomposition product p -vinylphenol ( p VP). We have addressed this issue by recording the fully resolved spectra of these two bands. The derived values of the rotational constants show unambiguously that the carrier of these bands is p VP; the two conformers are due to the two possible orientations of the -OH group with respect to the vinyl group. With the aid of theoretical calculations, the origin at 33,207.3 cm^-1 has been assigned to trans-p VP and the origin at 33,211.8 cm^-1 to cis-p VP. These results confirm the thermal decarboxylation of tP CA.