High-resolution excitation and absorption spectroscopy of gas-phase p-coumaric acid: unveiling an elusive chromophore.
We report on the first successful high-resolution spectroscopic studies on isolated para-coumaric acid, the chromophore of the photoactive yellow protein which has become a model system for studying biological light-induced signal transduction. Employing various double-resonance multiphoton ionization techniques in combination with mass-resolved ion detection and the results of quantum chemical calculations, we identify three conformations the molecule can adopt under our experimental conditions. The vibrational activity in the excitation spectra allows us to conclude that in the Franck-Condon region accessed from the ground state S(1) is the V'(pipi*) state. Interestingly, we find considerable out-of-plane vibrational activity, indicating that the molecule adopts a nonplanar geometry in S(1). The ionization requirements show that after excitation rapid internal conversion takes place to a lower-lying npi* state. Such a state has been postulated by ab initio calculations on para-coumaric acid and derivatives, but until the present study no direct evidence had been found for its presence.
Smolarek
Szymon
S
University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands.
Vdovin
Alexander
A
Perrier
Dayinta L
DL
Smit
Jorrit P
JP
Drabbels
Marcel
M
Buma
Wybren J
WJ