High-Resolution Excitation and Absorption Spectroscopy of Gas-Phase p-Coumaric Acid: Unveiling an Elusive Chromophore

Abstract

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.

The Art of Choosing the Right Quantum Chemical Excited‐State Method for Large Molecular Systems

“…Otherwise, solvent effects need to be taken into account, either explicitly by including a few solvent molecules in the excited-state calculation, or implicitly through a dielectric continuum model. In the following, pCA again serves as an educative example, since high-resolution gas-phase spectra are available revealing a weakly allowed excited pp à state at 4.1 eV as the lowest excited state, followed by two practically degenerate states -a strongly allowed pp à and a forbidden np à state [67]. For comparison, the lowest vertical excited states of pCA have been computed at several wavefunction-based (Table 3.1) and density-based (Table 3.2) levels of theory at the MP2 optimized ground-state equilibrium geometry.…”

Laboratory infrared spectroscopy of PAHs

“…Superfluid nanoscopic He droplets are known to provide an extremely noninteracting and cold (0.4 K) matrix to study the spectroscopy of molecular species (Goyal et al 1992;Hartmann et al 1996). Drabbels and coworkers are now applying these methods to study the spectroscopy of ions, including aromatic species, at high resolutions (Loginov et al 2008;Smolarek et al 2010).…”

Photo absorption of ‐coumaric acid in aqueous solution: RISM‐SCF‐SEDD theory approach

“…The chromophore shows large spectral shift in photo absorption, more than 1 eV, probably due to the chemical modification and environmental effect. Toward understanding the mechanism of the spectral shift, absorption spectra both in gas and aqueous solution phase have been observed . Putschögl et al reports that the photo absorption of the chromophore shows significant pH dependence due to its deprotonation and the maximum wavelengths of , and are located at 310 nm (4.00 eV), 283 nm (4.38eV), and 334 nm (3.72 eV), respectively .…”

“…Putschögl et al reports that the photo absorption of the chromophore shows significant pH dependence due to its deprotonation and the maximum wavelengths of , and are located at 310 nm (4.00 eV), 283 nm (4.38eV), and 334 nm (3.72 eV), respectively . The first high‐resolution absorption spectra of reveals that the peak is at 4.05 eV . From the absorption spectrum of both in vacuo and water, the solvent effect on the photo‐absorption is suggested not to be significant in .…”