Vibrational and electronic spectroscopy of the 4-hydroxystyrene-CO2 cluster and its hydrate: a para-coumaric acid impostor

We report on the results of high-resolution spec-troscopic studies on the 4-hydroxystyrene−CO2 cluster. We show that these clusters are generated upon heating of para-coumaric acid, the chromophore of the photoactive yellow protein (PYP), as the result of a thermal decarboxylation process. Since the mass of the cluster and the starting material are the same, standard mass-resolved multiphoton ionization spectroscopic methods do not suffice to distinguish these clusters from para-coumaric acid. Instead, more advanced methods that include various UV and IR depletion methods need to be applied. These methods, in combination with quantum chemical calculations, enable us to unravel the structural and spectroscopic properties of 4-hydroxystyrene−CO2 as well as of its hydrate, 4-hydroxystyrene−CO2−H2O.