A comprehensive spectroscopic study of urocanic acid: OVGF and EOM-CCSD approaches

The interest in the photobiology of the trans isomer of urocanic acid (UCA) is due to the important role of this
molecule as a major absorber of UV light in the human skin. So, urocanic acid can act as a natural sunscreen. UV
light induces photoisomerization of UCA into the cis- form. Herein, theoretical study of the electronic structure of
various tautomeric forms of cis and trans isomers of urocanic acid is reported together with modeling of the
photoelectron spectra of both isomers. The most stable conformers were used to calculate vertical valence
ionization energies. Two high-level methods were used for calculating ionization energies and photoelectron
spectra: Outer Valence Green’s Function (OVGF) and equation-of-motion couple cluster method (EOM-CCSD).
Total electron energies were calculated at M06-2X/6-311++G(d,p) level. The orbital character associated with
individual bands in the calculated photoelectron spectra was established using NBO method. This character was
then used to assign calculated photoelectron spectra. The theoretical photoelectron spectra of cis and trans
isomers were obtained and we found that orbitals localized on the C––
C and C––
O moieties are most responsible
for the lowest energy ionizations. The ionization energies obtained assuming Koopmans’ approximation were
also predicted for all tautomers and compared with OVGF and EOM-CCSD results. Also, we computed molecular
electrostatic potential (MEP) surfaces for urocanic acid tautomers. The molecular structures of two bio-active
conformers were based on X-ray diffraction data obtained for UCA bound to a receptor protein. These molecular
structures were subsequently used for calculating corresponding ionization energies. Ligand-receptor
interaction in the active site and HOMO orbitals for these two bioactive forms are also given.