Folic acid decomposes when exposed to UV light so understanding the effect of light on this biomolecule is very important. In this study, sixty conformers of folic acid were investigated by DFT calculations. Six most stable conformers were selected for study of their electronic structure and photoelectron spectra. The photoelectron spectra were simulated by two high level ab initio computational methods. Two methods used to obtain valence ionization energies were general-R symmetry adapted cluster-configuration interaction (general-R-SAC-CI) method and outer-valence Green's function (OVGF). The SAC-CI computations were performed with two different basis sets: D95 (df,pd) and 6-311G(d,p). Because of the size of the folic acid molecule, OVGF calculations were only performed with 6-311G(d,p) basis set. The calculated photoelectron spectrum of folic acid was obtained by superposition of weighted photoelectron spectra of individual conformers. Natural Bonding Orbitals (NBO) calculations were performed for the assignment of ionization bands of each conformer. It is shown that ionization occurs mostly from the orbitals localized on p-aminobenzoyl moiety in folic acid. Koopmans’ approximation is only applicable for describing the five lowest energy orbital ionizations (photoelectron bands). The in-situ molecular structures of two bio-active conformers were extracted from X-ray diffraction data obtained for folic acid bound to folate receptor protein. The unmodified molecular structures were then used for calculating ionization energies. We discuss the consequences for ligand-receptor binding of switching from the most stable conformation to bioactive conformation in FA.