One of the most important physico-chemical properties of biomolecules is their ionization potentials. For example, when a photoionization or photoabsorption process occurs in a biological system such as living cells, it may cause certain photobiological effects on the system such as damage, cell death or mutation. Therefore, knowing the values of ionization energies of biomolecules is important for unraveling the mechanism of protein, DNA and cell damage due to photoionization. Photoelectron spectroscopy is a powerful technique that is mostly used for determining ionization energies of atoms and molecules. Nevertheless, There are two limitations for directly extracting ionization energies from the observed features in the recorded photoelectron spectra, especially for biological molecules. The first limitation is related to the overlap of the ionization bands so that the spectrum exhibits broad overlapping bands with little evidence of vibrational structure. Therefore, the assignment of spectrum only based upon the ionization energies obtained for the observed features is complicated and ambiguous. The second limitation is related to the molecules with more than one stable conformers and tautomers. In this case, the recorded photoelectron spectrum is related to more than one conformer and tautomer and it is a weighted sum of the photoelectron spectra of the contributing tautomers and conformers. Considering these two limitations, it is difficult to obtain information from photoelectron spectrum without any theoretical calculations. These two limitations are more evident in the assignment and interpretation of the photoelectron spectra of biomolecules. To remove these limitations, high level ab initio calculations are necessary for describing and assigning the photoelectron spectra and revealing subtle differences caused by various chemical environments, conformations and tautomers. In this work, the valence vertical ionization energies (up to 5) of some important biological active molecules including 2,4-dinitrophenol, 2,4-dinitroanisole, nicotinic acid, nicotinic acid methyl ester, nicotinamide, N,N-diethylnicotinamide, barbituric acid, uric acid, cytosine, b -carotene and menadione were calculated in the gas phase and compared with the experimental data reported in the literature. The symmetry-adapted-cluster configuration interaction (SAC-CI) general-R method was used to calculate the ionization energies. The calculated energy positions and intensities of the ionization bands of each molecule were used to calculate its photoelectron spectrum. The Boltzmann population ratios of tautomers and rotational conformers of each molecule, obtained from the thermochemistry calculations, were considered in the calculation of its photoelectron spectrum. The calculated photoelectron spectrum of each molecule was compared with its corresponding experimental spectra reported in the literature. This allowed for assigning the photoelectron bands by natural bonding orbital (NBO) calculations even though some of the associated bands were significantly overlapped for some molecules. Among the considered molecules, there was no agreement between the experimental and calculated photoelectron spectrum of b -carotene. The reason for this disagreement was theoretically investigated, and attributed to the degradation and decomposition of b -carotene. The calculated first ionization energies of the considered molecules were correlated with the Hückel k -index to obtain coulomb ( a ) and resonance ( b ) integrals of the Hückel molecular orbital theory for the biomolecules considered in this study. A linear correlation was also found between the first ionization energy and Hückel k -index