In this work, the valence vertical ionization energies (up to 5) of some important biologically active molecules including 2,4-dinitrophenol, 2,4-dinitroanisole, nicotinic acid, nicotinic acid methyl ester, nicotinamide, N,N-diethylnicotinamide, barbituric acid, uric acid, cytosine, beta-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 intensity of each ionization band was evaluated using the monopole approximation. Comparison of the calculated photoelectron spectrum of each molecule with its corresponding experimental spectra 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 beta-carotene. The reason for this disagreement was theoretically investigated and attributed to the degradation and decomposition of beta-carotene. The calculated first ionization energies of the considered molecules were correlated with their Huckel k-index to obtain Coulomb (a) and resonance (beta) integrals of the Huckel molecular orbital theory for the biomolecules considered in this study. A linear correlation was found between the first ionization energy and the Hiickel k-index.