The Use of Gaussian to Explain Lack of Fluorescence and Calculate UV Spectra of 2- and 4-thiouracil

Undergraduate #143
Discipline: Chemistry and Chemical Sciences
Subcategory: Biochemistry (not Cell and Molecular Biology and Genetics)

Ian York - North Carolina Agricultural and Technical State University
Co-Author(s): George Agbeworvi, Mufeed M. Basti, and Kenneth M. Flurchick, North Carolina A&T University, Greensboro, NC

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A previous study has shown that 2- and 4-thiouracil (2TU and 4TU) molecules exhibit high absorbance in the ultraviolet (UV) range. However, fluorescence spectra were not observed in the solid phase, in acetonitrile, and in water, which rules out the quenching by solvent. The goal of this study is to use the computational chemistry program Gaussian to explain the lack of fluorescence and to calculate the UV spectra of these molecules in the three media. Explaining the lack of fluorescence can be achieved by calculating the contribution of the different atomic orbitals to the energy of the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of the molecules. The population difference between the HOMO and the LUMO determines the intensity of the fluorescence peaks. The difference in population is determined by the energy difference between the HOMO and LUMO. The energy difference, in turn, is determined by the identity of the atomic orbitals that contribute to the HOMO and the LUMO. The calculation results suggest that Pz atomic orbitals are the main contributors to the HOMO and LUMO orbitals which means that sigma (Ϭ) bonding is significantly more prevalent than pi (π) bonding. A Ϭ > Ϭ* transition requires more energy than a π > π* transition. Thus a higher sigma bonding contribution means that the HOMO > LUMO transition in 2TU and 4TU is not facile. The calculations were performed in vacuum, acetonitrile and water. In water, the molecules exist in monomeric and hydrogen-bonding initiated dimers. Thus the calculations were carried out on all monomeric and dimeric forms that exist in water. The calculated UV spectra were compared with the experimental data in the three corresponding media to validate the calculations. The calculated spectra of the monomeric forms of the molecules fit with the experimental ones. Future work will include the energy and UV spectra calculations of Hg+2 and Ag+1 complexes of 2TU and 4TU.

Funder Acknowledgement(s): This research was conducted as a part of the NC A&T Summer Biomathematics Institute sponsored by the Talent-21/HBCU-UP program and funded by the National Science Foundation HRD#1036299.

Faculty Advisor: Mufeed M. Basti,