First-Principles Modeling of the Electronic and Optical Properties of Biogenic Crystals: The Case of Anhydrous Guanine

Biogenic crystals and, in particular, guanine crystals are crucial for certain organisms in manipulating light. A systematic study of the properties of their different crystalline forms and the effect of the computational method considered is, however, currently lacking. In this study, we present a density functional theory (DFT)-based computational investigation using local basis set formalisms of the key properties of three crystalline forms of anhydrous guanine (AG). Their structural, electronic, vibrational Infrared, Raman, and refractive properties have been investigated. In addition, based on the computed data, we present for the first time a fourth AG form. In most cases, we find the computed data to be in very good to excellent agreement with the available experimental data. In particular, all AG forms are found to be thermodynamically accessible with differences in their computed lattice energies lower than ∼1 kcal/mol. However, the previously proposed orthorhombic γ form is found to systematically correspond to a second-order saddle point on the potential energy surface with all DFT models considered. On the other hand, the proposed γ′ form is a true minimum and displays a characteristic Raman signature already obtained experimentally for the β form and optical properties close to those provided by the other stable polymorphs. This indicates that γ′ might also contribute to AG polymorphism and offer an additional possibility for certain organisms to generate specific optical effects.