L‑Serine Anhydrous Crystals: Structural, Electronic, and Optical Properties by First-Principles Calculations, and Optical Absorption Measurement S. N. Costa, † F. A. M. Sales, † V. N. Freire, † F. F. Maia, Jr., ‡ E. W. S. Caetano,* ,§ L. O. Ladeira, ∥ E. L. Albuquerque, ⊥ and U. L. Fulco ⊥ † Departamento de Física, Universidade Federal do Ceara ́ , Centro de Ciê ncias, Caixa Postal 6030, Campus do Pici, 60455-760 Fortaleza-CE, Brazil ‡ Universidade Federal Rural do Semi-A ́ rido, UFERSA, Campus Angicos, 59515-000 Angicos-RN, Brazil § Instituto de Educaç ã o, Ciê ncia e Tecnologia do Ceara ́ , 60040-531 Fortaleza-CE, Brazil ∥ Instituto de Ciê ncias Exatas, Departamento de Física, Universidade Federal de Minas Gerais, Av. Antonio Carlos, 6627, Pampulha, 31340-550 Belo Horizonte-MG, Brazil ⊥ Departamento de Biofísica e Farmacologia, Universidade Federal do Rio Grande do Norte, 59072-970 Natal-RN, Brazil * S Supporting Information ABSTRACT: The X-ray diffraction data of L-serine anhydrous crystals was taken into account to initialize the total energy minimization process of their unit cell through density functional theory (DFT) computations, which were performed within both the local density and generalized gradient approximations with dispersion, LDA, and GGA+D, respec- tively. The calculated lattice parameters are in good agreement with the experimental results for the dispersion corrected generalized gradient approximation functional, with a unit cell volume larger by only about 0.32%; the Mulliken and Hirschfield charges show the zwitterionic state of the L-serine molecules in the DFT converged crystals. The electronic (band structure, density of states) and optical absorption properties were calculated to explain the light absorption of the L-serine anhydrous crystalline powder we have measured at room temperature. The optical absorption related to transitions between the top of the valence band and the bottom of the conduction band involves O-2p valence states and H-1s conduction states. The LDA (4.74 eV) and GGA+D (4.75 eV) estimated energy gaps are about 1 eV below the estimated value from optical absorption measurements (5.90 eV). Small values were obtained for the electron effective masses, which are almost isotropic, whereas large anisotropic values were found for hole effective masses, suggesting that the L- serine anhydrous crystal behaves like an n-type wide gap semiconductor. Different dielectric function profiles obtained for some of the most important symmetry directions also demonstrate the optical anisotropy of L-serine anhydrous crystals. 1. INTRODUCTION Serine (Ser, S, pK a = 2.21 for the α-carboxylic acid group, 9.15 for the α-ammonium ion, isoelectric point pI = 5.68, MW = 87.08), chemical formula C 3 H 7 NO 3 , is one of the 20 natural amino acids (a nonessential one since can be synthesized from metabolites, including glycine) whose polymerization gives rise to the proteins, biochemical compounds that rule the life- related biological functions. Among the 10 amino acids that can be formed in Miller’s atmospheric discharge experiments, it is ranked the sixth in order of decreasing abundance in prebiotic contexts, as predicted by thermodynamic arguments, which was likely reflected in the composition of the first proteins at the time the genetic code originated. 1 In respect to its abiotic synthesis, serine is found in meteorites, spark discharge experiments, and cyanide polymerization experiments. 2 It has two enantiomeric modifications: L-serine and D-serine. After pioneer works on the low-energy conformers of un- ionized serine, 3,4 Gronert and O’Hair 5 have spanned 51 serine conformers in vacuum at the highest theoretical ab initio level (up to MP2/6-31+G*) of that moment. The conformational equilibria of neutral serine was studied by Lambie et al. 6 through experimental matrix-isolation Fourier transform infra- red spectroscopy in combination with density functional theory (DFT) calculations. Besides, the multiple serine conformations of serine in the gas phase has been demonstrated with microwave Fourier transform spectroscopy and pulsed super- sonic expansion and laser ablation (LA-MB-FTMW). 7 On the other hand, Upadhyay et al. 8 performed an ab initio and density functional study of the L- and D-forms of serine in the gas phase Received: January 19, 2013 Revised: May 15, 2013 Published: June 7, 2013 Article pubs.acs.org/crystal © 2013 American Chemical Society 2793 dx.doi.org/10.1021/cg400111w | Cryst. Growth Des. 2013, 13, 2793−2802