Molecular structure and spectral properties of L-theanine under external electric field

L-theanine (N-ethyl-L-glutamine) is a class of non-protein amino acid with hypotension and antioxidation. It is helpful to understand the properties of L-theanine molecules by studying its structure and spectral characteristics under external electric field. The B3LYP method is used to optimize the geometric configuration of L-theanine molecules at the base ground level of 6-311g (d, p) based on the density functional theory in Gaussian 09W-D01 software. On this basis, the bond lengths and vibrational frequencies of L-theanine molecule are calculated under different external electric fields (0/0.0125 a.u.). The corresponding infrared spectra are obtained from above calculation. At the same time, the time-dependent density functional method (TD-DFT) is used to calculate the influence of external electric field on the excited states, frontier orbital and energy gaps of molecules under the same basis set. The results show that the molecular structure and its ultraviolet and infrared spectra change with the external electric field intensity. With the increase of external electric field, the frequency of stretching vibration is red-shift, and the frequency of bending vibration is blue-shift. The energy of the highest occupied molecular orbital (E HOMO) and the energy of lowest unoccupied molecular orbital (E LUMO) changes under external electric field. The energy gap of frontier orbital increases slightly at first and then decreases with the increase of electric field, and the molecular activity decreases first and then increases. The UV absorption spectrum shifts to blue and then to red. This study can provide a theoretical basis for the detection, synthesis and biological activity of L-theanine. (authors)