Research On Terahertz Theoretical Spectra Of Biomolecule

Terahertz(THz)waves usually refer to the electromagnetic waves with frequencies between 0.1 and 10 THz between microwave and infrared radiations.The substance contains rich physical and chemical information in the terahertz band,and among the applications of terahertz spectroscopy,the detection of biomolecules by means of THz spectroscopy is a very valuable basic research,because it not only enhances the understanding of interaction mechanism between terahertz waves and biomolecules,but also lays an important foundation for its application and development in safety detection,biomedicine,quantitative and qualitative analysis of substances and other fields.In recent years,most researches have focused on the experimental measurement,few studies have conducted in-depth simulations.However,through the theoretical simulation,we can not only understand the structural of the material in more detail,but also deeply analyze the vibration source of the characteristic peaks in the terahertz ba nd.Thus,this paper focused on terahertz theoretical spectra of amino acid and vitamin samples in combination with terahertz spectroscopy and quantum mechanics theory.The solid-state density functional theory(DFT)based on the periodic boundary condition was explored to accurately predict the amino acid and vitamin terahertz spectra.The main research contents and results of this paper include:(1)Firstly,L-alanine with relatively simple structure was used as the research object,and a periodic crystal model which is more suitable for simulating the THz spectrum of solid amino acid was constructed.The importance of dispersion correction and full structure optimization for the simulation of THz spectra of hydrogen-bonded solid state amino acid systems i s pointed out.Research shows that the model proposed in this paper has a higher accuracy than previous reports.(2)The advantages and disadvantages of Generalized-Gradient Approximation(GGA)functional and hybrid density functional(HF)B3LYP in the prediction of solid-state amino acid terahertz spectra were compared,and the reliability of the calculation method was evaluated from various aspects such as bond length,absorption peak position,and calculation time.In general,utilizing B3 LYP can improve the reproduction of both molecular structures and simulated spectra.However,it also greatly increases the cost of the calculations.It is evident by the results presented in this study that the combination of PBE-D functional with 6-311G(d,p)basis set is accurate enough to reproduce the experimental THz spectra of solid-state amino acids.(3)The improved model was applied to the simulation of vitamin THz spectrum,and the terahertz spectra of ascorbic acid and pyridoxine were obtained using PBE and B3 LYP density functionals,respectively.The results showed that for the vitamin samples,the PBE functional can be used to obtain theoretical spectra very close to the experimental results,however,the calculation results of B3 LYP are not satisfactory.By comparing the ground state structure s after optimization,it is found that the excessively changed lattice of B3 LYP after dispersion correction is an important cause of unsatisfactory results.This provides important reference and guidance for the future research of other THz theoretical spectra.