Theoretical Study Of Molecular Faces For Amino Acids

The molecular face theory?MFT?can not only show the shape,size and electron density of a molecule,but also indicate the active sites of intermolecular interactions.However,the MFT needs a large number of calculations of complex molecular integrals,which is not suitable for the study of macromolecular system.In this thesis,we developed a method for rapidly predicting the D_pbb and created a semi-empirical method for describing the molecular face of a molecule on the basis of MFT.By the study of 20 amino acids in the state of non-dissociation and zwitterionic ion,it is validated the new method can greatly speed the calculation with a very high accuracy.Specific research contents are as follows:?1?Taking the water molecule as an example,we derive the semi-empirical expression for the rapid prediction of D_pb.As applications,we respectively calculated the D_pbb of the non-dissociation state and zwitterion state of amino acid molecules at the level of MP2/6-311++G?d,p?with the ab initio method and semi-empirical method.The results show that the average relative error of D_pbb between the new method and the ab initio method is only0.236‰.According to that the bigger D_pbb value is,the stronger the chemical bonds is,it is shown that in the amino acid molecules,they always have the first dissociation of the?-COOH followed by the?-NH₂,then the N-H,and the last is the C=O.That is to say,carboxyl groups are more active than amine groups.This is consistent with the results of other scholars.?2?We set up a semi-empirical method to build molecular face rapidly.In order to verify the accuracy of this method,we calculated the volume and surface area,the boundary distance along the bond direction as well as its corresponding electron density of 20 amino acids in the state of non-dissociation and zwitterionic ion,and built their molecular faces with ab initio method and semi-empirical method.We found that the molecular faces of the two methods are very similar,and the relative errors of their volumes and surface areas are about 8.741%and 3.687%respectively.The horizontal distance along the bond direction to the shape boundary and the electron density near the boundary in new method is very close to the ab initio result.It intuitively indicates the active region of the reaction,which laid a foundation for study the properties of amino acids.In this thesis,we described the molecular faces of 20 amino acids in different states and studied their related properties by using the original ab initio calculations and the new method of rapid predicting the D_pbb and semi-empirical molecular face for the first time.It provides a new way to study the activity of biological macromolecular system.