Study On The Mechanism Of Molecular Ion Fragmentation Of Protonated Aspartic Acid And Glutamic Acid

In 2013,the experiment ionized aspartic acid and glutamic acid by the APCI technique,followed by a further investigation of the resulting deprotonated aspartic acid and glutamic acid in a mass spectrometer.They found deprotonated aspartic acid have three molecular ion products,i.e.[Asp-H-CO2]-(m/z 88),[Asp-H-NH3]-(m/z 115)and[Asp-H-H2O]-(m/z 114),respectively.Deprotonated glutamic acid have two molecular ion products,[Glu-H-CO2]-(m/z 102)and[Glu-H-H2O]-(m/z 128),respectively.Another interesting aspect should be given special attention;i.e.,the yield of[Asp-H-NH3]-is higher than that of[Asp-H-CO2]-when the voltage is less than 120 V.However,when the cone voltage is larger than 120 V,the yield of the latter becomes higher than the former.It seems that the kinetic processes and product distribution are strongly temperature-dependent when they arise from the voltage change.For glutamic acid,Regardless of how the voltage changes,[GIu-H-H2O]-yield is always higher than[GIU-H-H2O]-.In the CID process,the formation and distribution of different fragments are strongly related to the rearrangement and isomerization reactions of parent molecular ions and intermediates,imply that there will be different fragments of the channel and May get different products.So deprotonated aspartic acid and glutamic acid dissociation get structure of the products,dissociation paths and whether the temperature has a significant effect on the dominant pathway has become a fundamental scientific problems.In this thesis,the fragmentation processes of deprotonated aspartic acid and deprotonated glutamic acid to eliminate CO2,NH3,and H2O were investigated by a quantum-mechanism computation at the B3LYP/6-311++G(2df,2pd)and MP2/6-31+G(d,p)levels of theory.By constructing the fragmentation reaction potential energy profile using the Gibbs free energies of the located stationary poiints,we explored the preferred dissociation pathways,product distribution,and kinetic and thermodynamic processes.Furthermore,the thermal energy correction to key stationary points was computed to evaluate the temperature dependences of the dominant reaction channels and product distribution and to further compare our results with the available experiment.Additionally,the similarities and differences between the present theoretical investigation and available experiment were discussed in detail.