Studying On The Fragmenttaion Reaction Of Pyridinylamides In Mass Spectrometry

Amide bond is the basic skeleton structure of peptides and proteins but also functional group in many drug molecules.Therefore,a lot of mass spectrometry workers are interested in the gas phase chemistry of amides.In this work,the fragmentation mechanisms of two kinds of pyridinylamides have been investigated by high-resolution electrospray ionization high energy collision dissociation tandem mass spectrometry combined with isotopic labeling experiments and density functional theory(DFT)calculations.The main content includes two parts:1.Study on fragmentation mechanisms of protonated N-pyridinyl benzamides in mass spectrometryThe fragmentation of protonated N-pyridinyl benzamides gave rise to a substituted2-hydroxypyridine cation(ion a),benzoyl cation(ion b)and ion c.Denstity functional theory calculations indicated that pyridine N1 atom was the most favorable site.Under HCD,the amide group of protonated N-pyridinyl benzamides tautomerized to iminol.Then the iminol hydroxyl was transferred to the C2 atom of the pyridine ring through a intramolecular 1,3-OH-shift.Finally,a substituted 2-hydroxypyridine cation was gained by cleavage of C2-N3 bond with the neutral loss of substituted benzonitrile.Electron-withdrawing groups at pyridine ring or electron-donating groups at benzene ring facilitated product ion a.Furthermore,it was found that the fragmentation mechanism induced by amide/iminol tautomerization was also applicable to N-(pyridin-3-yl)benzamides.2.Study on fragmentation mechanisms of protonated benzopyridine amide derivatives in mass spectrometryThe gas phase fragmentation mechanisms of two kinds of benzopyridine amidederivatives were studied,which were 2-([1,1'-biphenyl]-2-yl)-N-quinolin-8-yl)acetamide derivatives and 2-([1,1'-biphenyl]-2-yl)-N-quinolin-8-yl)acetamide derivatives.In the high energy collision dissociation(HCD)spectra of the first kind of compounds,four major fragment ions were observed.Fragment ion a was gained by neutral loss of water of protonated ion.Fragment ion b was gained by cleavage of amide bond.Fragment ion c was gained by neutral loss of CO.Quinolin-8-amine ion was also observed.In the HCD spectra of the second kind of compounds,four major fragment ions were observed.Fragment ion b was gained by cleavage of amide bond.Fragment ion c was gained by neutral loss of water of protonated ion.Density functional theory calculations indicated that pyridine N1 atom was the most favorable site.When the additional proton was transfer to the dissociative protonation sites:N2 atom,the product ion b was formed by the charge induced heterolytic cleavages.Substituent effect suggested that electron-donating groups were in favor of generating ion b,whereas electron-withdrawing groups were in favor of generating ion c.Deuterated experiments further validated the proton migration of these compounds.