Quantum Chemistry Study On ESI-ITMS~n Fragmentation Of Caffeoylquinic Acids

Objective This paper aims to interpretate the fragmentation mechanisms and pathways of seventeen kinds of cinnamic acids?CAs?,quinic acid?QA?and nine caffeoylquinic acids?CQAs?in electrospray ion trap mass spectrometry?ESI-ITMS?in the negative ion scan mode with the aid of quantum chemistry,and summarize the fragmentation patterns as well.The results could provide reference for interpretation of MS fragmentation of this kind of compounds or structural analogues,and also lay a foundation for other soft ionizaiton methods.Methods In ESI-MS negative ion scan mode,the MS and MSn spectrum of seventeen CAs,QA and nine CQAs was obtained by selecting precursor ions both automatically and manually.The generation order of product ions from all the species is needed to know before the affiliation of these product ions could be maked clear.The density functional theory?DFT?method at B3LYP/6-311g?d,p?level was used to optimize the structures of all the compounds,then relevant equilibrium geometries,structural properties and other parameters could be obtained.The computed results were helpful to predict ionization sites,simulate the fragmentation processes in ESI-MS,conclude the characteristics fragmentation pathways and compare different fragmentation among these species.Results The experiments in negative ion mode provided the better sensitivity and selectivity,?1?proton transfer always occurred during fragmentation of hydroxycinnamic acids?HCAs?.The fragmentation pattern was that CO₂ was removed from carboxylic group first after deprotonation,and then CO was removed from benzene to form a five-membered structure,the ring contraction reaction by CO elimination from benzene was in an increasingly difficult order: m-phenolic hydroxyl > p-phenolic hydroxyl > o-phenolic hydroxyl.CO was not only removed from diHCAs,but also CO₂ was removed to form fragment ions.Ortho effect occurred in odiHCAs,i.e.one phenolic hydroxyl group offered H+,which combined with the other one to lose H2 O.?2?When radical anions were generated in methoxycinnamic acids?MCAs?,the single electron on their oxygen atoms readily obtained one hydrogen radical.Except 6-exo-trig cyclization reaction of 2,4-diMCA and 2,5-diMCA,MCAs had the following descending sequence of ionization efficiency in ESI-MS because of different substituent positions: 4-OCH3 > 3-OCH3 > 2-OCH3.The fragmentation pattern was that carboxyl deprotonated to generate [M-H]-,after CO₂ lost,a part of mono-MCAs generated a radical anion by lossing methyl;the other underwent 4-exo-tet cyclization reaction.Methyl radical or methoxyl group could be lost in diMCAs after CO₂ lost from [M-H]-,and corresponding fragments were generated.Except ortho steric interference in 2,3-diMCAs,diMCAs could undergo 6-exo-trig cyclization reaction when 2-OCH3 exists.?3?In nine CQAs,QA part was in a chair conformation.Caffeoyl had strong electron activity because of conjugation,and played an important role in MS fragmentation.Acyl-oxygen bond and alkyl-oxygen bond at both sides of oxygen atom connected to carbonyl CQAs could either broke,and mono-CQAs easily fragmentated in the following descending sequence: 4-CQA > 3CQA > 5-CQA > 1-CQA.An aromatization reaction,which occurred during fragmentation,existed in mono-CQAs by losing H2 O to form double bonds successively on cyclohexane until one conjugated six-membered ring structure was formed.There were similar fragmentation in diCQAs with the same deacylation degree according to the substituent positions of caffeoyl.Conclusions In this study,ESI-MS and quantum chemical method were combined to successfully obtain MSn data and equilibrium geometries of seventeen CAs,QA,nine CQAs and their product ions generated in ESI-MS.Their mass spectral fragmentation pathways in ESI-MS negative ion mode were experimentally and theoretically explained.The research results could provide a theoretical basis for other species.