Interaction Of Metal Cations With Nucleosides: Mass Spectrometry And Quantum Chemical Calculation Study

It’s well known that metal ions play an important role in vivo, for example,inDNA, the metal ionscan affect the stability of the DNA double helix structure; Asthe basic structural unit of DNA and RNA, the nucleoside compounds are involved inalmost all of the biochemical reaction process in vivo, and the reactions often dependson the metal ions, whcihplay an important role in biological synthesis, conformationmaintain nucleoside, function and regulation. Thus, the studies of the interactionsbetween metal cations and nucleosides have attained widespread attention, but fewwere studied in the gas phase.Electrospray Ionization mass spectrometry （ESI-MS） has become the importantmeans to anlyze the small biological molecules such as amino acids, and evencomplex biological molecules such as peptides and proteins; and the quantumchemistry calculationcan be used to study the chemical related issues which couldexplain the experimental phenomena, forecast the reaction process and result, andgive some reaction intermediates or reaction transition state structures which was veryhard to be obseved in experiment.In this disseration, ESI-MS and density functional methods （DFT） was used toinvestigate ithe interactions between environmental heavy metal copper ions and fivekinds of nucleoside molecules in the gas phase. The studies include the interactionproduct configurations, the interaction mechanism, and the products of collisioninduced dissociation （CID） which would provides some scientific guidance for therelevant life phenomena.The main research contents and conclusions as follows:1. Five Nucleosides and Cu（NO₃）₂water/methanol （30/70） solutions were seperatelyanalyzed by ESI-MS. The meatal cations of [CuLn]²⁺,[Cu（L-H）Ln]⁺,[LH]⁺,[L2H]⁺,[BH]⁺,[CuL（MeOH）_n]²⁺ and [CuL（NO₃）_n]⁺could be clearly showed in the massspectra; the covalent coordination bond had showed evidently effect on theinteraction of the copper ions with cytidine and guanosine; and the spectra strengthof the complex of copper ions with cytidine was high.2. Further analysis for the product of the divalent copper ions and cytidine of itscomplex structure and characteristics was described on a lower collision energy: NH₃, H₂O and CONH stripped off could be observed in the [L+H]⁺CID progress. agroup of small molecules broken off after the sugar ring off could be observed inthe [Cu（L-H）Ln]⁺（n=0,1） CID progress. Ligand proton transfer could be occured inall the[CuLn]²⁺（n=2-6） CID process, and other channels including the charge toreduce cracking and neutral molecules group off. Neutral methanol moleculesstripped off and three charge reduced processes could be observed in the[CuL（MeOH）]²⁺ CID progress. Proton cytidine dimmers [L2H]⁺could be observedin the [CuLn]²⁺（n=4-6） CID process, which might be bonded by the hydrogenbonding interactions of two cytidine molecules.3. DFT study for complex configuration of [CuLn]²⁺ and [LnH]⁺was described. It wasfound that the Cu²⁺ was easily liganded with cytidine molecules by covalentcoordination bond to form stable complexes on chelation sites N（3）and O（7） ofcytosine at the same time on a plane. And the proton H+was easily bonded cytidinemolecular by hydrogen bonds to form a protonated poly [LnH]⁺（n=1-4）. Thecalculated stability orders are [CuL2]²⁺>[CuL]²⁺>[CuL4]²⁺>[CuL3]²⁺ and[L2H]⁺>[LH]⁺>[L3H]⁺>[L4H]⁺which are consistent quite well with the peakintensity distribution in the mass spectrum. The [CuL]²⁺ was not observed mightbecause that the high strength of [CuL（MeOH）1-2]²⁺ was easily observed as [CuL]²⁺ liganded with methanol molecules; While high strength of [LH]⁺might be thebivalent Cu²⁺-cytidine complexes CID products.4. Combining with the stability analysis of the spatial structure of the isomers[CuLn]²⁺（n=2-4） of cytidine and guanosine complexes, CID fragmentationpathway analysis and calculation results showed that the guanosine ligand site wasN（7） with copper ions which preferred Cu-N ligand bond, and then the guanosinecomplex molecules would ligand the third or more guanosine by hydrogen bonds toform a more stable copper-guanosine complexes; while cytidine ligand complexespreferred ligand number4or6at the ligand site both of the N（3） and carbonyl O（7）,which showed a higher peak intensity of the ESI-MS. Protonated cytidine andradical guanosine cationic products were respectively removed on the Low energyCID process. CID of Cytidine ligand dissociation channel was mainly showedProton Transfer process, while the guanosine ligand dissociation was the electrontransfer process after the electronic rearrangement for [CuGn-1]⁺.。5. Combined with the stability analysis of the spatial structure of the isomeric by DFT method, the protonation of cytosine isomers of cation configurations was analyzed.the overall structure of the protonation cytosine cations and the neutral cytosinemolecule were very close, but the hydrogen proton changed and the cytosine in thecationic charge redistributed. bond length of the two C-N connected with N（3）would be increased slightly as hydrogenation happened on the protonation of thepyrimidine ring, while the pyrimidine ring on other bond length was tightened. Inthe gas phase, sigma transformation process of the O-H bond,[1,3]-proton transferand [1,4]-proton transfer of the pyrimidine ring would observed on differentprotonation of cytosine cationic configurational. CID fragmentation pathwayanalysis of [LH]⁺and [BH]⁺and calculation results showed that the N-glycosidicbond broken in LH, the sugar ring was also broken off in CID dissociate reactionï¼›After dissociation, some little molecules of HNCO、H₂O、CN2H₂、NH₃and COwould be stripped off on the following channels of the CID process of nucleobase[BH]⁺; In addition, CO stripped process would happened on the multi-stage CIDchannels of the nulceoside base（cytosine, guanine, thymine and uracil）, and theHCN stripped process would happened on the multi-stage CID channels of adenine.The innovation points of this paper:1．ESI-MS was used for the analysis of the products of different nucleotidemolecules with copper ions in the gas phase, and also combined with highprecision accurate DFT calculation, the product configurations were sucessfulprovided.2．For the first time, the statbility of the complexes of copper ion with the nucleotidemolecules were investigated in the gas phase, and the characteristics of theinteractions were confirmed.3．The multiple mass spectrometry method and DFT calculation was used to analyzethe CID of the protonated cytosine, adenine, etc., and the their dissociationmechanism was investigated.