A Study On The Characteristic Ions Of Fragmented Protein By Mass Spectrometry

There are plenty of proteins in the bamboo,but the function of many proteins is not clear.The sequence imformation of proteins and peptides in the bamboo is vital,and it is necessary to identify the sequence of purified proteins and peptides from bamboo by mass spctrometry.The modified peptides are fragmented and produce additional sequence-characteristic ions,which is valuable for the identification of the peptides.In this paper,the peptides were modified by carbamylation(-NH2 group of both the N-termini and lysine were carbamylated)and dual-modification(N-termini was carbamylated and lysine residue was guanidinylated),individually.The b-and y-series ions of the native and modified peptides were studied.In the other hand,an overall understanding of the variation of the abundance of immonium ions and immonium-related ions is beneficial for the identification of unknown peptides.Thus,the abundance of the immonium ions and immonium-related ions was investigated.The details are as followed.(1)The model peptide VQGESNDLK was carbamylated under different p H levels(7,8,9),different temperature levels(70 ?,80 ?,90 ?),and at different reaction time(30,60 min),individually.The result is as followed.The model peptide was modified quickly in a buffer of p H 7 and at a temperature of 90 ?,and the reaction time was 60 min.Thus,the optimal conditions for carbamylation of the peptides were as follows: the temperature was 90 ?,p H was 7,and the reaction time was 60 min.(2)The model peptide VQGESNDLK and BSA tryptic peptides were all carbamylated,and their b-and y-series ions were studied.The result is as followed.The model peptide and most of tryptic peptides produced b1 ion and more b-series ions.But the longer peptide(>10 amino acid residues)produced less y-series ions.Thus,the shorter carbamylated peptides(?10 amino acid residues)produced b1 ion and more b-series ions.The longer carbamylated peptides(>10 amino acid residues)produced additional b1 ion,but less y-series ions(especially in the high-mass region).(3)The model peptide VQGESNDLK and BSA tryptic peptides were dual-modified,and their b-and y-series ions were studied.The result is as followed.The dual-modified model peptide and most of the tryptic peptides produced b1 ion and more y-series ions,and their peptide sequence coverage also increased.Therefore,the shorter dual-modified peptides produced more b-series ions(containing b1 ion)and y-series ions,and their peptide sequence coverage was almost 100%.The longer dual-modified peptides produce b1 ion and more y-series ions,and their peptide sequence coverage was almost above 90%.(4)The immonium ions and immonium-related ions of the model peptide VQGESNDLR and sucrose phosphorylase were studied.And the frequency of the immonium ions and immonium-related ions of 130 tryptic peptides was also investigated.The result is as follows.The immonium ions and immonium-related ions of the model peptide and four typical peptides showed low abundance at 20 e V.At the optimal collision energy,the immonium ions of leucine at m/z 86,isoleucine at m/z 86,glutamine at m/z 101,arginine at m/z 129,tryptophan at m/z 159,proline at m/z 70,valine at m/z 72,glutamic acid at m/z 102,phenylalanine at m/z 120,and tyrosine at m/z 136,as well as the immonium-related ions of methionine at m/z 61,lysine at m/z 84,glutamine at m/z 84,and tyrosine at m/z 91 existed in high abundance.However,the immonium ions of serine at m/z 60 and threonine at m/z 74,asparagine at m/z 87,methionine at m/z 104 showed low abundance,but still kept stable at medium(40-70 e V)or high collision energy(70-90 e V).Thus,those immonium and immonium-related ions above could indicate the presentence of the corresponding amino acid residues well.