Electrospray ionization mass spectrometry and its application in protein chemistry

Electrospray ionization mass spectrometry (ESI MS) and its application in protein chemistry are reviewed in this paper Because of the multiple charge peaks produced by electrospray ionization mass spectrometry (ESI), the molecular mass range of detection is greatly expanded and the sensitivity is high In addition, it can be used in conjunction with HPLC and capillary electrophoresis to expand the application of mass spectrometry in protein chemistry Keywords electrospray ionization mass spectrometry; protein chemistry in organic compound structure identification, mass spectrometry, nuclear magnetic resonance, infrared and ultraviolet analysis methods provide structural information of compounds from different sides Mass spectrometry is based on mass analysis and has high sensitivity It can provide molecular weight, molecular formula (high resolution mass spectrometry) and some related structural information The classical organic mass spectrometry requires that the substance to be measured can be gasified, with certain purity and good thermal stability Therefore, the compounds with high polarity, difficult to gasify, thermal instability and impure are difficult to be determined by classical mass spectrometry In recent years, with the development of organic mass spectrometry in hardware, software and ionization technology, as well as the continuous improvement of the interface with various separation methods (such as chromatography-mass spectrometry), the detection range of compounds has been expanded In the aspect of molecular weight determination, it has expanded from small chemical molecules to large biological molecules, and the molecular weight can be measured up to several hundred thousand daltons There are many ionization methods for mass spectrometry, including field desorption, plasma desorption, laser desorption, rapid particle bombardment, thermo spray ionization and atmospheric ionization Each ionization method has a certain molecular weight detection range It is generally considered that the molecular weight of thermal spray is about Zui 8ku, and the fast atom bombardment is 25ku However, with the increase of molecular weight, the sensitivity of all analytical methods decreased Electrospray ionization mass spectrometry (ESI-MS) can produce multiple charge peaks Compared with the traditional mass spectrometry, it expands the detection range of molecular mass and improves the sensitivity A M/Z limited to a certain range of quadrupole mass spectrometry can be used to analyze the protein [1] with molecular weight exceeding 200ku In addition, ESI-MS method can produce a series of multi charge peaks, which can get accurate molecular weight It can also be connected with HPLC and CE separation methods, expanding the application of mass spectrometry in biological field Electrospray phenomena can be traced back to two centuries ago, but the innovative research on electrospray as an ionization method started at about 30 before Dole The aim of their research is to produce gaseous large ions by electrospray In 1984, Yamashita combined the technology of atmospheric pressure electrospray ionization with quadrupole mass spectrometry In the same year, Alexandror combined it with magnetic mass spectrometry In 1988, Fenn research group reported that 45 proteins with a positive charge molecular weight of 40 Ku were obtained by ESI-MS, and then ESI-MS entered a new development stage in the field of biomacromolecule research So far, the method has been able to analyze proteins with a mass range of about 200 Ku 1 electrospray ionization process As shown in the figure, a high voltage is applied to the capillary orifice, acting on the solution which enters the ionization chamber through the spray head, and then the voltage of 3~6kV is added to the capillary and the relative electrode The voltage causes the charge enhancement on the surface of the droplet at the end of the capillary The high voltage leads to the split of the liquid surface and the formation of the multi charge droplets, and the positive or negative electricity carried by the electrode relative to the capillary tube Charge to produce a positively or negatively charged droplet For electrospray as a whole, the formation of charged droplets is the * step of the whole electrospray process, and the next ionization is the key to electrospray analysis However, the mechanism of the formation of molecular ions by charged droplets is not clear Iribane and Thomson proposed the hypothesis of auxiliary ion evaporation In this model, the ions on the surface of the droplet are due to the evaporation of the solvent of the charged droplet in the air When the surface of the droplet reaches the critical point, the ions directly enter the gas phase from the liquid phase to complete the ionization process However, rollgen et al Put forward different hypothesis mechanisms He thought that when the droplet evaporated at atmospheric pressure, with the evaporation of solvent, the surface charge density of the droplet increased as the droplet diameter became smaller, and the surface charge of the droplet reached the Raleigh limit Limit), the droplet further fission, once again reaching the Rayleigh limit, once again "explosion", such a cycle, when the solvent is completely evaporated from the droplet, molecular ions are formed Abbas and Latham's experiments confirm the hypothesis that molecular ions are formed from droplets The formation of macromolecular gas-phase ions can be explained by the theory of charged residue model The theory holds that the formation of macromolecular gas-phase ions is based on solvent evaporation, and the formation of gas-phase ions containing only one molecular ion is caused by Coulomb burst and the mutual repulsion of smaller droplets The energy required for this process is very low and will not lead to molecular cracking However, some effective fragment ions can be obtained by setting the collision induced dissociation (CID) voltage in the ion transport region of the ion source, but this is only effective for some unstable structures, and there is a certain difference between the CID mass spectrum of ESI-MS and the EI and Fab mass spectra, which may be due to the fact that the cracking environment of the former is more complex than that of EI and Fab sources There are some restrictions on the acquisition of complex structure With the development of modern technology, electrospray ionization and tandem mass spectrometry (MS-MS) can provide many structural information for compounds 2 the detection of molecular weight of electrospray ionization mass spectrometry is because the molecular state of ESI-MS is not due to cracking (unless induced by collision induced dissociation into the vacuum system at high energy), so it is easier to determine the molecular weight of biological macromolecules ESI-MS can obtain a cluster of multi charge mass spectrum peaks, and its molecular weight can be determined from the following assumptions: (1) the difference between two adjacent peaks is one charge; (2) the charge is formed by molecular ion protonation Any two peaks can be used to measure molecular weight effectively Equations (1) and (2) describe the relationship between molecular mass MR and polycharged ions (P1, P2) and their respective charges (Z1, Z2) P1z1 = Mr + maz1 = Mr + 1.0079z1 (1) p2z2 = Mr + 1.0079z2 (2) when P2 > P1, the (1) and (2) equations are solved, and Z1 = J (p2-1.0079) / (p2-p1) (3) the accurate molecular mass can be obtained by Z1 The mass spectra of a cluster of multi charge peaks can then be transformed into mass spectra of [M + H] + or [M + H] -.