Biomolecules electrospray ionization

Advances in mass spectrometry have made it a tool of exceptional power for analysis of large biomolecules. Electrospray ionization, MALDI, and other soft ionization techniques for nonvolatile compounds and macromolecules make possible analyses of proteins, nucleic acids, and other biologically relevant compounds with molecular weights up to and in excess of 100,000 daltons. Electrospray ionization with quadrupole mass analysis is now routine for biomolecule analysis as is analysis using MALDI-TOF instruments. Extremely high resolution can be achieved using Fourier transform-ion cyclotron resonance (FT ICR, or FTMS). We shall discuss ESI and MALDI applications of mass spectrometry to protein sequencing and analysis in Sections 24.5E, 24.13B, and 24.14. 

Matrix-assisted laser desorption mass spectrometry (MALDI-MS) is, after electrospray ionization (ESI), the second most commonly used method for ionization of biomolecules in mass spectrometry. Samples are mixed with a UV-absorbing matrix substance and are air-dried on a metal target. Ionization and desorption of intact molecular ions are performed using a UV laser pulse. 

Fenn J.B., Mann M., Meng C.K., Wong S.F., and Whitehouse C.M. (1989), Electrospray ionization for mass spectrometry of large biomolecules, Science 246, 64-71. 

Numerous workers have demonstrated the applicability of electrospray ionization mass spectrometry (ESI/MS) for the detection and analysis of biomolecules with highly electronegative groups (reviewed by Wood et al., 2003, and for neutral steroids by Higashi and Shimada, 2004). The sensitivity of detection of neurosteroids can also be enhanced by derivatization when they are analyzed by nano-electrospray/mass spectrometry procedures. Neurosteroid sulfates can be easily prepared in a single-step reaction in pyridine with the N,N-dimethylformamide complex of sulfur trioxide (Chatman et al., 1999). Another elegant... 

Choudhary, G., Horvath, C., and Banks, J. E (1998). Capillary electrochromatography of biomolecules with online electrospray ionization and time-of-flight mass spectrometry.. Chromatogr. A 828, 469—480. 

Hutton, T. and Major, H.J., Characterizing biomolecules by electrospray ionization-mass spectrometry coupled to liquid chromatography and capillary electrophoresis, Biochem. Soc. Trans., 23, 924, 1995. 

Two recently developed mass spectrometric techniques have had a major impact on the analysis of large biomolecules matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and electrospray ionization mass spectrometry (ESI-MS). MALDI-MS was first introduced by Karas and Hillenkamp66 and Tanaka et al.61 in 1988 and has experienced an exponential development. It has become a widespread soft ionization technique for bioorganic samples, especially large biomolecules. Fenn and co-workers68 first published the successful soft ionization technique for... 

Electrospray ion source electrospray ioniz., Coulomb repulsion M"+ atmospheric pressure ToF-MS, FTICR-MS tandem MS biomolecules... 

ToF mass spectrometers as dynamic instruments gained popularity with the introduction of matrix assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) as effective pulsed ion sources for the soft ionization of large biomolecules (up to 10s dalton) due to their high ion transmission.38 ToF mass spectrometers, quadrupole analyzers and/or magnetic sector fields can be combined in tandem mass spectrometers (MS/MS) for the analysis of organic compounds. 

Mass spectrometry has been revolutionalized by the advent of electrospray ionization, but, before we concentrate on this relatively recent addition to the array of ionization methods, we will first discuss two other techniques which are routinely used for the ionization of biomolecules matrix assisted laser desorption ionization (MALDI) and fast atom bombardment (FAB). These techniques share common features in that ... 

Electrospray ionization (ESI) was first employed more than 20 years ago, but it is fairly recently that it became a routine technique for the soft ionization of a wide range of polar analytes, including biomolecules. For this technique, the analyte is usually dissolved in a mixture of an organic solvent (most commonly acetonitrile or methanol) and water with a pH modifier [e.g. formic (methanoic) or acetic (ethanoic) acid for positive ion mode]. The presence of the pH modifier ensures that ionization takes place in the solution state. This is the only common case where ionization occurs before ion vaporization the exact mechanism of the vaporization (Figure 5.6) is still not clearly understood in ESI. 

Reversed-phase HPLC is widely utilized to generate a peptide map from digested protein, and the MS online method provides rapid identification of the molecular mass of peptides. The HPLC-MS-FAB online system is a sensitive and precise method for low-MW peptides (<3000 Da) even picomol quantities can be detected. However, as the MW of the analytes increases, the ionization of peptides becomes more difficult and decreases the sensibility of the FAB-MS (112). Electrospray ionization (ESI-MS) was found to be an efficient method for the determination of molecular masses up to 200,000 Da of labile biomolecules, with a precision of better than 0.1%. Molecular weights of peptide standards and an extensive hydrolysate of whey protein were determined by the HPLC-MS-FAB online system and supported by MALDI-TOF (112). Furthermore, HPLC-MS-FAB results were compared with those of Fast Performance Liquid Chro-motography (FPLC) analysis. Mass spectrometry coupled with multidimensional automated chromatography for peptide mapping has also been developed (9f,l 12a). 

Electrospray ionization and matrix-assisted desorption ionization were both introduced around the same time, in the late 1980s. In fact matrix-assisted laser desorption ionization (MALDI) was first mentioned in the literature in 1987 (Karas et al., 1987). In the years prior to that, there were limited reports of the application of laser desorption MS. Early developments in MALDI focused primarily on macromolecules, particularly peptides and proteins. Historically, MALDI ion sources have predominantly been coupled to time-of-flight (TOF) instruments. TOF requires precise timed ionization events, and since ions are generated in MALDI by a pulsed desorption, this combination is complementary. Mass spectra generated by MALDI can be relatively simple, containing predominantly singly charged ions. The importance of both ESI and MALDI are well proven in the analysis of biomolecules, and both techniques were awarded the Nobel Prize for chemistry in 2002 (Chapter 1). 

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