Electrospray-ionization protein adducts

A characteristic feature of ESMS is the detection of multiply charged analytes. Macromolecules, such as proteins have multiple sites where protonation or deprotonation (the two most common charge inducing mechanisms in electrospray—other routes to charge induction include, ionization through adduct formation, through gas-phase reactions, and through electrochemical oxidation or reduction) occur. These are desorbed effectively in ESMS and... 

The mechanism of zinc deprivation by 3-nitrosobenzamide was elucidated most recently. When the reconstituted nucleocapsid protein p7 of HIV-1 (15 i-M) was incubated with 3-nitrosobenzamide (300 iM) at pH 7.5, three disulfide bonds per protein molecule were formed while 3-nitrosobenzamide was reduced to the hydroxylamine. Molecular masses of p7 adducts augmented by one or two 3-nitrosobenzamide residues were observed by electrospray ionization MS, consistent with covalent bond formation between cysteine sulfur and the nitroso nitrogen atom127. 

Elaboration of nerve agent toxicokinetics requires sophisticated analytical tools to detect and, if possible, to quantify the free toxicants as well as adducts with proteins and enzymes. Analysis of OP nerve agents has been performed by capillary electrophoresis (CE), biosensors, matrix-assisted laser desorption/ionization (MALDI) MS, desorption electrospray ionization MS (DESI MS), ion mobility time-of-flight MS (IM-TOF MS), nuclear magnetic resonance spectroscopy (NMR), LC-UV, gas chromatography (GC), and many more techniques (Hooijschuur et al, 2002 John et al, 2008). 

Both quantitative approaches have drawbacks. For instance, the current mass spectrometric method does not distinguish between the various isomers of IsoK/LG adducts and thus provides no information on the pathway leading to adducts. Because complete proteolyis is required, it is also not possible to specifically determine the identity of the adducted protein. The method is also very time-consuming and limited to labs with electrospray ionization tandem mass spectrometers and associated expertise. In contrast to mass spectrometry methods, antibody based methods are relatively easy and inexpensive however. 

Owing to its compatibility with solution samples, ESI is preferred over other ionization methods in many MS fields. Applications of metal ion adducts have been reported for ESI [55-57]. For example, ESI can be used to produce alkali-metal adducts of antibiotics that do not form abundant [M+H]+ ions. Informative adducts between alkali-metal ions and peptides have been observed under a variety of conditions of electrospray ionization mass spectrometry (ESI-MS). It should be noted, however, that the presence of salt ion adducts cause the signal suppression and interference with the interpretation of the mass spectra, particularly in analytical MS of proteins and other biological molecules. 

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