In this technical article the use of multiple-charge ions in protein and peptide analysis, such as the determination of intact protein molecular mass is discussed.
In order to form a multiple-charge ion, an analyte molecule must have more than one site which can be ionized. In electrospray analyses, these charge locii may be any of the functional groups that we normally associate with protonation (positive charge) or deprotonation (negative charge). Amines are excellent proton acceptors and a very common site for positive charges. Carboxylic acids can donate the acidic proton from the –OH group and form a negative ion. While negative ion formation at acidic sites can occur for proteins, we most typically evaluate protein molecules in electrospray as positive ions.
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