Origin of Supercharging in Electrospray Ionization of Noncovalent Complexes from Aqueous Solution

The use of m-nitrobenzyl alcohol (m-NBA) to enhance charging of noncovalent complexes formed by electrospray ionization from aqueous solutions was investigated. Addition of up to 1% m-NBA can result in a significant increase in the average charging of complexes, ranging from similar to 13% for the homo-heptamer of NtrC4-RC (317 kDa; maximum charge state increases from 42+ to 44+) to similar to 49% for myoglobin (17.6 kDa; maximum charge state increases from 9+ to 16+). Charge state distributions of larger complexes obtained from heated solutions to which no m-NBA was added are remarkably similar to those containing small amounts of ni-NBA. Dissociation of the complexes through identical channels both upon addition of higher concentrations of ni-NBA and heating is observed. These results indicate that the enhanced charging upon addition of ni-NBA to aqueous electrospray solutions is a result of droplet heating owing to the high boiling point of m-NBA, which results in a change in the higher-order structure and/or dissociation of the complexes. For monomeric proteins and small complexes, the enhancement of charging is lower for heated aqueous solutions than from solutions with m-NBA because rapid folding of proteins from heated solutions that do not contain m-NBA can occur after the electrospray droplet is formed and is evaporatively cooled. (J Am Soc Mass Spectrom 2009, 20, 1933-1943) (C) 2009 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry