Electrospray mass spectrometric study of anionic complexes of enkephalins with Cu(II): regioselective distinction of Leu/Ile at the C‐terminus induced by metal reduction

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Abstract
The yield of metallation of methionine‐enkephalin and leucine‐enkephalin isomers by copper(II) chloride was investigated by electrospray ionization ion trap mass spectrometry (ESI‐ITMS) in negative ionization mode. Binary ([(M3H)+CuII]−) and ternary ([(M2H)+CuIICl]−) complexes were observed. Soft and hard desolvation conditions (by changing the declustering voltage) were applied to study their influence on the metallation yield and on the observed deprotonated and metallated species. Structures of the binary complexes with defined charge locations are proposed, based on the observed in‐source fragmentations. It was demonstrated that the in‐source fragmentations under hard desolvation conditions could differentiate the Leu/Ile isomers if located at the C‐terminal position but not at the N‐terminal position. This behavior was also observed using a triple quadrupole analyzer. This facile distinction, due to a different radical loss from the [(M3HCO2)+CuII]− species (loss of [C3H7]. for YGGFL and [C2H5]. for YGGFI), was facilitated by the reduction of the oxidation state of Cu(II). This in‐source differentiation of YGGFI and YGGFL was also implemented in LC/ESI‐MS analysis by post‐column addition of the copper salt with a syringe pump. Copyright © 2003 John Wiley & Sons, Ltd.

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