Distinction of Leu and Ile Using a Ruthenium(II) Complex by MALDI-LIFT-TOF/TOF-MS Analysis

The novel N-terminal labeling method using a ruthenium(II) complex derivative characteristically indicated a n and d n (N-terminal) fragment ions in high sensitivity by MS/MS analysis (MALDI-LIFT or ESI-CID). Although these fragment ions depended on a fragmentation process by MS/MS analytical methods to some degree, each case indicated similar side-chain cleavage patterns. The labeling method allows accurate distinction of amino acid residues by MS/MS analysis even if the residues are structural isomers such as leucine and isoleucine. The method was applied to long-chain peptides and provided easy and rapid N-terminal sequencing.