Characterization of metal-labelled peptides by matrix-assisted laser desorption/ionization mass spectrometry and tandem mass spectrometry.

Metal labelling of peptides and proteins using high-affinity metal-chelating compounds has found widespread applications in the medical and bioanalytical fields. In the present study we investigated the analysis of peptides derivatized either with cysteine- or amino group-directed metal-bound DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) chelators in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The metal complexes of DOTA were shown to be stable under MALDI-MS conditions. The introduction of the metal label led in a number of cases to significantly increased signal-to-noise (S/N) values and thus improved sensitivity of the labelled peptides compared to their unlabelled counterparts, especially for multiply labelled peptides. The presence of the labels did alter the tandem mass spectrometric (MS/MS) behaviour, namely the formation of sequence specific a-, b- and y-ion series, in dependence of the position of the label within the peptide sequence. For cysteine-derivatized peptides several label-specific reporter ions and characteristic immonium ions could be identified. Amino-directed labelling led only to the formation of characteristic immonium ions in ε-amino groups of lysine, whereas N-terminal labelling in some cases led to the formation of a(1)- and b(1)-ions. The results clearly show that MALDI-MS is suitable for the analysis of metal-labelled peptides, which was also confirmed in liquid chromatography (LC)/MALDI-based identification of proteins in a model protein mixture labelled with Cys-reactive DOTA. Here, in comparison to a run with alkylated cysteines, more than 50% more cysteine-containing peptides were identified.