Rational selection of the optimum MALDI matrix for top-down proteomics by in-source decay

In-source decay (ISD) in MALDI leads to c- and z-fragment ion series enhanced by hydrogen radical donors and is a useful method for sequencing purified peptides and proteins. Until now, most efforts to improve methods using ISD concerned instrumental optimization. The most widely used ISD matrix is 2,5-dihydroxybenzoic acid (DHB). We present here a rational way to select MALDI matrixes likely to enhance ISD for top-down proteomic approaches. Starting from Takayama's model (Takayama, M. J. Am. Soc. Mass Spectrom. 2001, 12, 1044-9), according to which formation of ISD fragments (c and z) would be due to a transfer of hydrogen radical from the matrix to the analyte, we evaluated the hydrogen-donating capacities of matrixes, and thus their ISD abilities, with spirooxazines (hydrogen scavengers). The determined hydrogen-donating abilities of the matrixes are ranked as follows: picolinic acid (PA) > 1,5-diaminonaphtalene (1,5-DAN) > DHB > sinapinic acid > alpha-cyano-4-hydroxycinnamic acid. The ISD enhancement obtained by using 1,5-DAN compared to DHB was confirmed with peptides and proteins. On that basis, a matrix-enhanced ISD approach was successfully applied to sequence peptides and proteins up to approximately 8 kDa. Although PA alone is not suitable for peptide and protein ionization, ISD signals could be further enhanced when PA was used as an additive to 1,5-DAN. The optimized matrix preparation was successfully applied to identify larger proteins by large ISD tag researches in protein databases (BLASTp). Coupled with an adequate separation method, ISD is a promising tool to include in a top-down proteomic strategy.