title: Matrix effect on in-source decay products of peptides in matrix-assisted laser desorption/ionization.
abstract: MALDI-ISD of peptides were studied using several salicylic acid derivatives, 2,5-dihydroxybenzoic acid (2,5-DHB), 5-aminosalicylic acid (5-ASA), 5-formylsalicylic acid (5-FSA), and 5-nitrosalicylic acid (5-NSA) as matrices. The difference in the nature of the functional group at the 5-position in the salicylic acid derivatives can dramatically affect the ISD products. The use of 2,5-DHB and 5-ASA leads to "hydrogen-abundant" peptide radicals and subsequent radical-induced N-Cα bonds cleavage. N-Cα bond cleavage gave a c'/z (·) fragment pair and radical z (·)-series fragments gain a hydrogen radical or react with a matrix radical. In contrast, the use of 5-NSA resulted in the production of a "hydrogen-deficient" peptide radical that contained a radical site on the amide nitrogen in the peptide backbone. Subsequently, the radical site on the amide nitrogen induces Cα-C bond dissociation, leading to a (·)/x fragment pair. The a (·)-series ions undergo further hydrogen abstraction to form a-series ions after Cα-C bond cleavage. Since the Pro residue does not contain a nitrogen-centered radical site, Cα-C bond cleavage does not occur. Alternatively, the specific cleavage of CO-N bonds leads to a b (·)/y fragment pair at Xxx-Pro which occurs via hydrogen abstraction from the Cα-H in the Pro residue. The use of 5-FSA generated both a (·)/x- and c'/z (·)-series fragment pairs. An oxidizing matrix provides useful complementary information in MALDI-ISD compared to a reducing matrix for the analysis of amino acid sequencing and site localization in cases of phosphopeptides. MALDI-ISD, when used in conjunction with both reducing and oxidizing matrices is a potentially useful method for de novo peptide sequencing.
authors.affiliation: Mass Spectrometry Laboratory, Graduate School in Nanobioscience, Yokohama City University ; General and Physical Chemistry Department, Mass Spectrometry Laboratory, University of Liège.
authors.forename: Daiki
authors.initials: D
authors.lastname: Asakawa
authors.affiliation: Mass Spectrometry Laboratory, Graduate School in Nanobioscience, Yokohama City University.
authors.forename: Motoshi
authors.initials: M
authors.lastname: Sakakura
authors.affiliation: Mass Spectrometry Laboratory, Graduate School in Nanobioscience, Yokohama City University.
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Takayama
keywords: MALDI in-source decay; c′/z· and a·/x fragment pair; de novo peptide sequencing; hydrogen radical transfer; site localization of post translational modifications