Negative ion electrospray mass spectra of the maculatin peptides from the tree frogs Litoria genimaculata and Litoria eucnemis

Abstract

Collision-induced fragmentations of deprotonated maculatin 1 peptides provide significant sequencing information. When the peptide lacks those residues which can fragment through their alpha side chains (e.g. Thr, Ser, Glu and Gln in this study) the basic α and β′ backbone cleavages occur from the [M-H]- anion. When Thr, Ser, Glu and Gln are present, the ease of side-chain fragmentation of these residues is: Thr (loss of MECHO) > Ser (CH2O) > Glu (H2O) > Gln (NH3). When one of more of these residues is (are) present, the α and β′ cleavages often occur from a fragment rather than the [M-H] - anion, e.g. for Thr, the [(M-H)- MeCHO]- anion. These four residues also initiate γ backbone cleavage reactions. The relative abundances of peaks resulting from γ cleavage are Glu > Ser = Thr > Gln for maculatin 1 spectra. An unusual Gln19/Ile17 cyclisation/cleavage reaction occurs in maculatin spectra: the peptide [M-H]- anion must adopt a helical conformation in order for these two groups to interact. Analogous fragmentations have been reported previously in the negative ion spectra of the caerin 1 peptides. © 2003 John Wiley & Sons, Ltd.