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

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 alpha and beta' backbone cleavages occur from the [Mbond;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 (CH(2)O) > Glu (H(2)O) > Gln (NH(3)). When one of more of these residues is (are) present, the alpha and beta' cleavages often occur from a fragment rather than the [Mbond;H](-) anion, e.g. for Thr, the [(Mbond;H)(-)bond;MeCHO](-) anion. These four residues also initiate gamma backbone cleavage reactions. The relative abundances of peaks resulting from gamma cleavage are Glu > Ser = Thr > Gln for maculatin 1 spectra. An unusual Gln19/Ile17 cyclisation/cleavage reaction occurs in maculatin spectra: the peptide [Mbond;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.