A comparison of the positive- and negative-ion mass spectra of bio-active peptides from the dorsal secretion of the Australian red tree frog, Litoria rubella.

The collision-induced tandem mass spectral data for MH+ and [M-H]- ions from six bio-active peptides from Litoria rubella are compared. Backbone cleavages of [M-H]- ions provide sequencing information for five of the peptides [e.g. Phe Pro Trp Leu (NH2) and pGlu Phe Pro Trp Leu (NH2)] and in these cases, the negative-ion spectra are as informative as the positive-ion spectra. Side-chain cleavages are also noted in these spectra. For example, (i) when Trp is present, the loss of C9H7N (129 u) competes with the backbone cleavages, and (ii) the [M-H]- ion of Ile Glu Phe Phe Thr (NH2) undergoes facile side-chain fragmentation [loss of H2O (from Glu) and MeCHO (from Thr)], but does not form any conventional backbone-cleavage ions.