Diagnostic di- and triphosphate cyclisation in the negative ion electrospray mass spectra of phosphoSer peptides

It has been shown previously that [M-H](-) anions of small peptides containing two phosphate residues undergo cyclisation of the phosphate groups, following collision-induced dissociation (CID), to form a characteristic singly charged anion A (H3P2O7(-), m/z 177). In the present study it is shown that the precursor anions derived from the diphosphopeptides of caerin 1.1 [GLLSVLGSVAKHVLPHVVPVIAEHL(NH2)] and frenatin 3 [GLMSVLGHAVGNVLGGLFKPKS(OH)] also form the characteristic product anion A (m/z 177). Both of the precursor peptides show random structures in water, but partial helices in membrane-mimicking solvents [e.g. in d3-trifluoroethanol/water (1:1)]. In both cases the diphosphopeptide precursor anions must have flexible conformations in order to allow approach of the phosphate groups with consequent formation of A: for example, the two pSer groups of 4,22-diphosphofrenatin 3 are seventeen residues apart. Finally, CID tandem mass spectrometric (MS/MS) data from the [M-H](-) anion of the model triphosphoSer-containing peptide GpSGLGpSGLGpSGL(OH) show the presence of both product anions A (m/z 177) and D (m/z 257, H4P3O10(-)). Ab initio calculations at the HF/6-31+G(d)//AM1 level of theory suggest that cyclisation of the three phosphate groups occurs by a stepwise cascade mechanism in an energetically favourable reaction (ΔG = -245 kJ mol(-1)) with a maximum barrier of +123 kJ mol(-1).