FAST-ATOM-BOMBARDMENT MASS-SPECTRA OF SOME T-BUTOXYCARBONYL)-O-(DIORGANYLPHOSPHONO)-L-SERINES AND O-(DIORGANYLPHOSPHONO)SERYL-CONTAINING DIPEPTIDES AND TRIPEPTIDES

Positive and negative ion fast atom bombardment (f.a.b.) mass spectrom etry were found to be useful methods for the analysis and structural c haracterization of five t-butoxycarbonyl)-O-(diorganylphosphono)-L-ser ines (organyl = Ph, Et, Me, Bzl, Bu(t)), especially in the case of the sensitive benzyl and t-butyl phosphono derivatives. Under positive io n operating conditions, high intensity pseudo-molecular ions were obta ined in the f.a.b. mass spectra, and the fragmentation pathway of the phenyl, ethyl and methyl derivatives was established by parent/daughte r linked scanning studies to involve (a) the two-step loss of the t-bu toxycarbonyl group, (b) loss of the amino acid as the neutral fragment from the [MH]+, [MH-56]+, [MH - 100]+ and [MH - 146]+ ions by a four- centred beta-elimination rearrangement, and (c) cleavage of the phosph ono phenyl and ethyl groups from only the [(RO)2P(OH)2]+ and [NH=CHCH2 PO3R2+H]+ fragments. Parent/daughter linked scanning studies of the be nzyl derivative showed that the prominent fragmentation involved loss of the benzyl group as the tropylium ion and that the 'apparent' [MH - 90]+ peak observed in its f.a.b. mass spectrum resulted from cleavage of the phosphono benzyl group in the matrix during the bombardment pr ocess. In the case of the t-butyl derivative, parent/daughter linked s canning studies showed that the prominent fragmentation involved succe ssive 'in-flight' loss of the phosphono t-butyl groups as isobutene. N egative ion f.a.b. mass spectrometry of the five derivatives gave f.a. b. mass spectra which displayed distinct [M - H]- anions along with hi gh intensity [M - H - R]- and [(RO)2PO2]-fragment anions, the f.a.b. m ass spectrum of the t-butyl derivative containing an additional [M - H - Bu(t) - Bu(t)]- fragment anion. Parent/daughter linked scanning stu dies established that the majority of the observed fragment anions res ulted from extensive fragmentation of the Boc-Ser(PO3R2)-OH derivative s in the matrix phase followed by sputtering of the resultant fragment s into the gas phase. In addition, positive ion f.a.b. mass spectromet ry was found to be useful for the analysis of a series of protected O- (diorganylphosphono)sery]-containing dipeptides and tripeptides (organ yl = Ph, Et, Me, Bzl). The obtained spectra showed that beta-eliminati on fragmentation of the Ser(PO3R2) residue was more pronounced with th e tripeptide series and indicated that there was increased sensitivity of the O-(diorganylphosphono)seryl residue with replacement of the Bo c group by an amino acyl residue at its N-terminus.