LINEAR FREE-ENERGY CORRELATION IN THE LOW-ENERGY TANDEM MASS-SPECTRA OF SODIATED TRIPEPTIDES GLY-GLY-XXX

The backbone cleavages of 14 sodiated tripeptides of the series Gly-Gly-Xxx, where Xxx = Gly, Ale, Val, Leu, lie, Phe, Tyr, Met, Glu, Pro, Trp, Lys, His and Arg, were studied in a hybrid tandem mass spectrometer. A C-terminal y-type ion of the form y(1) + Na + H was noted in all cases studied. N-Terminal b(n) + Na + 17, b(n) + Na - H and a(n) + Na - H ions, along with internal fragments, were also noted. Because information on sodium affinities of amino acids is limited, the sodiated tripeptides studied were compared not only with the rank ordering of amino acid sodium affinities, but also on the basis of available proton affinities. A linear relationship between log[(y(1) + Na + H)/(b(2) + Na + 17)] and the proton affinity of the C-terminal amino acid substituents was found: as the proton affinity of the C-terminal residue increases, the fraction of y(1), ion formation increases. When the C-terminal substituent was more basic than Trp, the correlation does not hold, probably because the highly basic amino acids, Lys, His, and Arg, are cationized on the side-chain instead of on the terminal amino group.