The formation and the fragmentation of the sodium adduct ions for tertiarybutoxycarbonyl-
L-prolyl-
L-proline ethyl ester (Boc-
L-Pro-
L-Pro-OEt) were compared with those for Boc-
D-Pro-
L-Pro-OEt on the addition of sodium chloride in positive-ion fast atom bombardment (FAB) mass spectrometry. The abundance of the [M+Na]
+ ion was about 2 times lower than that of the [M+H]′ ion on the addition of sodium chloride in FAB mass spectrum for Boc-
L-Pro-
L-Pro-OEt, while the former was almost same as the latter in the spectrum for Boc-
D-Pro-
L-Pro-OEt, suggesting that Boc-
D-Pro-
L-Pro-OEt has a stronger Na′ ion affinity than Boc-
L-Pro-
L-Pro-OEt. On the other hand, in the collision-induced dissociation (CID) mass spectra of the [M+Na]′ ions, the abundance of the [M+Na-C(CH
3)
3+ H]
+ ion, which is due to the loss of a tertiarybutyl group from the [M+Na]
+ ion for Boc-
D-Pro-
L-Pro-OEt, was higher than that for Boc-
L-Pro-
L-Pro-OEt. These results indicate that Boc-
L-Pro-
L-Pro-OEt was distinguished from Boc-
D-Pro-
L-Pro-OEt by the CID mass spectra of the [M+Na]
+ ions, and the difference of the cleavage patterns between Boc-
L-Pro-
L-Pro-OEt and Boc-
D-Pro-
L-Pro-OEt when the carboxyl-terminals were protected with ethyl ester, was smaller than that between tertiarybutoxycarbonyl-
L-prolyl-
L-proline (Boc-
L-Pro-
L-Pro) and Boc-
D-Pro-
L-Pro when they were free reported previously.
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