Differentiation of Diastereomeric Tertiarybutoxycarbonylprolylproline Ethyl Esters by Collision-Induced Dissociation of Sodium Adduct Ions in Fast Atom Bombardment Mass Spectrometry.

Abstract

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(CH3)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.1)