Figure 1. The structure of deprotonated cellobiose showing the loss of 60 mass units by elimination of the C1/C2 or C5/C6 parts of the reducing ring.
Tandem mass spectrometry and isotopic labelling have been used to probe the fragmentation behaviour of glucopyranosyl disaccharide anions. Based on these experiments several reaction mechanisms have been proposed, involving ring opening by cleavage of the bond between C1 and the ring oxygen followed by fragmentation of the reducing sugar ring [2].
In this study we have used quantum mechanical molecular orbital calculations to investigate the energetics of disaccharide fragmentation. This was done using the semiempirical Austin Model 1 (AM1) and at the Hartree-Fock level using the 3-21G basis set. AM1 was used to calculate the transition state between proposed fragments. The products, reactants and the transition state determined using AM1 were subsequently used as input for ab initio calculation. At the ab initio level the transition state was refined using quadratic synchronous transit guided transition state optimisation (QST3). The loss of 60 mass units from deprotonated cellobiose has been reported to be possible by loss of either the C1/C2 or the C5/C6 parts of the reducing ring. By calculating the energies of the corresponding transition states the energetics of the two pathways can be compared.