2.0 Computational Methods
Twenty two molecules with the potential to react as Schiff bases (Scheme
1) were taken from data reported by ICCVAM (Interagency Coordinating
Committee on the Validation of Alternative
Methods)[57] and Kern et
al. [58] as described
previously.[54] All SB compounds with
non-zero EC3 values were chosen, covering a different chemical
substructures (Table 1). Some of the substrates contained chemical
functionality associated with other chemical domains
(SNAr, SN2 and Acyl) or had more than
one position capable of forming a SB product. The compounds can be
sub-classified as aliphatic and aromatic aldehydes, ketones, 1,2 diones
and 1,3 diones. In the case of 1,2 and 1,3 diones, each molecule
contains two carbonyl groups leading to two plausible points for
nucleophilic attack.
Calculations were performed using a model consisting of the substrate,
two water molecules and two methylamine molecules to approximate the
biological reaction. The simulated reaction leading to the imine product
(Schiff base) involves three distinct steps as shown in Scheme 2. The
first step involves nucleophilic attack of the substrate by methylamine
giving rise to a carbinolamine or hemiacetal-like intermediate (INT1).
Proton transfer from the protonated intermediate, via water,
leads to the elimination of water and the formation of the protonated
imine product (INT2). The final step involves the deprotonation of INT2
to give the deprotonated imine (PROD).