Controllable Synthesis and Biological Application of Schiff Bases from D-Glucosamine and Terephthalaldehyde

Theoretically, the two aldehydes of terephthalaldehyde (TPA) are equivalent, so the single or double Schiff base from TPA and D-glucosamine (Glc) may be formed at the same time. However, it is preferred to produce separately a single Schiff base (L-1) or double Schiff base (L-2) for different synthesis systems of anhydrous methanol or water-methanol. We calculated the Delta(r)G of the formation of compounds L-1 and L-2 by density functional theory (DFT). In an anhydrous methanol system, the Delta(r)G values of L-1 and L-2 are both below zero and L-2 is lower, suggesting the spontaneous formation of the two Schiff bases. Though adjusting the molar ratio of Glc to TPA, L-1 and L-2 both were separately formed in anhydrous methanol. However, in the water-methanol system, L-2 was absent, which is most likely due to higher Delta(r)G (4.95 eV) and better water solubility. The results also exhibits that the positive charge of C in -CHO for TPA is smaller in a mixed solvent than that in methanol, which confirms that the nucleophilic reaction of the Schiff base is more difficult in a mixed solvent. Therefore, we could realize to control the synthesis of a pure single or double Schiff base from Glc and TPA by adjusting the molar ratio and solvent. The as-prepared two kinds of Schiff bases have strong optical properties, high bacteriostatic activity, and can be used as fluorescent probes for tumor cell imaging.