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 (L1 ) or double Schiff base (L2 ) for different synthesis systems of anhydrous methanol or water–methanol. We calculated the Δr G of the formation of compounds L1 and L2 by density functional theory (DFT). In an anhydrous methanol system, the Δr G values of L1 and L2 are both below zero and L2 is lower, suggesting the spontaneous formation of the two Schiff bases. Though adjusting the molar ratio of Glc to TPA, L1 and L 2 both were separately formed in anhydrous methanol. However, in the water–methanol system, L2 was absent, which is most likely due to higher Δr 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.