At first sight, the presence of the aspartic acid residue in the consensus peptides for the Hf-BTB-NH2 nanosheets appears to be counter-intuitive given the overall anionic surface character of this MON. However, Lan et al. demonstrated that various amino acids can bind to the surface of closely-related Hf12-cluster nanosheets via ligand exchange. In principle, the anionic carboxylate form of aspartic acid (which has a sidechain pKa of 3.90) may similarly displace the capping formate ligand (pKa 3.75) to bind to the Hf clusters.