The effect of earth metal ion on the property of peptide-based metal–organic frameworks

A series of alkaline earth metal-based bio-analogous metal–organic frameworks (1–4) based on a cyclic dipeptide (2,5-piperazinedione-1,4-diacetic acid, H2PODC) were prepared under the same synthetic conditions and the effect of earth metal ion on the (geometrical and electronic) property of these MOFs was investigated. Crystal analysis demonstrates that the metal ion radii play an important role in coordination number, coordination modes of the ligand and bond length. The binding mode between metal ion and peptide can be divided into two groups: (1) Mg2+ and Sr2+; (2) Ca2+ and Ba2+ ions, where the difference is whether the carbonyl group participates in coordination. The result may be relevant to the different biophysical phenomena of protein between alkaline earth metal ions. UV-vis spectra show two adsorption peaks for compounds 1–4, while fluorescence spectra display one emission peak. Via first-principles density functional theory (DFT) calculation, we found that two adsorption peaks should be attributed to the transition of valence band (VB) → two empty bands and VB → conduction band (CB), while one emission peak may result from the transition of CB → VB.