Histidine Tethered Dipeptide Based Zn-Coordinated Organohydrogel: Potential Biomimic for CO ₂ Capture and Conversion

Carbonic anhydrase (CA) is an efficient enzyme for mitigating the emission of CO2. Herein, a Zn-coordinated supramolecular organohydrogel has been developed from histidine-containing dipeptide gelators to mimic the activity of CA for efficient CO2 capture and its conversion. By changing the headgroup at the C-terminal and protection at the N-terminal of phenylalanine–histidine dipeptides, organohydrogels were developed in DMSO/water in the absence and presence of Zn salts (Cl–, OH–, NO3–, OAc–). Gelation efficiency of gelators (G1–G4) varied from 1.0 to 10.0% w/v in different v/v compositions of DMSO–water. Gelation ability in the presence of varying Zn salts and their rheological studies showed that gelator G4 (Palm(C16)-Phe-His-OH) formed the most efficient organohydrogel along with all four Zn salts in 2:1 (v/v) DMSO–water, having a minimum gelation concentration (MGC) 0.8% w/v. The 1H NMR spectra, mass spectroscopy, elemental analysis in the XPS study, EPR, and FTIR studies confirmed the formation of Zn-coordinated organohydrogel of G4–ZnCl2 at a 3:1 mol ratio of the gelator and Zn salt. Lowering of MGC in the presence of Zn salt and the corresponding XRD analysis assured the contribution of the metal ion in improving the gelation efficiency. The histidine moiety of gelators might have made efficient coordination with the metal center and can act as a mimic of the active site of metalloenzyme CA. Subsequently, the CA-mimicking activity of Zn-coordinated gels of G4 was monitored by measuring the hydrolysis of p-nitrophenyl acetate (p-NPA). Among all Zn-coordinated gels of G4 with varying Zn salts, G4–ZnCl2 exhibited the highest conversion rate (2.53 ± 0.05 μM/min), which was 2.14–6.17 times higher than those of other Zn salt-coordinated gels and 8.4–21 times higher than that of only gel or Zn salt. Importantly, GC-TCD experiments confirmed that G4–ZnCl2 can efficiently absorb ∼43% CO2 within its three-dimensional (3D) porous structure, which was further converted into CaCO3 upon addition of CaCl2. Hence, G4–ZnCl2 provides a sustainable platform that can both absorb and transform CO2.