Supramolecular assembly of l-Lysine on ZSM-5 zeolites with different Si/Al ratio

The adsorption of the amino acid, L-lysine, on two ZSM-5 having different silicon to aluminium ratio (i.e. Si/Al = 15 and 37) has been investigated by experiments at macroscopic and at atomistic level resolution. The kinetic and equilibrium constants of the adsorption processes obtained for both the adsorption materials, show that Al content is an important factor in tuning the adsorption of the amino acid on the zeolite. Both the saturation capacity and the binding constant decrease as Si/Al ratio increases. The refinements of high resolution X-ray diffraction data obtained from synchrotron radiation, indicate that adsorption of L-lysine in ZSM-5 is a surface-confined supramolecular self-assembly process. L-lysine molecules at the intersection of the sinusoidal and the straight channels, are arranged in a α-helical conformation stabilized by the simultaneous occurrence of strong H-bonds among the tail of L-lysine molecules, water molecules and framework oxygens. L-lysine molecules within the sinusoidal channel are arranged as β-sheets forming infinite hydrogen-bonding β-strands with the framework oxygens. Our results suggest that L-lysine conformation strongly depends on both pH and hydration state influencing amino acids physico-chemical properties and intermolecular bond energies.