Exploring the effect of pore size on the activity of superoxide dismutase mimics immobilized in mesoporous spherical silica particles

The imidazolate-bridged diCu(ii) complex [Cu2(dien)2(mu-Im)]3+ (Im = imidazolate, dien = diethylenetriamine) was encapsulated in mesoporous spherical silica particles (MSSP) with retention of the morphology and mesostructure of the guest-free silica particles. This hybrid material catalyzes the dismutation of superoxide efficiently, with second-order catalytic rate constant of 8.28 x 107 M-1 s-1, obtained from the nitro blue tetrazolium photoreduction inhibition superoxide dismutase assay, at pH = 7.8. The small silica channels of MSSP (wp = 2.7 nm) enhance the distortion of the Cu(ii) local environment compared to mesoporous silicas with wider pores and ensure the two metal moieties of the reduced catalyst remain in close proximity to react with superoxide. These two combined effects increase the SOD activity of the hybrid material ten times over the free catalyst. The small channel size is also relevant for the salpn derived functional SOD mimic [CuZn(salpn)Cl2] (H2salpn = 1,3-bis(salicylideneamino)propane), which upon encapsulation in MSSP doubles its SOD activity. The close fitting of the small pores of mesoporous spherical silica particles (MSSP) to the catalyst dimensions improves the stability of the encapsulated complexes and increases up to ten times their superoxide dismutase activity.