Construction of a hybrid biocatalyst containing a covalently-linked terpyridine metal complex within a cavity of aponitrobindin

A hybrid biocatalyst containing a metal terpyridine (tpy) complex within a rigid beta-barrel protein nitrobindin (NB) is constructed. A tpy ligand with a maleimide group, N-[2-([2,2':6',2 ''-terpyridin]-4'-yloxy)ethyl]maleimide (1), was covalently linked to Cys96 inside the cavity of NB to prepare a conjugate NB-1. Binding of Cu2+, Zn2+, or Co2+ ion to the tpy ligand in NB-1 was confirmed by UV-vis spectroscopy and ESI-TOF MS measurements. Cu2+-bound NB-1 is found to catalyze a Diels-Alder reaction between azachalcone and cyclopentadiene in 22% yield, which is higher than that of the Cu2+-tpy complex without the NB matrix. The results suggest that the hydrophobic cavity close to the copper active site within the NB scaffold supports the binding of the two substrates, dienophile and diene, to promote the reaction. (C) 2015 Elsevier Inc All rights reserved.