Cooperation of Macrocyclic Metal Centers with 4-Dialkylaminopyridines in Catalysis by Poly(ethylenimine)-Based Artificial Metalloesterases

Poly(ethylenimine) (PEI) was modified by laurylation, introduction of 4-N,N-di(3-carboxy-propyl)aminopyridyl (DAP) group, NiII-template condensation of the polymer with glyoxal, and/or acetylation of primary and secondary amines, to produce Ni1II[Lau11-PEI-GO-DAP5]Ac and several other related derivatives of PEI. The hydrolysis of anionic ester 2-nitro-4-carboxyphenyl acetate (NCPA) and hydrophobic ester 4-nitrophenyl caproate (NPC) was effectively catalyzed by Ni1II[Lau11-PEI-GO-DAP5]Ac and turnover kinetics due to regeneration of the catalyst was observed. Artificial metalloesterase Ni1II[Lau11-PEI-GO-DAP5]Ac manifested characteristics of actions of metalloenzymes such as complex formation with substrates, considerable degree of catalysis, selectivity toward hydrophobic or anionic esters, and cooperativity among multiple catalytic elements including metal ions. In the hydrolysis of NCPA, the metal center anchors the anionic ester and the DAP moiety makes nucleophilic attack at the bound substrate. Electrostatic attraction between the metal center and NCPA is strengthened by the hydrophobic microenvironment provided by the lauryl groups. In the hydrolysis of NPC, the substrate is bound by the catalyst primarily due to the hydrophobic effects exerted by the lauryl groups and the DAP moiety makes nucleophilic attack at the bound substrate. In NPC hydrolysis, the intrinsic nucleophilicity of the DAP is enhanced by the medium effects exerted by the macrocyclic metal centers.