Conformational Flexibility of Poly(ethylenimine) and Its Derivatives

Degree of protonation of the amino groups and the equilibrium constant for binding of Ni(II) ion by the amino groups were measured for poly(ethylenimine) (PEI), the PEI derivative containing lauryl groups (Lau-PEI), the PEI derivative containing β-cyclodextrin (CD-PEI), and the PEI derivative containingo,o′-dihydroxyazobenzene (DHAB-PEI). The amino groups of Lau-PEI, CD-PEI, and DHAB-PEI resisted protonation much more strongly compared with those of PEI. In addition, Ni(II) binding by the amino groups of Lau-PEI, CD-PEI, and DHAB-PEI was much weaker than that of PEI. These are taken to indicate that Lau-PEI, CD-PEI, and DHAB-PEI possess compact conformations in order to minimize the hydrocarbon–water interfacial area. The high conformational flexibility of the polymer backbone results in the formation of hydrophobic microdomains by aggregation of hydrophobic moieties. Implications of the conformational flexibility on the design of artificial enzymes are also discussed.