Metal ions are essential participants for the catalytic activities or structural stabilities of metalloenzymes. Metalloenzymes use the cooperative action between the substrate binding site and catalytic metal center. β-Cyclodextrin is a naturally occurring cyclic heptamer of D-(+)-glucopyranose units that are linked by $\alpha$(1→4) glucopyranose bonds.
In this study, new supramolecular β-CD monomer 1 in which Cu(Ⅱ) ion binding sites are attached to the primary side of hydrophobic β-CD pockets, was prepared. β-CD dimers possessing tridentate 7- (2), pentadentate 13- (3) and 15-membered (4) pyridine diamide chelators were designed, synthesized and characterized by MALDI-MS, NMR, IR and UV-Visible spectroscopy. Fluorescence and pH-metric titration were carried out in order to ascertain their behavior as bifunctional hosts for fluorescence guests and metal ions. As expected, supramolecular β-CD dimers have high binding affinity for fluorescence guests, e.g. TNS and TPPS. Supramolecular β-CD monomer and dimers chelate Cu(Ⅱ) ion by forming amidate-Cu(Ⅱ) complexes. The $pK_{a1}$ values for the Cu(Ⅱ) promoted deprotonation of amide ligands from 1, 3 and 4 were determined to be 4.3, 6.3 and 6.3, respectively. Above pH 8.0, supramolecular ligands 3 and 4 bind fluorescent guest and Cu(Ⅱ) ion simultaneously. 1-Cu(Ⅱ), 3-Cu(Ⅱ) and 4-Cu(Ⅱ) complexes were isolated as blue or purple solids. Cu(Ⅱ) complexes catalyze the hydrolysis of p-nitrophenyl acetate, adamantate and amino acids in an enzyme-like manner; displaying Michaelis-Menten kinetics, substrate specificity and competitive inhibition. In this context, these complexes are regarded as artificial metalloenzyme. Artificial metalloenzymes hydrolyze the p-nitrophenyl esters by the nucleophilic addition of a Cu(Ⅱ)-coordinated hydroxide ion to the carbonyl carbon of substrate included in hydrophobic β-CD pocket. These results support the zinc hydroxide mechanism of carboxypeptidase A.
β-Cyclodextrin의 primary OH기에 금속 이온 chelator를 도입시킨 β-CD monomer 1, β-CD dimer 2, 3와 4 거대분자를 제조하였다. MALDI-MS, NMR, IR and UV-Visible spectroscopy를 사용하여 거대분자의 화학 구조적 특성을 분석하였다. 거대분자들은 organic guests와 Cu(Ⅱ) ion을 동시에 결합할 수 있는 능력을 보여주었다. UV-titration을 이용하여 monomer 1, dimer 3와 dimer 4 거대분자들의 Cu(Ⅱ) chelation 능력을 확인하였다. Monomer 1, dimer 3와 dimer 4의 deprotonation $pK_{a1}$ 값은 각각 4.3, 6.2와 6.3 이었다. 재결정 방법을 이용하여 blue 또는 purple 색을 띠는 1-Cu(Ⅱ), 3-Cu(Ⅱ)와 4-Cu(Ⅱ) complex를 순수 분리하였다. 이들 complex들은 p-nitrophenyl acetate, adamantate 또는 amino acids ester들의 가수분해 반응의 촉매제 역할을 하였다. Cu(Ⅱ) complex들은 자연계의 금속효소와 유사한 Michaelis-Menten kinetics, competitive inhibition kinetics와 substrate specificity를 나타내었다. 이러한 관점에서 Cu(Ⅱ) complex들은 artificial metalloenzyme 이라 할 수 있다. Artificial metalloenzyme의 가수분해 촉매작용은 Cu(Ⅱ)-coordinated hydroxide ion이 β-CD pocket에 결합하고 있는 substrate의 carbonyl carbon을 nucleophilic attack하는 metal-hydroxide mechanism 이었다. 이러한 실험 결과는 carboxypeptidase A의 zinc-hydroxide mechanism을 지지하는 것이다.
