Allosteric Binding of an Ag+ Ion to Cerium(IV) Bis-porphyrinates Enhances the Rotational Activity of Porphyrin Ligands.
A series of cerium(IV) bis-porphyrinate double-deckers [Ce(bbpp)(2)] (BBPP=5,15-bis(4-butoxyphenyl)porphyrin dianion), [Ce(tmpp)(2)] (TMPP=5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin dianion), [Ce(tfpp)(2)] (TFPP=5,10,15,20-tetrakis(4-fluorophenyl)porphyrin dianion), [Ce(tmcpp)(2)] (TMCPP=5,10,15,20-tetrakis(4-methoxycarbonylphenyl)porphyrin dianion), and [Ce(tmpp)(tmcpp)] was prepared. They bind three Ag(+) ions to their concave porphyrin pi subunits (pi-clefts) according to a positive homotropic allosteric mechanism with Hill coefficients (n(H)) of 1.7-2.7. The rotation rates of the porphyrin ligands in [Ce(bbpp)(2)] were evaluated to be 200 s(-1) at 20 degrees C (DeltaG$\rm{^{\ne }_{293}}$=14.1 kcal mol(-1)) and 220 s(-1) at -40 degrees C (DeltaG$\rm{^{\ne }_{233}}$=11.0 kcal mol(-1)) without and with Ag(+) ions, respectively. These results consistently support our unexpected finding that Ag(+) binding can accelerate rotation of the porphyrin ligand. On the basis of UV-visible, (1)H NMR, and resonance Raman spectral measurements, the rate enhancement of the rotational speed of the porphyrin ligands is attributed to conformational changes of the porphyrin in cerium(IV) bis-porphyrinate induced by binding of Ag(+) guest ions in the clefts. This novel concept of positive homotropic allosterism is applicable to the molecular design of various supramolecular and switch-functionalized systems.
Ikeda
Masato
M
Department of Chemistry and Biochemistry Graduate School of Engineering, Kyushu University Fukuoka 812-8581, Japan, Fax: (+81) 92-642-3611.
Takeuchi
Masayuki
M
Shinkai
Seiji
S
Tani
Fumito
F
Naruta
Yoshinori
Y
Sakamoto
Shigeru
S
Yamaguchi
Kentaro
K