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Allosteric Binding of an Ag + Ion to Cerium( IV ) Bis‐porphyrinates Enhances the Rotational Activity of Porphyrin Ligands
Author(s) -
Ikeda Masato,
Takeuchi Masayuki,
Shinkai Seiji,
Tani Fumito,
Naruta Yoshinori,
Sakamoto Shigeru,
Yamaguchi Kentaro
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20021216)8:24<5541::aid-chem5541>3.0.co;2-x
Subject(s) - porphyrin , chemistry , allosteric regulation , supramolecular chemistry , ligand (biochemistry) , ion , cerium , crystallography , stereochemistry , photochemistry , crystal structure , inorganic chemistry , organic chemistry , biochemistry , receptor , enzyme
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 π subunits (π‐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 °C (Δ G $\rm{^{\ne }_{293}}$ =14.1 kcal mol −1 ) and 220 s −1 at −40 °C (Δ G $\rm{^{\ne }_{233}}$ =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.