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π–π Stacking Interaction in an Oxidized Cu II –Salen Complex with a Side‐Chain Indole Ring: An Approach to the Function of the Tryptophan in the Active Site of Galactose Oxidase
Author(s) -
Oshita Hiromi,
Suzuki Takashi,
Kawashima Kyohei,
Abe Hitoshi,
Tani Fumito,
Mori Seiji,
Yajima Tatsuo,
Shimazaki Yuichi
Publication year - 2019
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/chem.201900733
Subject(s) - indole test , stacking , chemistry , moiety , ring (chemistry) , side chain , galactose oxidase , stereochemistry , photochemistry , crystallography , galactose , organic chemistry , polymer
In order to gain new insights into the effect of the π–π stacking interaction of the indole ring with the Cu II –phenoxyl radical as seen in the active form of galactose oxidase, we have prepared a Cu II complex of a methoxy‐substituted salen‐type ligand, containing a pendent indole ring on the dinitrogen chelate backbone, and characterized its one‐electron‐oxidized forms. The X‐ray crystal structures of the oxidized Cu II complex exhibited the π–π stacking interaction of the indole ring mainly with one of the two phenolate moieties. The phenolate moiety in close contact with the indole moiety showed the characteristic phenoxyl radical structural features, indicating that the indole ring favors the π–π stacking interaction with the phenoxyl radical. The UV/Vis/NIR spectra of the oxidized Cu II complex with the pendent indole ring was significantly different from those of the complex without the side‐chain indole ring, and the absorption and CD spectra exhibited a solvent dependence, which is in line with the phenoxyl radical–indole stacking interaction in solution. The other physicochemical results and theoretical calculations strongly support that the indole ring, as an electron donor, stabilizes the phenoxyl radical by the π–π stacking interaction.