Issue 5, 2009

Galactose oxidase models: insights from 19F NMR spectroscopy

Abstract

19F labelled tripodal ligands that possess a N3O donor set (one phenol, one tertiary amine and either two pyridines or one pyridine and one quinoline) have been synthesized. The fluorine is incorporated either at the phenol O-donor (HLF and HLCF3) or at the quinoline N-donor (HLqOMe and HLqNO2). The copper(II)–phenol complexes (2H)2+, (1H)2+, (3H)2+ and (4H)2+ as well as the corresponding copper(II)–phenolate complexes have been characterized. X-Ray diffraction reveals an increase in the oxygen–copper bond distance of more than 0.4 Å upon protonation of the phenolate moiety of (4)+. Protonation is accompanied by an axial to equatorial isomerization of the quinoline group. DFT calculations show that stretching of the Cu–O(phenol) bond, π-stacking interactions and rotation of the pyridine are key steps in this isomerization process. Protonation, and thus changes in the oxygen–copper bond distance induce either a decrease ((1H)2+, (2H)2+) or an increase ((3H)2+ and (4H)2+) in the copper–fluorine distance that could be monitored by 19F NMR. In the former case, a broadening of the 19F NMR signal is observed, whereas a sharpening is observed in the latter case. Temperature dependent 19F NMR measurements on equimolar mixtures of the phenol and phenolate complexes of (3)+ and (4)+ reveal rate constants for proton transfer and/or isomerization of 3000 ± 100 s−1 and 2900 ± 100 s−1, respectively, at the coalescence temperature. This temperature was found to be strongly affected by the phenolpara-substituent as it is 226 K and ca. 330 K for (3)+ and (4)+, respectively. A phenoxyl radical species ((3˙)2+) could be generated and characterized for the first time by 19F NMR spectroscopy.

Graphical abstract: Galactose oxidase models: insights from 19F NMR spectroscopy

Supplementary files

Article information

Article type
Paper
Submitted
29 Jul 2008
Accepted
02 Oct 2008
First published
28 Nov 2008

Dalton Trans., 2009, 832-842

Galactose oxidase models: insights from 19F NMR spectroscopy

F. Michel, S. Hamman, C. Philouze, C. P. Del Valle, E. Saint-Aman and F. Thomas, Dalton Trans., 2009, 832 DOI: 10.1039/B813036A

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