Galactose oxidase models: insights from 19F NMR spectroscopy.
(19)F labelled tripodal ligands that possess a N(3)O 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 (HL(F) and HL(CF3)) or at the quinoline N-donor (HLq(OMe) and HLq(NO2)). 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 A 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, pi-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 (19)F NMR. In the former case, a broadening of the (19)F NMR signal is observed, whereas a sharpening is observed in the latter case. Temperature dependent (19)F 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 phenol para-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 (19)F NMR spectroscopy.
Michel
Fabien
F
Département de Chimie Moléculaire, Chimie Inorganique Redox Biomimétique (CIRE), UMR CNRS 5250, Université J. Fourier, B. P. 53, 38041 Grenoble cedex 9, France.
Hamman
Sylvain
S
Philouze
Christian
C
Del Valle
Carlos Perez
CP
Saint-Aman
Eric
E
Thomas
Fabrice
F
D003198:Computer Simulation; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D005461:Fluorine / Q000737:chemistry; D005692:Galactose Oxidase / Q000378:metabolism*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D012015:Reference Standards
D008024:Ligands; D005461:Fluorine; D005692:Galactose Oxidase