Mechanism of nitric oxide reactivity and fluorescence enhancement of the NO-specific probe CuFL1.
The mechanism of the reaction of CuFL1 (FL1 = 2-{2-chloro-6-hydroxy-5-[(2-methylquinolin-8-ylamino)methyl]-3-oxo-3H-xanthen-9-yl}benzoic acid) with nitric oxide (NO) to form the N-nitrosated product FL1-NO in buffered aqueous solutions was investigated. The reaction is first-order in [CuFL1], [NO], and [OH(-)]. The observed rate saturation at high base concentrations is consistent with a mechanism in which the protonation state of the secondary amine of the ligand is important for reactivity. This information provides a rationale for designing faster-reacting probes by lowering the pK(a) of the secondary amine. Activation parameters for the reaction of CuFL1 with NO indicate an associative mechanism (DeltaS(double dagger) = -120 +/- 10 J/mol.K) with a modest thermal barrier (DeltaH(double dagger) = 41 +/- 2 kJ/mol; E(a) = 43 +/- 2 kJ/mol). Variable-pH electron paramagnetic resonance experiments reveal that, as the secondary amine of CuFL1 is deprotonated, electron density shifts to yield a new spin-active species having electron density localized on the deprotonated amine nitrogen atom. This result suggests that FL1-NO formation occurs when NO attacks the deprotonated secondary amine of the coordinated ligand, followed by inner-sphere electron transfer to Cu(II) to form Cu(I) and release of FL1-NO from the metal.
McQuade
Lindsey E
LE
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Pluth
Michael D
MD
Lippard
Stephen J
SJ
D003300:Copper / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D005456:Fluorescent Dyes / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009569:Nitric Oxide / Q000737:chemistry*; D013816:Thermodynamics
D005456:Fluorescent Dyes; D009569:Nitric Oxide; D003300:Copper