Photooxidation of metal-bound thiolates: reactivity of sulfur containing peroxidic intermediates

Abstract

A variety of reactions of singlet oxygen with metal-bound thiolates are described, and contrasted with the photooxidation of organic sulfides. Superficially, these two processes appear to involve similar mechanisms, but there are important differences: unlike the photooxidation of organic sulfides, the rate of the initial reaction of metal-thiolates with singlet oxygen (k t ) appears to be affected by protic solvents and acids. The nucleophilicity of the thiolate moiety is reduced by addition of acids or in protic solvents, leading to significantly lower k t values. The primary intermediate in the photooxidation of organic sulfides is a nucleophilic persulfoxide, which can be stabilized by protic solvents or by addition of acid. However, the primary intermediate in the photooxidation of metal thiolates cannot be trapped with phosphite, suggesting that it may be less nucleophilic than its organic counterpart. Support for this hypothesis is also derived from the rather modest (compared with organic sulfides) acceleration of the rate of product formation by addition of acid.

Model peptide for anti-sigma factor domain HHCC zinc fingers: high reactivity toward 1O2 leads to domain unfolding

“…This was confirmed by the loss of 66 mass units corresponding to H 2 SO 2 upon MS/MS fragmentation. The formation of a sulfinate upon photooxidation of metal-bound thiolate has already been reported in many instances for Pt,35–37 Ni,38 Pd,38 Co39–41 and Cd42 complexes thereby supporting the formation of a sulfinate for Zn-bond cysteines in Zn·L ASD (HHCC) but the product with a 48-mass unit increase is more intriguing. As F4 does not contain any amino acid prone to oxygen incorporation upon photooxidation other than Cys34 ( i.e.…”

Reactivity of a Zn(Cys)2(His)2 Zinc Finger with Singlet Oxygen: Oxidation Directed toward Cysteines but not Histidines

“…The rare studies on the oxidation of coordinated thiolates were performed with Ni II , Pd II ,P t II ,a nd Co III complexes of small organic ligands, but not Zn II . [41][42][43][44][45][46][47] Studies on zinc finger proteins are even more scarce. The sole example, to our knowledge, demonstrates that 1 O 2 can inhibit DNA binding of nuclear receptors, which are transcriptionf actors containing Zn(Cys) 4 zinc fingers.…”

“…Despite abundant reports on the photooxidation of amino acids,15 almost nothing is known about the photooxidation of metal‐bound cysteines or histidines. The rare studies on the oxidation of coordinated thiolates were performed with Ni II , Pd II , Pt II , and Co III complexes of small organic ligands, but not Zn II 41–47. Studies on zinc finger proteins are even more scarce.…”

Sequential Photooxidation of a Pt(II) (Diimine)cysteamine Complex: Intermolecular Oxygen Atom Transfer versus Sulfinate Formation

“…12 It is not known whether or not the reaction of singlet oxygen with metal thiolates proceeds via an analogous mechanism. 13 It is striking that complex 1 (which undergoes intramolecular O-atom transfer) does posses two hydrogen atoms on the α carbon atom while complex 4 does not. However, no H/D exchange was observed at the methylene site adjacent to the sulfur atom during the photooxidation of 1 in D 2 O, in contrast with the photooxidation of organic sulfides which does proceed via a hydroperoxysulfonium ylide and concomitant H/D exchange at the methylene group adjacent to the sulfur atom.…”