A Mass Spectrometric Approach for Probing the Stability of Bioorganic Radicals

Abstract

Glycyl radicals are important bioorganic radical species involved in enzymatic catalysis. Herein, we demonstrate that the stability of glycyl-type radicals (X-(.) CH-Y) can be tuned on a molecular level by varying the X and Y substituents and experimentally probed by mass spectrometry. This approach is based on the gas-phase dissociation of cysteine sulfinyl radical (X-Cys SO .-Y) ions through homolysis of a Cα Cβ bond. This fragmentation produces a glycyl-type radical upon losing CH2 SO, and the degree of this loss is closely tied to the stability of the as-formed radical. Theoretical calculations indicate that the energy of the Cα Cβ bond homolysis is predominantly affected by the stability of the glycyl radical product through the captodative effect, rather than that of the parent sulfinyl radical. This finding suggests a novel experimental method to probe the stability of bioorganic radicals, which can potentially broaden our understanding of these important reactive intermediates.

Radical Formation in the Gas‐Phase Ozonolysis of Deprotonated Cysteine

“…Ozonolysis of cysteine-containing peptides also yields abundant odd-electron [M-2H+O] •-ions (see Supporting Information Table S3). Similar sulfinyl radicals have been reported from radical-driven cleavage of disulfide linkedpeptides in the gas phase 12 and have also been detected as reactive intermediates in some biochemical transformations. 13 These labeling experiments together with known thermochemistry can be used to infer a mechanism for free radical formation in the ozonolysis of [Cys-H] -(Scheme 2).…”

Amino Acid Oxidation: A Combined Study of Cysteine Oxo Forms by IRMPD Spectroscopy and Simulations

“…Recently, a variety of mass spectrometric studies have been able to identify the reactive intermediates formed by (interfacial) ozonolysis of deprotonated cysteine at the air–liquid interface, to probe the stability and fragmentation behavior of intact and modified sulfinyl radical ions, and elucidate the intrinsic mechanism involved in the formation of cysteine oxo forms [cysSO x ] − ( x =1, 2, 3) and sulfenate radical anions in the gas phase . The unambiguous and detailed characterization of the structural and dynamic behavior of these transient intermediates in isolated environments, in which external interferences are absent, will help to reveal intrinsic properties relevant to their biological activity.…”

“…[14][15][16] These modifications,i nw hich sulfur becomes gradually less nucleophilic with increasing oxidation state, involve as ignificant fraction of protein sulfidrylsi nt he cell [10] and can increase functional diversity in proteins, which modulates their metal binding ability [17] and chemical behavior. [18] Recently,avariety of mass spectrometric studies have been able to identify the reactive intermediates formed by (interfacial) ozonolysis of deprotonated cysteineatthe air-liquid interface, [5,19] to probe the stability and fragmentation behavior of intact and modified sulfinyl radical ions, [20,21] and elucidate the intrinsic mechanism involved in the formationo fc ysteine oxo forms [cysSO x ] À (x = 1, 2, 3) and sulfenate radicala nions in the gas phase. [22] The unambiguous and detailed characterization of the structurala nd dynamic behavior of theset ransient intermediates in isolated environments, in which external interferences are absent,w ill help to reveali ntrinsic properties rele-vant to their biological activity.T he inherentb onding and conformationalf eatureso fc ysteine in its (de)protonated, [23,24] nitrosated, [25] and metal-tagged [26] forms have been previously interrogated both in the free amino acid and in the residue embodied in the glutathione peptide [27] by aj oint computational and experimental infrared multiple photon dissociation (IRMPD) assay.I RMPD spectroscopy,c oupled with ESI tandem mass spectrometry (MS/MS), is ap ivotalt ool to provide direct structural clues about gaseous (bio)molecular ions, [28][29][30][31][32][33] including (de)protonations ite, metal-binding patterns,a nd local intramolecular interactions in both native and modified amino acids and peptides, [34][35][36][37][38][39][40][41][42] DNA/RNA bases and nucleotides, …”

The Trifluoromethyl Sulfinyl and Oxathiyl Radicals

“…[4] The smaller sulfinyl radicals CH 3 SOC [5] and HSOC [6] play important roles in the atmospheric oxidation of volatile sulfur compounds such as dimethyl sulfide( DMS) and H 2 S. Very recently, the spectroscopy, [7] photochemistry, [7b] oxidation, [8] and electronic structures [9] of CH 3 SOC have been studied in detail. [4] The smaller sulfinyl radicals CH 3 SOC [5] and HSOC [6] play important roles in the atmospheric oxidation of volatile sulfur compounds such as dimethyl sulfide( DMS) and H 2 S. Very recently, the spectroscopy, [7] photochemistry, [7b] oxidation, [8] and electronic structures [9] of CH 3 SOC have been studied in detail.…”