title: The cysteine radical cation: structures and fragmentation pathways.
abstract: A theoretical study on the structures, relative energies, isomerization reactions and fragmentation pathways of the cysteine radical cation, [NH(2)CH(CH(2)SH)COOH].+, is reported. Hybrid density functional theory (B3LYP) has been used in conjunction with the 6-311++G(d,p) basis set. The isomer at the global minimum, Captodative-1, has the structure NH(2)C.(CH(2)SH)C(OH)(2)+; the stability of this ion is attributed to the captodative effect in which the NH(2) functions as a powerful pi-electron donor and C(OH)(2)+ as a powerful pi-electron acceptor. Ion Distonic-S-1, H(3)N(+)CH(CH(2)S.)COOH, in which the radical is formally situated on the S atom, is higher in enthalpy (DeltaH degrees (0)) than Captodative-1 by 6.1 kcal mol(-1), but is lower in enthalpy than another isomer Distonic-C-1, H(3)N(+)C.(CH(2)SH)COOH, by 8.2 kcal mol(-1). Isomerization of the canonical radical cation of cysteine, [H(2)NCH(CH(2)SH)COOH].+, (Canonical-1), to Captodative-1 has an enthalpy of activation of 25.8 kcal mol(-1), while the barrier against isomerization of Canonical-1 to Distonic-S-1 is only 9.6 kcal mol(-1). Two additional transient tautomers, one with the radical located at C(alpha) and the charge on SH(2), and the other a carboxy radical with the charge on NH(3), are reported. Plausible fragmentation pathways (losses of small molecules, CO(2), CH(2)S, H(2)S and NH(3), and neutral radicals COOH. , HSCH(2). and NH(2).) from Canonical-1 are examined.
authors.affiliation: Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario, Canada.
authors.forename: Junfang
authors.initials: J
authors.lastname: Zhao
authors.forename: K W Michael
authors.initials: KW
authors.lastname: Siu
authors.forename: Alan C
authors.initials: AC
authors.lastname: Hopkinson
mesh_terms: D001665:Binding Sites; D002412:Cations; D003198:Computer Simulation; D003545:Cysteine / Q000737:chemistry* / Q000378:metabolism; D004583:Electrons; D004735:Energy Transfer; D005609:Free Radicals / Q000737:chemistry / Q000378:metabolism; D006860:Hydrogen Bonding; D007536:Isomerism; D008958:Models, Molecular; D011789:Quantum Theory; D050052:TRPC Cation Channels / Q000737:chemistry* / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D002412:Cations; D005609:Free Radicals; D050052:TRPC Cation Channels; D003545:Cysteine