Mechanistic examination of Cα -Cβ tyrosyl bond cleavage: Spectroscopic investigation of the generation of α-glycyl radical cations from tyrosyl (glycyl/alanyl)tryptophan

The reactivity of the radical cations of methyl 2,2-diphenylcyclohexyl ether (7), 6,6-diphenyl-1,4-dioxaspiro[4.5]decane (8), methyl cis- and trans-2-phenylcyclohexyl ether (9cis and trans), and 6-phenyl-1,4-dioxaspiro[4.5]decane (10), generated by photosensitized (electron transfer) irradiation, has been studied. Solutions of the ethers and acetals in acetonitrile–methanol (3:1), with 1,4-dicyanobenzene (2) serving as the electron acceptor, were irradiated with a medium-pressure mercury vapour lamp through Pyrex. The diphenyl derivatives 7 and 8 were reactive; 7 gave 6,6-diphenylhexanal dimethyl acetal (11) and 8 gave 2-methoxy-2-(5,5-diphenylpentyl)-1,3-dioxolane (12). These are the products expected from the intermediate 1,6-radical cation, formed upon carbon–carbon bond cleavage of the cyclic radical cation. The monophenyl derivatives 9cis and trans and 10 were stable under these irradiation conditions. The mechanism for the carbon–carbon bond cleavage and for the cis–trans isomerization is discussed. An explanation, based upon conformation, is offered for the lack of reactivity of 9 and 10. Molecular mechanics (MM2) calculations were used to determine the preferred conformation of 9cis and trans, and 10. Key words: photosensitization, electron transfer, radical cation, carbon–carbon bond cleavage, conformation.

Download Full-text

Theoretical examination of competitive β-radical-induced cleavages of N–Cα and Cα–C bonds of peptides

Canadian Journal of Chemistry ◽

10.1139/cjc-2015-0208 ◽

2015 ◽

Vol 93 (12) ◽

pp. 1355-1362 ◽

Cited By ~ 3

Author(s):

Wai-Kit Tang ◽

Chun-Ping Leong ◽

Qiang Hao ◽

Chi-Kit Siu

Keyword(s):

Amino Acid ◽

Density Functional ◽

Bond Cleavage ◽

Radical Cations ◽

Radical Center ◽

Amino Acid Residues ◽

Mobile Proton ◽

Aromatic Side Chain ◽

Heterolytic Cleavage ◽

Terminal Fragment

Selective cleavages of N–Cα and Cα–C bonds of β-radical tautomers of amino acid residues in radical peptides have been examined theoretically by means of the density functional theory at the M06-2X/6-311++G(d,p) level. The majority of the bond cleavages are homolytic via β-scission. Their energy barriers depend largely on the ability of the radical being stabilized in the transition structures and the availability of a mobile proton in the vicinity of the β-radical center. The N–Cα bond is less favorably cleaved than the Cα–C bond (except Ser and Thr) for systems without a mobile proton. It is because, firstly, the homolytic cleavage is less favorable for the more polar N–Cα bond than for the less polar Cα–C bond. Secondly, a less stable σ-radical localized on the amide nitrogen atom of the incipient N-terminal fragment is formed for the former, while a more stable radical delocalized in a π*(CO)-like orbital of the incipient C-terminal fragment is formed for the latter. In the presence of a mobile proton N-terminal to the β-radical center, some degrees of heterolytic cleavage character, as preferred by the polar N–Cα bond, are observed. Consequently, its barrier is reduced. If the mobile proton is located at the C-terminal amide oxygen of the β-radical center, the Cα–C bond cleavage will be significantly suppressed. It is because the radical in the incipient C-terminal fragment becomes more localized as a σ-radical on the carbon atom of its protonated amide group. With basic amino acid residues, the Cα–C bond cleavage can be reactivated. Heterolytic cleavage of the polar N–Cα bond can be largely facilitated if a mobile proton N-terminal to the β-radical center is available and the radical in the incipient C-terminal fragment is sufficiently stabilized, for instance, by the aromatic side chain of Trp and Tyr. Therefore, cleavages of the N–Cα bond induced by the β-radical tautomer of Trp and Tyr are often preferred as compared with cleavages of the Cα–C bond in peptide radical cations containing mobile protons.

Download Full-text

ChemInform Abstract: ESR-SPECTROSCOPIC INVESTIGATION OF RADICAL CATIONS, RADICAL ANIONS, AND RADICAL TRIANIONS OF DIDEHYDRO(N)ANNULENES

Chemischer Informationsdienst ◽

10.1002/chin.198546039 ◽

1985 ◽

Vol 16 (46) ◽

Author(s):

W. HUBER

Keyword(s):

Spectroscopic Investigation ◽

Radical Cations ◽

Radical Anions

Download Full-text

ChemInform Abstract: First Characterization of Zirconium Enolate Radical Cations in Solution and Their Mesolytic Bond Cleavage to Zirconocene Cations.

ChemInform ◽

10.1002/chin.199825068 ◽

2010 ◽

Vol 29 (25) ◽

pp. no-no

Author(s):

M. SCHMITTEL ◽

R. SOELLNER

Keyword(s):

Bond Cleavage ◽

Radical Cations

Download Full-text

The effect of meta- and para-methoxy substitution on the reactivity of the radical cations of arylalkenes and alkanes. Radical ions in photochemistry. Part 26

Canadian Journal of Chemistry ◽

10.1139/v91-124 ◽

1991 ◽

Vol 69 (5) ◽

pp. 839-852 ◽

Cited By ~ 20

Author(s):

Donald R. Arnold ◽

Xinyao Du ◽

Kerstin M. Henseleit

Keyword(s):

Electron Transfer ◽

Radical Cation ◽

Bond Cleavage ◽

Cation Radical ◽

Radical Cations ◽

Addition Product ◽

Molecular Orbital Calculations ◽

Carbon Carbon Bond ◽

Markovnikov Addition ◽

Cis And Trans

The effect of meta- and para-methoxy substitution on the reactivity of some radical cations has been determined. The compounds chosen for study were 1-(3-methoxyphenyl)-1-phenylethylene (7), 1-(4-methoxyphenyl)-1-phenylethylene (8), 3-(3-methoxyphenyl)indene (9), 3-(4-methoxyphenyl)indene (10), methyl 2-(3-methoxyphenyl)-2-phenylethyl ether (11), methyl 2-(4-methoxyphenyl)-2-phenylethyl ether (12), cis- and trans-2-methoxy-1-(3-methoxyphenyl)indane (13), and cis- and trans-2-methoxy-1-(4-methoxyphenyl)indane (14). The radical cations of these compounds were generated by photosensitization (electron transfer) using 1,4-dicyanobenzene (3) as the electron acceptor. The three reactions studied were: (1) The addition of nucleophiles (methanol) to the radical cation of the arylalkenes, a reaction that yields the anti-Markovnikov addition product. (2) The carbon–carbon bond cleavage of radical cations, which yields products derived from the radical and carbocation fragments. (3) The deprotonation of the radical cation, a reaction that can be used to invert the configuration at a saturated carbon centre. The mechanisms of these reactions are discussed and the factors that need to be considered in order to predict reactivity are defined. Molecular orbital calculations (UHF/STO-3G) were carried out on the radical cations of the model compounds 3- and 4-vinylanisole and 3- and 4-methylanisole. Key words: photochemistry, photosensitize (electron transfer), radical cation, radical.

Download Full-text