Indolyl Radical Research Articles

Unpaired electron migration between aromatic and sulfur peptide units.

Cysteine thiyl radicals (Cys/S.) were found capable of one-electron oxidation of tyrosine. Equilibration occurred, using Cys and Gly-Tyr, with an equilibrium constant of K5 = 20 +/- 4 at pH 9.15: Cys/S. + Tyr in equilibrium Cys + Tyr/O. (5) Hence the reduction potentials of these couples differ at pH 9.15 by E(Cys/S., Cys) - E(Tyr/O., Tyr) = 80 mV. Oxidation of Trp-Gly by Cys/S. was not detectable from pH 7 to 12. The methionyl radical cation (Met/S.N), formed via .OH-attack on Met-Gly, reacts with Trp-Gly to generate the indolyl radical (Trp/N.). New results on intramolecular Trp/N.----Tyr/O. transitions indicate that the reaction requires direct contact between the two redox centers. Various possible pathways for migration of unpaired electrons between peptide units are compiled in a scheme.

Synthesis and properties of azoles and their derivatives. 36. Synthesis of 2-amino-1,3,4-oxadiazoles, 2-amino-1,3,4-thiadiazoles, and 1,2,4-triazoline-3-thiones containing indolyl radicals

On the heterocyclization of 1-(indol-3-ylcarbonyl)thiosemicarbazides, depending on the conditions of performing the reaction, either 1,2,4-triazoline-3-thiones, 2-amino-1,3,4-thiadiazoles, or 2-amino-1,3,4-oxadiazoles containing an indolyl residue in position 5 are obtained. 1,2,4-Triazoline-3-thiones and 2-amino-1,3, 4-thiadazoles have also been synthesized by the reaction of the hydrochloride of ethyl indol-3-carboximidate with thiosemicarbazides.

ChemInform Abstract: Synthesis and Properties of Azoles and Their Derivatives. Part 33. Synthesis ofl,3,4‐Oxadiazoles Containing Indolyl Radical.

AbstractDurch Umsetzung von Indolylcarbonsäureiminoesterhydrochloriden (I) mit Hydraziden (II) oder auch durch Umsetzung von Indolylcarbonsäurehydraziden (IV) mit Iminoestern (V) werden Indo‐ 74 lylalkyl‐oxadiazole (III) hergestellt.

Charge transfer in peptides. Effects of temperature, peptide length and solvent conditions upon intramolecular one-electron reactions involving tryptophan and tyrosine

Tryptophyl-tyrosyl peptides, TrpH-(Gly)n-TyrOH and cyclic c-TrpH-TyrOH, were oxidised selectively at the indole group with electron accepting radicals (N˙3 or Br˙–2) in aqueous solution, using pulse radiolysis techniques. Subsequent to the primary oxidation an efficient synchronous transformation of indolyl into phenoxyl radicals occurred, both in the linear peptides (n= 0, 1 and 2), [graphic omitted] and in c-TrpH-TyrOH. Rate constants of the reactions [1(n)], obtained by kinetic absorption spectroscopy, follow an empirical inverse-square distance relationship, k1(n)/k1(0)= 1/(1 +n2/2), k1(0)= 7.3 × 104 s–1. Thermal activation of reaction [1(0)] was observed with an apparent activation energy of 0.22 eV. The addition of 1.5 mol dm–3 urea (a modifier of hydrogen-bond structures) or of 60 mmol dm–3 SDS (a detergent shielding the hydrophobic indole and phenol group) did not affect reaction [1(0)]. Deprotonation of the OH-group of TyrOH (pKa= 10.1) and protonation of the indolyl radical (pKa= 4.3) efficiently enhanced the radical transformation rates (k1(0) > 106 s–1 at pH 11 and k1(0)≈k1(2) > 2 × 105 s–1 at pH = 4). Intermolecular reactions between Trp and TyrOH were comparatively slow in neutral as well as in acid or alkaline solution. The low activation energy suggests that the observed intramolecular radical transformations occur by an electron-tunnelling mechanism. The (tentative) inverse square relationship and the pH effects indicate, on the other hand, that electronic (inductive) interactions between the reactants are involved. Selective one-electron oxidation of Cys-bis-TyrOH, His-TyrOH and Ser-TyrOH-βNA at the tyrosyl residue, and of Dansyl-TyrOH at the dansyl-group, did not induce comparable charge-transfer reactions.

Charge Transfer in Peptides

One-electron oxidation of TyrOH-TrpH or TrpH-TyrOH in aqueous solutions by N3 radicals occurs predominantly at the tryptophyl residue. The corresponding indolyl radicals (absorbing at 510 nm) are subsequently transformed into phenoxyl radicals (absorbing at 390/405 nm): TyrOH-Trp leads to TyrO-TrpH, k5 = 5.4 x 10(4)s-1, (5), Trp-TyrOH leads to TrpH-TyrO, k7 = 7.3 x 10(4)s-1. (7) The first-order radical transformation rates are independent of the (initial) concentration of N3 or peptide and unaffected by urea (as a modifier of hydrogen bond structures). Intermolecular conversion of indolyl into phenoxyl radicals, e.g. by reaction of GlyH-Trp with TyrOH-GlyH, is very slow and inefficient. It is concluded that reactions (5) and (7) occur by intramolecular charge transfer across the peptide bond.

Theoretical analysis of the photophysical properties of indole, the indolyl radical, and the indole radical cation

Theoretical analysis of the photophysical properties of indole, the indolyl radical, and the indole radical cation

INTERMEDIATES IN THE PHOTOLYSIS OF INDOLE: ROOM TEMPERATURE AQUEOUS SOLUTIONS AND LOW TEMPERATURE GLASSES

Abstract— Flash photolysis of indole aqueous solutions produces four transients with absorption maxima, in their order of appearance in time, at 720, 370, 380 and 520 nm. These correlate with low temperature photolysis observations. The results are interpreted in terms of electron‐radical cation generation and subsequent production of the indolyl radical.

Reactions of halogenometalalkoxides

Investigation of the thermal stability of halogenometalalkoxides obtained by reacting Grignard compounds with esters of oxalic acid and containing an indolyl group, is continued. Despite the aromatic nature of the indolyl radical, the ordinary decomposition of the halogenomagnesiumalkoxides with splitting off of aldehyde (ketone) does not take place at 40‡–110‡ C. This is due to hydrogen bonding between hydrogen on the nitrogen and the alkoxy group. The IR spectra of the esters of N-indolyl- and α-indolylglyoxylic acids synthesized are investigated. The latter are shown to contain a hydrogen bond between the NH group hydrogen and the carbonyl group.