127605-72-5Relevant academic research and scientific papers
Peptide Peroxyl Radicals: Base-Induced O2*- Elimination versus Bimolecular Decay. A Pulse Radiolysis and Product Study
Mieden, Oliver J.,Schuchmann, Man Nien,Sonntag, Clemens von
, p. 3783 - 3790 (2007/10/02)
Radiolytically generated OH radicals react with the cyclic dipeptides glycine anhydride (1) and alanine anhydride (2), forming a single type of peptide radical in each case by abstracting a carbon-bound H atom at the ring.In the case of sarcosine anhydride (3), besides the C(3) or C(6) atoms (78percent), the H atoms at the N-methyl groups are also targets of the OH radical attack (22percent).In N2O/O2 (4:1 v/v) saturated solutions these peptide radicals add oxygen (k ca. 2*109 dm3 mol-1 s-1) to form the corresponding peroxyl radicals 6 (from 1), 7 (from 2), and 12 and 13 (from 3).The kinetics of O2*- elimination from the radicals 6 and 7 has been monitored by pulse radiolysis techniques.The pKa values of the peroxyl radicals 6 and 7 have been determined to be 10.8 and 11.2, respectively.The anions of these peroxyl radicals (6a and 7a) rapidly eliminate O2*- with the rate constants 1.6*105 and 3.7*106 s-1, respectively.In contrast, the spontaneous HO2* elimination reactions of the peroxyl radicals 6 and 7 are very slow, with rate constants of -1 as estimated from product analysis.The overall bimolecular decay rate constants of the cyclic dipeptide peroxyl radicals have been determined by pulse radiolysis measurements (2k(6) = 8.6*108 dm3 mol-1 s-1; 2k (7) = 1.6*108 dm3 mol-1 s-1; 2k(12/13) = 4.0*108 dm3 mol-1 s-1).The main products (G values) in the γ radiolysis of glycine anhydride in N2O/O2-saturated solution at pH 6 are 3-hydroxy-2,5-dioxopiperazine (14) (3.5*10-7 mol J-1), 2,3,5-trioxopiperazine (17) (1.6*10-7 mol J-1), and 2,5-dioxo-2,3,4,5-tetrahydropyrazine (8) (0.4*10-7 mol J-1).The product 8 is in equilibrium with its hydrate 14, which is again in equilibrium with its ring-opened form N-glyoxylyl glycinamide (16).In acidic solutions radical 6 decays essentially bimolecularly, giving equal amounts of 14 (present in an equilibrium mixture of 8, 14, and 16) and 17.In basic solution 14 becomes the single main product as the OH(-) -induced O2*- elimination reaction becomes much faster than the bimolecular decay.This is in good agreement with the yield of O2*- formation as monitored by its reaction with tetranitromethane under various pH conditions.With alanine anhydride (2), the main reaction under γ radiolysis conditions even at pH 6 is still the OH(-) -induced O2*- elimination reaction with 3-hydroxy-2,5-dioxo-3,6-dimethylpiperazine (15) as the single main product (G(15) = 4.7*10-7 mol J-1.The major peroxyl radical in sarcosine anhydride system (12) only decays bimolecularly either by self-termination or by cross-termination with the minor peroxy radical (13) (products at pH 6 in γ radiolysis: 2,3,5-trioxo-1,4-dimethylpiperazine (20), G = 4.0*10-7 mol J-1; 3-hydroxy-2,5-dioxo-1,4-dimethylpiperazine (21), G = 0.4*10-7 mol J-1; 1-formyl-2,5-dioxo-4-methylpiperazine (22), G = 0.6*10-7 mol J-1; and 2,5-dioxo-1-methylpiperazine (24), G = 0.6...
Oxidation of Cyclic Dipeptide Radicals in Aqueous Solution: The Rapid Hydration of the Intermediate 1,6-Dihydropyrazine-2,5-diones (Cyclic Dehydrodipeptides). A Pulse-radiolysis Study
Mieden, Oliver J.,Sonntag, Clemens von
, p. 2071 - 2078 (2007/10/02)
The lifetimes of radiolytically generated 1,6-dihyropyrazine-2,5-diones (cyclic dehydrodipeptides) in aqueous solution have been studied by pulse radiolysis.Hydroxyl radicals were allowed to react with the 2,5-dioxopiperazines glycine anhydride (1) and alanine anhydride (2).The resulting 3-yl radicals (4) and (5) have been oxidized by IrCl62- in a diffusion-controlled reaction 9; k(5)=3.0x109 dm3mol-1s-1>, which yielded the corresponding carbocations which rapidly deprotonate at N-4 (pKa-1> prevents the isomerisation of (6) and (7) into their tautomeric 2,5-dihydroxypyrazines.In acidic and basic solutions a proton-catalysed 6, k(7)=3.7x106 dm3mol-1s-1> and a base-catalysed 7; k(7)=2.0x105 dm3mol-1s-1> addition of water is observed.The 3-yl (8) (79percent) and N-methyl (9) (21percent) radicals resulting from H abstraction from sarcosine anhydride are also readily oxidized by IrCl62- (k=3.6x109 dm3mol-1s-1).Blocking of N-1 and N-4 by methyl groups prevents deprotonation of the resulting carbocations (12) and (14) and their reaction with water can be monitored directly by pulsed-conductivity measurements.The positive charge on the exocyclic carbocation (14) is effectively stabilized by nitrogen and thus addition of water proceeds only slowly -1>.However, in the case of the endocyclic carbocation (12), which is destabilized by the carbonyl group next to C-3, the reaction with water is fast 6 s-1>.The instability of (6) towards attack by water (possibly attended by a general sensitivity towards nucleophiles), may be the reason why no succesful synthesis of the tautomeric parent 2,5-dihydroxypyrazine has been reported so far.
