EPR spectroscopic study of the radical oxidation of hydroxypurines in aqueous solution: acid–base properties of the derived radicals

Abstract

EPR spectroscopy has been employed to study the reactions of SO,'-and OH' radicals with xanthine, %methylxanthine, hypoxanthine and 2-hydroxypurine. from one-electron oxidation of either the parent compound or its anion, followed by deprotonation. For xanthine and 8-methylxanthine the respective radical anion and radical dianion were observed, for hypoxanthine the species detected correspond to the neutral radical and radical anion, whereas for 2-hydroxypurine only its radical anion was observed.12.1 k 0.5, respectively, whereas the pKa value of the hypoxanthine neutral radical was 7.7 k 0.1. Radicals obtained on the reaction of SO,'-with these hydroxypurines resulted, depending on the pH, The pKa values of the radical anion of xanthine and 8-methylxanthine were estimated as 12.0 k 0.1 and In the reaction with OH', only the radical anion of xanthine was detected. Paper 5/07893H

Structure and redox properties of radicals derived from one-electron oxidised methylxanthines

“…On the other hand, at pH 7 the radical species involved in these repair equilibria can be deprotonated, as demonstrated for the xanthine neutral radical which has a pK a value of 5.0. 9 The changes in the order of antioxidant capacities presented in Scheme 1 show that, in neutral medium, the following repair reactions occur (Eqs 6 and 7): 9-MeX •-+ 1-MeX → 9-MeX -+ 1-MeX •-+ H + Eq. 6 1,9-DiMeX • + 1-MeX → 1,9-DiMeX -+ 1-MeX •-+ 2H + Eq.…”

“…4,9,19 By in situ photolysis of an aqueous solution of 1-MeX or 3-MeX at pH values between 9 and 13 containing potassium persulphate, weak ESR spectra were obtained (Fig. 8).…”

“…In addition to the antioxidant effect of uric acid and its derivatives, it has been shown that xanthines have a significant antioxidant effect and can protect biological targets against damage by ROS. 9,10 In the present work, the antioxidant effect of several xanthine derivatives (some examples are shown in Table 1) is studied by evaluating their redox potential and their respective ability to reduce other radicals under equilibrium conditions, by using a strategy already applied for adenine and its derivatives. 11 Xanthine radical anions are also studied by ESR spectroscopy and the experimental results compared with those obtained by DFT calculations.…”

Oxidation of Purine Nucleotides by Triplet 3,3′,4,4′-Benzophenone Tetracarboxylic Acid in Aqueous Solution: pH-Dependence

“… 7 − 10 We found only limited examples of the detection of radicals derived from purines in solution by means of EPR. 11 , 12 In order to study the chemical reactivity of two purine nucleotides, adenosine-5′-monophosphate (AMP) and guanosine-5′-monophosphate (GMP), toward 3,3′,4,4′-benzophenone tetracarboxylic acid (TCBP) in the triplet-excited state in aqueous solutions of different pH at room temperature and to provide further information on the acid–base properties of radical intermediates observed in the photo-oxidation reactions of purines in aqueous solution, we have combined two techniques, namely the time-resolved laser flash photolysis (LFP) and the time-resolved chemically induced dynamic nuclear polarization (CIDNP). The CIDNP is the name given to the nonequilibrium nuclear spin state populations produced in chemical reactions that involve radical pair intermediates.…”

“…Unfortunately, optical techniques often suffer from insufficient spectral resolution and do not allow unambiguous identification of the radical intermediates of purine nucleotides, − while their direct EPR detection under physiological conditions is usually problematical because of very short lifetimes of such radicals. Most of the observations of the radicals derived from purine bases are performed by EPR techniques at low temperatures. − We found only limited examples of the detection of radicals derived from purines in solution by means of EPR. , In order to study the chemical reactivity of two purine nucleotides, adenosine-5′-monophosphate (AMP) and guanosine-5′-monophosphate (GMP), toward 3,3′,4,4′-benzophenone tetracarboxylic acid (TCBP) in the triplet-excited state in aqueous solutions of different pH at room temperature and to provide further information on the acid–base properties of radical intermediates observed in the photo-oxidation reactions of purines in aqueous solution, we have combined two techniques, namely the time-resolved laser flash photolysis (LFP) and the time-resolved chemically induced dynamic nuclear polarization (CIDNP). The CIDNP is the name given to the nonequilibrium nuclear spin state populations produced in chemical reactions that involve radical pair intermediates.…”