Poster
# 55
Poster |
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6th Internet World Congress for Biomedical Sciences |
Bartolomé Quintero(1), María del Carmen Cabeza(2)
(1)(2)Dpt. Physical Chemistry. Faculty of Pharmacy. University of Granada - Granada. Spain
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[Biophysics] |
[Cell Biology & Cytology] |
PDQ degradates in a neutral aqueous medium. By keeping the samples in darkness, the process originates the appearance of the p-hydroxyphenyl radical both in aerobic and anaerobic conditions. However, the appearance of hydroquinone and quinone is observed after 24 hours only when the reaction is carried out in aerobic conditions. These data appears to point out a homolytic reaction induced by the solvent wherein oxygen plays a catalytic role since the heterolytic process leading to the aryl cation should have produce the appearance of hydroquinone in anaerobic conditions. Taking into account the ability of the aryl radicals to react with oxygen affording the corresponding peroxyl radicals, we have proposed the pathways outlined in Figure 6 in order to explain the results found in the analysis of the dediazoniation of PDQ. So, the primary reaction could be induced by hydroxyl anion giving the non-protonated p-hydroxyphenyl radical (-Oph·). Subsequent reaction with oxygen would lead to the corresponding peroxyl radical (-OphO2·). The transformation of this latter radical into the anion semiquinone (SQ-·) could take place via tetroxide as an intermediate. The dediazoniation rate would increase because of the presence of this reducing species since two additional pathways in the reduction of PDQ can be considered. One of them involving PDQ direct reduction to form quinone (Q) and the other pathway starting from the disproportionation of semiquinone to give quinone (Q) and hydroquinone (H2Q). We have checked that hydroquinone can efectively reduce PDQ.
The chelating agent DTPA appears to decrease the PDQ degradation process rate. In principle, chelation of reducing metal cation by DTPA would prevent a possible direct reduction of PDQ. This is mainly the expected effect in adding DTPA to the system, however no significant difference in the degradation rate was observed as a result of either the presence or the absence of DTPA in anaerobic conditions. The possibility of another involvement of the reducing metal ions in the redox cycles shown in Figure 6 appears apparentely ruled out. In fact, concentrations of ferrous ion up to 0.01 mM do not modify the PDQ degradation rate although a clear increase is observed following the addition of hydroquinone. Influence of other ions apart from Fe2+ has not been analyzed. Nevertheless, it would seem rather improbable that PDQ should be reduced by means of these ions via an independent degradation pathway since the quantity of reducing ions, mainly Fe2+, in the buffers used is very low [around 0.17 ppm for phosphate buffer (0.05 M), 0.35 ppm for acetate buffer (0.2 M) and 0.003 ppm for HCl solution at pH 1] and the DTPA concentration required to stop the degradation reaction in our experimental conditions is about 1.5 mM. Moreover, the presence of DTPA appears to be associated to a decrease in the oxygen consumption. The possible formation of a complex such as DTPA-Fe(II)-O2 observed in other system could reduce the PDQ degradation rate by disminishing the amount of the molecular oxygen avalaible for the formation of peroxyl radical. However, the formation of the above mentioned complex should be accompanied by an increase in the oxygen consumption. In addition, a possible interaction between DTPA and PDQ in the ground state blocking the reduction of this latter compound, appear also ruled out because of the absence of significant changes in the absorption spectrum of PDQ in the presence of DTPA.
It is known that hydroquinone can undergo an autooxidation reaction in aqueous medium. It has been reported that 2,3-dimethyl-1,4-naphthohydroquinone undergoes autoxidation to the corresponding quinone at pH 7.4, with stoichiometric consumption of oxygen and formation of hydrogen peroxide. In the presence of trace metals in the buffer the rate of oxidation was low, but it increased when trace metals were removed from the buffer by treatment with Chelex resin, DTPA or bathophenanthroline sulfonate [Munday, 1999]. Likewise it has been pointed out that DTPA does not affect the autooxidation of tetrachlorhydroquinone [Zhu et al., 1998]. Our results indicate the p-hydroquinone protolytic equilibria could be altered (and subsequently the autooxidation reaction) by the presence of DTPA. In Figure 5 is noticeable how in the presence of DTPA the absorption at 288 increases and simultaneously decreases about 311 nm. In addition, the spectrum registered with aqueous solution of hydroquinone 24 h after preparing the solution and in the absence of DTPA (Fig. 4) shows the appearance of an absorption band located about 244 nm likely originates by quinone. This band is clearly less intense as DTPA is present. Bearing in mind those results and the equilibria shown in Figure 7 a tentative explanation is proposed. .At pH 9 hydroquinone is partially dissociated (pK = 9.9) and the absorption about 311 nm could be attributed in part to HQ- (absorption maximum at 307 nm). The reaction of H2Q with molecular oxygen is very slow at neutral pH [Roginsky et al., 1999] therefore the increase in the reaction rate at pH 9 seems to be related with the faster oxidation of HQ-. We propose that the spectral changes observed in basic medium in the presence of DTPA are a consequence of the protonation of HQ- to give H2Q. So that, apparentely the influence of relatively high concentrations of DTPA appears to be related with the shift of the equilibrium in the system hydroquinone/semiquinone/quinone. Somehow, DTPA interfers as well at neutral pH decreasing the rate of appearance of quinone (Fig. 4) probably by decreasing the concentration of anion semiquinone. Admitting that the concentration of anion semiquinone is a critical factor in the analyzed processes, a decrease in the concentration of anion semiquinone would justify the observed decrease in the H2Q degradation rate in a neutral medium as well as the decrease in the PDQ degradation rate wherein is also assumed that semiquinone plays an essential role. We are currently making other experiences in order to check the points included in our hypothesis.
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[Biophysics] |
[Cell Biology & Cytology] |
