Abstract
The photooxidation of free guanosine and of guanine in the DNA double helix by thiopyronine wasstud iedus ing physico-chemical methods to gain further insight into the mechanism and structural results of the damage of the double helix. After excitation of the sensitizer the key reaction takes place between the oxidized free radical of sensitizer TP+ with G giving the unstable G+ leading to fission products of G and moreover to single and double strand breaks. The life-times of the triplet state (TP T) and TP+ increase by intercalation because the faster side reactions become prevented as has been ascertained by flash photolysis. The CD-spectra show no significant changes as it should be the case for certain conformational transitions. However, the mean molecular weight of DNA was found to decrease vs. irradiation time immediately and the flexibility increases. This increase of chain flexibility is also to be seen from the response of relaxation curves of melting and from the rise of peaks in the pulse polarogram. Moreover, at higher dose range one has to assume that an additional denaturation occurs. The oxidation potentials of some sensitizers and bases were determined. As it will be pointed out more clearly in part II the whole pathway from the excitation to the strand fission represents a redox chain-reaction.
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