Edwin E. Budzinski

Free radical-induced double lesions in DNA

This review surveys the work that has been done on free radical-induced DNA double lesions. Double lesions consist of two modifications of the DNA in close proximity. Double lesions can be generated by a single free radical-initiating event and the mechanism of formation often involves the participation of guanine. The identification of double lesions in oligomer and polymer DNA is reviewed and possible mechanisms of formation are outlined. The potential biological significance of double lesions is discussed. Double lesions induced by UV light are outside the scope of this review.

Free radical-induced tandem base damage in DNA oligomers.

A new tandem base lesion has been identified in two DNA oligomers, namely d(GpT) and d(CpGpTpA), exposed to X-irradiation in deoxygenated aqueous solution. In this lesion the C6 carbon atom of thymine is hydroxylated and a covalent link is formed between the C5 carbon atom of thymine and the C8 carbon atom of the adjacent guanine base. In addition, further evidence in the form of mass spectrometric data is presented confirming the structures of previously reported tandem base lesions that are produced by ionizing radiation in the presence of oxygen. New data is presented on the prevalence of a previously reported tandem base lesion in which the methyl carbon atom of thymine is covalently linked to the C8 carbon atom of the adjacent guanine base. The free radical-initiated processes by which tandem base damages are generated are discussed. To date four different radiation-induced tandem base lesion have been identified. The evidence suggests that tandem base damage is a significant component of free radical-induced DNA damage.

Free radical-induced double base lesions

Evidence is presented for the formation of products in irradiated dinucleoside monophosphates in which both bases are damaged. The dinucleoside monophosphates d(GpT), d(GpC), d(TpG) and d(CpG) were X-irradiated in oxygenated aqueous solution. Product identification was by NMR spectroscopy. In products containing double base lesions, guanine is converted to 8-hydroxyguanine and the pyrimidine base is degraded to a formamido remnant.

Tandem lesions and other products in X-irradiated DNA oligomers

Free radicals interact with DNA bases to produce secondary radicals. The secondary radicals are reactive species and tend to interact with neighboring bases, resulting in DNA lesions with two adjacent modified bases. In this study the DNA oligomers d(CpApTpG) and d(CpGpTpA) were exposed to free radicals generated in anoxic aqueous solution by X irradiation. Four new lesions were identified in which adjacent guanine and pyrimidine bases are covalently bonded. One of the tandem lesions formed in d(CpGpTpA) has the C5 carbon atom of cytosine covalently bonded to the C8 carbon atom of guanine. Interestingly, the same bond is formed between the terminal bases in d(CpApTpG), resulting in a cyclized molecule.

Primary radiation damage in thymidine

Several primary radiation products produced by ionizing radiation in the nucleoside thymidine have been stabilized at 4.2°K; two have been identified by ESR−ENDOR spectroscopy. One of these products is formed by reduction of the thymine ring. The other product results from the loss of a hydrogen atom from the 5′−hydroxy group.

Free radical formation in x‐irradiated histidine HCl

ENDOR spectroscopy was used to identify three primary radicals in single crystals of histidine hydrochloride x irradiated at 4.2°K. Oxidation leads to decarboxylation producing the radical R–CH2–CH–NH3+, where R stands for the protonated imidazole ring. Another oxidation product is produced by removal of an electron from the imidazole ring. The reduction process yields a carboxyl anion by addition of an electron to the carbonyl oxygen. ENDOR spectroscopy was also used to characterize more completely the final hydrogen‐adduct radical that is obtained on warming the crystals to room temperature.

Radiation-Induced Formation of a Crosslink between Base Moieties of Deoxyguanosine and Thymidine in Deoxygenated Solutions of d(CpGpTpA)

A new type of tandem base lesion has been observed in d(CpGpTpA) X-irradiated in aqueous solution. The lesion is attributed to the formation of a covalent bond between the C8 carbon atom of guanine and the methyl carbon atom of thymine. This tandem base lesion is formed in the absence of oxygen. It is the main product produced by ionizing radiation under these conditions.

Double Base Lesions in DNA X-Irradiated in the Presence or Absence of Oxygen

Abstract Box, H. C., Patrzyc, H. B., Dawidzik, J. B., Wallace, J. C., Freund, H. G., Iijima, H. and Budzinski, E. E. Double Base Lesions in DNA X-Irradiated in the Presence or Absence of Oxygen. Previously, double lesions in which two adjacent bases are modified were identified in DNA oligomers exposed in solution to ionizing radiation. However, the formation of such lesions in polymer DNA had not been demonstrated. Using reference oligomer containing a specific double lesion and employing liquid chromatography-mass spectrometry (LC-MS), it was possible to show directly that double lesions are formed in irradiated calf thymus DNA. The double lesion in which a pyrimidine base is degraded to a formamido remnant and an adjacent guanine base is oxidized to 8-oxoguanine was detected in DNA X-irradiated in oxygenated aqueous solution. The double lesion in which the methyl carbon atom of a thymine base is covalently linked to carbon at the 8-position of an adjacent guanine base was detected in DNA irradiated in a deoxygenated environment.

ENDOR Study of X‐Irradiated Single Crystals of Ice

ENDOR measurements were made on x‐irradiated single crystals of hexagonal ice. The ENDOR frequencies yield the hyperfine coupling for the OH radicals produced in irradiated ice. The method is capable of distinguishing between OH radicals produced in three different environments. The principal values of the hyperfine coupling tensors in megacycles per second of the three distinct OH radicals are as follows: AxxAyyAzz(I)− 80.0− 124.4+ 9.2(II)− 81.2− 124.4+ 19.4(III)− 78.2− 126.2+ 15.6 Using ESR spectra in conjunction with ENDOR data, the g tensors were also determined with the following results: gxxgyygzz(I)2.00282.00892.0597(II)2.00312.00892.0571(III)2.00272.00882.0581

Characteristics of trapped electrons and electron traps in single crystals

Two additional carbohydrates are reported whose crystal structures trap electrons intermolecularly in single crystals x irradiated at low temperature, namely sucrose and rhamnose. Five carbohydrate and polyhydroxy compounds are now known which exhibit this phenomenon. The following characteristics of the phenomenon were investigated: (1) the hyperfine couplings of the electron with protons of the polarized hydroxy groups forming the trap; (2) the distances between these protons and the trapped electron; (3) the spin density of the electron at the protons and (4) the relative stabilities of the electron trapped in various crystal structures.