Harold G. Freund

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.

Paramagnetic Absorption of Single Crystals of Succinic Acid Irradiated at Low Temperature

Electron spin resonance absorption was observed in single crystals of (–CH2–COOH)2, (–CH2–;COOD)2, and (–CH2–13COOH)2 irradiated at 77°K. The principal paramagnetic species observed at low temperature has an unpaired electron in a 2pπ orbital on the carbon atom of the carboxyl group. It is probably ionized succinic acid with the oxygen of the carboxyl group carrying a negative charge.

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

Conformations of the Free Radical in Irradiated α‐Aminoisobutyric Acid

The electron paramagnetic resonance spectrum of x‐irradiated α‐aminoisobutyric acid was studied in single crystals irradiated at 77°K and allowed to warm up gradually. Different conformations of the free‐radical (CH3)2CCOOH were observed. The g tensor and the hyperfine splitting tensor were determined for each conformation.

Trapped electrons in irradiated single crystals of polyhydroxy compounds

The intermolecular trapping of electrons has been observed in single crystals of dulcitol and L(+) arabinose x‐irradiated at 4.2 °K. Attribution of a major component of the ESR absorption to trapped electrons is based upon the character of the hyperfine pattern, which arises from multiple anisotropic hyperfine interactions with exchangeable protons, and on the g value of the absorption, which is always less than the free spin value. The removal of the trapped electron absorption upon irradiation with visible light has also been demonstrated. In these experiments all of the electrons are trapped in identical sites. This circumstance provides some important advantages in the study of the factors affecting the stabilization of charge in an environment of polarizable molecules.

Vicinal Lesions in X-irradiated DNA?

Irradiation of the dinucleoside monophosphate d(GpT) in an oxygenated solution gives products characterized by damage on one or both guanine and thymine bases, the yields of which were proportional to radiation dose.

ENDOR Study of Irradiated Histidine HCl

Free‐radical formation was studied in x‐irradiated single crystals of histidine HCl using conventional electron spin resonance (ESR) as well as double‐resonance (ENDOR) spectroscopy. The spectra are attributed to a free radical formed by the addition of hydrogen to the C2 position on the imidazole ring of histidine.

Radiation effects on amino acids: valine.

Single crystals of dl‐valine were x‐irradiated at liquid‐nitrogen temperature. The ESR absorption spectrum was observed as the crystal warmed. After warming to 225°K, a definitive spectrum was obtained which is attributed to the formation of the free radical (CH3)2CHCHCOOH. A transformation of the absorption occurs at room temperature due to the formation of the free radical (CH3)2CCH(NH3+)COO−. This radical was examined by ESR and ENDOR spectroscopy.