DNA damage induced by secondary electrons and x-rays
X-rays are widely used in medical diagnosis and radiotherapy. The effects of X-rays on cells are dominated by their actions on the cellular DNA. The interaction of X-rays with DNA produces photoelectrons, Auger electrons, Compton electrons and large quantities of secondary electrons (SE), the majority of which have energies lower than 70 eV. This thesis aims to compare the DNA damage induced by SE and X-rays. A dosimetry method to measure the absorbed dose of Al[subscript k alpha] X-rays (1.5 keV) in ultrahigh vacuum chamber (UHV) using radiochromatic dosimetry films and a color scanner was developed. This provided the value for the absorbed dose in dry biological samples irradiated under UHV, allowing direct comparison of DNA damage induced by Al[subscript k alpha] X-rays and SE. The induction of covalent interduplex DNA crosslinks (CL) by X-rays was detected using restriction enzyme digestion and alkaline agarose gel electrophoresis. The G value for CL induction in DNA in UHV by Al[subscript k alpha] X-rays is 16 « 4 nmol J[superscript -1], 2.8 fold that of double strand breaks (DSB). G(CL) in air by soft X-rays (14.8 keV) is 3 « 1([gamma] [approximate] 6, [gamma] is defined as the number of water molecules per nucleotide,) and 3.2 « 0.4 ([gamma] [approximate] 21) nmol J[superscript -1], 61 and 89% of those for DSB. These results suggest that CL is an important type of DNA damage, possibly as important as DSB in radiobiology. A new method of studying SE-induced DNA damage in comparison to Al[subscript k alpha] X-rays under identical experimental conditions was developed, using thin and thick films of DNA deposited on tantalum. X-ray induced SE emission from the tantalum serves as a source of SE in UHV. The lower limits of G values for DNA SSB and DSB induced by SE were derived to be: 86 « 2 and 8 « 2 nmol J[superscript -1], respectively. These values are 1.5 and 1.6 times larger than those obtained with 1.5 keV photons. The enhanced DNA damage by SE emission from tantalum exposed to soft X-rays (14.8 keV) in air was studied as a function of DNA hydration levels. The enhancement factor for SSB, DSB and CL induced by SE vs X-rays were derived to be: 5.0 « 0.8, 3.6 « 1.0 and 9 « 4 at [gamma] [approximate] 6, and 8 « 1, 9 « 2 and 5.0 « 1.4 at [gamma] [approximate] 21, respectively. This study provides a molecular basis for understanding the enhanced interface biological effects during X-ray diagnostic examination and radiotherapy. A self-assembled monolayer (SAM) of a 5'-32 P-labelled 3'-thiolated oligonucleotide chemisorbed on gold was bombarded by low energy electrons (LEE) of 8 to 68 eV."--Résumé abrégé par UMI.