The Biophysics Research Program at ETSU


Radiation Damage to DNA:

DNA is known to be the critical cellular target for the damaging effects of ionizing radiation. Radiation damage to DNA can be initiated by both direct and indirect effects. Direct effects involve the formation of ionic, radical and excited intermediates as a result of the deposition of energy within the DNA. Indirect effects involve the interaction of the DNA with water radiolysis products such as OH radicals, H-atoms or hydrated electrons.

Radiation products, in order to be biologically effective, must have life times at room temperature long enough to perturb the biological system. Such products are among the terminal events in a sequence of events initiated by the ionizing radiation. The initial events in the sequence involve the production of free radicals. Subsequent events involve the reactions and decay of free radical intermediates to more stable non-radical products. This sequence of events can be experimentally determined by irradiating the sample at temperatures low enough to trap the initial products (usually 4 K or 77 K) and observing the changes in these products as the sample is warmed.

When possible single crystals are used because the structural information about the free radical which can be obtained is maximized. The single most sensitive technique for studying free radicals is electron spin resonance (ESR). My research involves the use of ESR and Electron Nuclear Double Resonance (ENDOR) spectroscopy to study the direct effects of radiation damage to nucleic acid constituents.


Collaborations:

A major portion of this research involves a collaborative effort with William Nelson at Georgia State University, and Einar Sagsuten and Eli Hole at the University of Oslo. This research is supported by PHS Grant CA 36810-15, awarded by the National Cancer Institute, DHHS.


Here we are at the 8th International Congress of Radiation Research in Edinburgh. From left to right, Bill Nelson, me, Bill and Pat Bernhard, Eli Hole and Einar Sagstuen.


Review Article:

A good summary of the scope of my research is contained in a recent review article "Radical Ions and Their reactions in DNA Constituents: ESR/ENDOR Studies of Radiation Damage in the Solid State", by David M. Close, Radiation Research 135, 1-15 (1993). This article summarizes most of the work that I've done on the radiation chemistry of nucleic acids.

A list of my Most recent publications or All publications can be found at this site.


Facilities:


EPR Spectroscopy:

The EPR laboratory is located in the basement of Brown Hall, Rm B-24. The equipment shown in the photograph below consists of a Varian E-line EPR X-band spectrometer and a 15" Magnion magnet. The original magnet field control has been replaced with a Bruker BH-15 field control unit.

Microwave accessories consists of a dual Varian rectangular cavity, the Varian variable temperature controller, and a Varian E-233 rotating circular cavity.

Equipment for doing ENDOR consists of a Wavetek Model 3000 signal generator and an ENI 350 L rf power amplifier. The Wavetek is frequency swept with a homemade digital programer.

Data acquisition is under the control of a computer interfaced to the BH-15 and to the output of the spectrometer. All software for data acquisition and post manipulation was developed locally.


Recent photograph of the Varian E-line EPR spectrometer with 15" magnet.


X-ray diffraction:

The Crystallographic Laboratory has an Enraf Diffractis 601 x-ray power supply. The normal x-ray tube in use has a molybdenum target, most suited for single crystals of organic compounds. The is a copper target also available.

Most diffraction experiments are done on a Buerger camera equipped with a Polaroid 4x5" film holder.


Recent photograph of the Buerger Precession x-ray camera in the Crystallographic Laboratory at ETSU


How would you like to live in Looking-glass House, Kitty? I wonder if they'd give you milk, there? Perhaps Looking-glass milk isn't good to drink--but oh, Kitty! now we come to the passage. You can just see a little peep of the passage in the Looking-glass House, if you leave the door of our passage as far as you can see, only you know it may be quite different beyond.

Oh, Kitty! how nice it would be if we could only get through into the Looking-glass House! I'm sure it's got, oh! such beautiful things in it! Let's pretend the glass has got soft like qauze, so that we can get through. Why, its turning into a sort of mist now, I declare! It'll be easy enough to get through---". She was up on the chimney-piece while she said this, though she hardly knew how she had got there. And certainly the glass was beginning to melt away, just like a bright silvery mist.

Through the Looking Glass and What Alice Found There, Lewis Carroll , 1871.


X-ray irradiation:

The ESR laboratory has a Phillips XRG-2500 x-ray power supply that supplies 50 kV at 30 ma to a side window Phillips x-ray tube. This apparatus is used to x-irradiate samples.


Other activities:

Part of the research project on radiation damage involves the identification of free radicals. Often there are several possible free radical models to choose from.. To help decide which radical model is the most likely choice, molecular orbital calculations are often performed. Two types of calculations are used.

Semi-empirical SCF calculations are done either at the INDO level of approximation, or with the MOPAC and AMPAC packages on the departments SUN 4 graphics workstation. We also have the ability to do ab initio SCF calculations using the Gaussian 98 program on several Pentium IV workstations (Dell Precision 530) running LINUX.


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