ACHARIYA ARTS AND SCIENCE COLLEGE (Affiliated to Pondicherry University)

Topics: Irradiation products of water & Fricke’s dosimetry

Gopi Krishna Giri

Irradiation products of water

• Indirect Ionization Effects

When the initial ionization event begins with water, to form free radicals, that cause a cascade of biological responses in macromolecules, the mechanism is collectively called an indirect effect.

• The primary mechanism of biological damage to macromolecules from ionizing radiation is an indirect interaction that begins with the radiolysis of water.

Radiolysis

• Ionization of water molecules cause them to split by a process called radiolysis.

• The event is a cascade of chemical transformations that result in the formation of free radicals.

• Free radicals are highly reactive particles that can indirectly harm the DNA causing "hit" that inactivates that cell.

Net products

• The net products of radiolysis of water molecules are the formation of highly reactive free radicals, namely

• hydrogen free radical (H˙), • hydroxyl free radical (OH˙). • hydroperoxyl radical (HO2˙).

These three free radicals are the results of ionization of water molecules and radiolysis.

• electron (e-) combines with water and forms the negative water molecule called "heavy water," a precursor to the hydroxyl radical (OH˙)

POTENTIAL OUTCOMES

The Fricke Dosimeter

• Originally developed as a dose-measuring device.

• Most useful as a method to directly measure the numbers of reactive species in solution.

Components

• The standard solution: 1 mM FeSO4 in 0.8 N H2SO4

• When irradiated, the Fe2+ is oxidized to Fe3+.

• Fe3+ generates a blue color that can be quantified with a spectrophotometer.

• The colorimetric dose response is linear up to 400 Gy.

• The oxidation is complete at ~ 700 Gy.

The chemical reactions involved in Fricke dosimetry

• H· + O2 → HO2· • HO2· + Fe2+ → HO2¯ + Fe3+ • HO2¯ + H+ → H2O2• HO· + Fe2+ → HO¯ + Fe3+• H2O2 + Fe2+ → HO¯ + Fe3+ + HO· • H· + H2O → HO· + H2 (only in the absence

of oxygen)

• Each H· will produce 3 Fe3+

• Each H2O2 (radiolytic) will produce 2 Fe3+

• Each HO· (radiolytic) will produce 1 Fe3+

Overall, when O2 is present,

G(Fe3+) = 2 G(H2O2) + 3 G(H·) + G(HO·)

Uses

• Demonstrates that scavenger molecules will react, cell components should react similarly.

• Can be used to test other competing scavengers.

• Demonstrates that not all energy deposition translates into scavengable species, i.e., at high LET there is considerable intratrack recombination.