Hisaaki Kudo

The formation and properties of the melatonin radical: a photolysis study of melatonin with 248 nm laser light

The photolysis of melatonin in aqueous solution has been studied spectrometrically with a 248 nm laser. The formation of hydrated electrons in a monophotonic process has been confirmed in neutral solution with a quantum yield of 0.22. Two main absorption bands at 340 and 460 nm plus an absorption shoulder resulted from the counterpart of the ejected electron, a melatonin radical, in solution. The big difference for the relative intensity of the absorption peaks under various pH conditions reveals that the melatonin radical exists in the solution through an acid-base equilibrium. In support from the pH dependence of the spectrum of the intermediate, the pKa1 for the doubly-protonated melatonin radical against the mono-protonated melatonin cation radical was estimated to be -0.95 and the pKa2 for the mono-protonated melatonin cation and melatonin neutral radical was 4.5 +/- 0.5. This work will benefit the basic understanding about melatonin as a UV-light protector, as a light receptor and the antioxidation functions of melatonin.

Laser photolysis study on the reaction of nitrate radical with tributylphosphate and its analogues—comparison with sulfate radical

Time resolved laser photolysis was carried out to study the reaction of NO3˙ radical toward a series of organic phosphates (tributylphosphate (TBP) and its analogues) with different length of side alkyl groups such as CH3, C2H5, C3H7, CH(CH3)2 and C4H9 in water, acetonitrile, and the mixtures of water and acetonitrile, respectively. For comparison, a similar investigation on SO4˙− radical was also conducted. The oxidation reaction of NO3˙ and SO4˙− radical with the phosphates was attributed to mainly an H-abstract reaction. The kinetics study shows that the rate constants of the reaction between the two radicals and phosphates increase remarkably with increasing of the length of the alkyl groups, e.g. in the case of NO3˙ radical in acetonitrile, the rate constant increases by almost 4 orders of magnitude from TMP (–CH3) to TBP (–C4H9). The composition of the solvent also greatly affects the reactivity but it exhibits an opposite effect of the two radicals, the rate constant of NO3˙ increases with the mole fraction of acetonitrile while that of SO4˙− decreases. The present study would be helpful for understanding the mechanism of deterioration of TBP in the Purex process.

Radiation: Types and Sources

This chapter describes what the radiation is; chemical action of radiation; historical developments of researches on radiation chemistry; and type, characteristics, and sources of radiations. Radiation is the flow of energetic electromagnetic wave or particle in the space. There are many kinds of radiations. The researches started from finding of X-ray by Roentgen in 1895, followed by many achievements by different researchers. The radiation can cause chemical/biological action/changes on systems irradiated. For example, water can be decomposed and gas evolution can be found; organic polymeric compounds can be cross-linked or degraded. This chapter provides introduction of the subsequent chapters. There are many kinds (types) of sources of radiation: natural (or artificial) radioisotopes and artificial radiation generators (particle accelerator). They are also briefly introduced.

Nuclear Engineering and Effects of Radiation

This chapter describes the relationship between radiation (or radiation chemistry) and nuclear engineering, and the importance of radiation chemistry in nuclear engineering. Water is used as coolant in current most widely operational nuclear power plants (NPP). Coolant is subjected to intense radiation and suffer the radiolysis. It triggers corrosion of structural materials of NPP and can lead to break/accident. Trials have been made to reduce corrosion in many plants of different countries. Examples are injection of H2 gas, adding of some chemicals into coolant water, etc. these trials, simulations and actual practices are introduced. Furthermore, some countries (including Japan) take policy to reprocess of spent fuel. The promised procedure is Purex (Plutonium Uranium Reduction Extraction) method, and it uses organic solvent such TBP (tri butyl phosphate) diluted in normal-dodecane (C12H26). Its radiation degradation is important issue and knowledge of radiation chemistry on organic compound is essential. Decomposition of and radiolysis products are described. In the terrestrial disposal of high level radioactive waste, the knowledge of radiation chemistry is essential because the interaction between groundwater and radioactive waste is anticipated. The under-ground environment is very complicated, regarding to pH pressure, temperature, etc.

Interactions Between Radiation and Matter

This chapter describes the interaction between radiation and substances/material. It depends on the type of radiation; electromagnetic wave (gamma ray or X-ray) or particle beams. However, in the case of gamma ray or X-ray, the interaction depends on energy; and in the case of particle beam, it depends on mass and electronic charge (negative or positive or neutral). In the former, microscopically the dominant phenomenon changes from photoelectric effect, Compton effect, to pair production with increasing energy. Macroscopically, its intensity decays exponentially with thickness of substance. In the latter, for the case of negatively or positively charged material, Colonm b interaction (scattering) is dominant, though the collision can occur with less probability; for the case of neutron, collision or capture is a dominant process. These schemes are explained.

Radiation Chemistry of Aqueous Solutions

This chapter describes the detail of radiation chemistry of water and aqueous solution. Aqueous solution of less concentration can be attributed as water solvent and solute. Water is, in a sense, one of the most important material, constituting coolant of nuclear reactor, biological bodies, etc. Radiation can ionize water molecule, leaving electron (where electron can be solvated); decompose water molecule into •H radical and •OH radicals. Consequently, hydrated electron, •H radical, and •OH radical are present in the solution. They diffuse, recombine, and react with each other or solute, and eventually final products such as H2O2 and H2 are formed. The yields of these chemical species, reaction rate constants, are dependent on many factors, such as pH, type of radiation, temperature, etc. Moreover, change of solute or biological effect can happen through the radiolysis of water or direct action of radiation. The former is called indirect action and the latter is called direct action. Dominance of the two processes also depends on the abovementioned factors. Change of solute can be applied to, for example, industrially, the treatment of wastewater. An example of purification of wastewater using radiation is introduced.

Radicals and ESR

This chapter describes radicals and ESR (electron spin resonance). Upon irradiation, short-lived chemical species such as electron, cation (positive ion), and excited states are produced. Subsequently, through the dissociation of bonds in compound, they can form radical, atoms, or molecules having unpaired electron. Radical is also short-lived, namely, reactive, in principle. Radicals can react in various ways. Radicals can be formed not only by radiation but also by other methods such as pyrolysis and photolysis. The detection method of radical is ESR. ESR uses electromagnets, and energy level of the unpaired electron splits into two stages under magnetic field, so-called Zeeman effect. The split width is proportional to the intensity outer magnetic field. Under such situation, if microwave is irradiated and if energy of the microwave is equal to the energy level, the microwave is absorbed. This is the resonance and coexistent nuclide; the absorption spectrum shows complicated pattern. The outlines of the radical and ESR are described.