Biological systems, irradiation

Ion beam induced ablation is one of the most important electronic excitation effects [1,2]. Ablation phenomena occur both thermally and photochemically in many kinds of materials including polymers and biological systems irradiated by both ion beams and high power laser pulses. The mechanisms of ablation of polymers induced by high density electronic excitation have not been made clear yet. 

Due to the intensive investigations more is known about radiation chemistry of water than any other liquid (Buxton 1987). Many of the principles of radiation chemistry as applied to the liquid phase have been developed in the course of studies on the radiation chemistry of water and aqueous solutions. Water and aqueous solutions have been studied because of the part they play in chemistry in general and in radiochemistry in particular, because they are readily available and not too difficult to work with, and because water is a polar Uquid that responds in characteristic ways to radiation. A practical motivation for the studies has been the desire to understand the effect of radiation on biological systems. Irradiation of water and several aqueous systems is an important consideration in various aspects of nuclear technology (Swallow 1973). 

The hydroxyl radical plays two essentially different roles (a) as a reactant mediating the transformations of xenobiotics and (b) as a toxicant that damages DNA. They are important in a number of environments (1) in aquatic systems under irradiation, (2) in the troposphere, which is discussed later, and (3) in biological systems in the context of superoxide dismutase and the role of iron. Hydroxyl radicals in aqueous media can be generated by several mechanisms ... 

It is well known that pyrimidine bases convert to photodimers upon irradiation to UV light near the X max( > 270 nm). This photochemical reaction has a lethal effect in biological systems due to the photochemical transformation of pyrimidine bases of nucleic acids. However the photodimerization is a reversible reaction and the photodimers split to afford the original monomers very efficiently upon irradiation at a shorter wavelengths as shown in Scheme 1(1). 

So, fullerenes C60 and their derivatives may be potential damaging agents of biologic systems upon PDT, because upon irradiation C60 molecule is able to generate singlet oxygen, selectively penetrate tumor cells, and be accumulated there and could be easily removed from biologic medium. 

On many occasions, and this is certainly correct for 2-nitrobenzyl derivatives, photocleavable groups can be eliminated under mild conditions (aqueous media, neutral pH, and low temperature). Frequently, at a sufficiently long wavelength, only the bond designated for cleavage is affected by irradiation, so that, whenever the irradiation itself and the photoproducts formed are not harmful to the biological system (such as cells,... 

Recently it has been reported (3 ) that in a triad molecule where a porphyrin is juxtaposed between a carotenoid and a quinone, a charge transfer donor-acceptor pair with a lifetime similar to that found experimentally in biological systems was produced on light irradiation. It was suggested that an electrical potential similar to the type developed in this donor-acceptor pair may be important in driving the chemical reactions in natural photosynthesis. 

The repair of macromolecules and biological systems by electromagnetic irradiation. This process is wavelength dependent. An example is DNA repair by certain enzymes. 

The irradiation of water is immediately followed by a period of fast chemistry, whose short-time kinetics reflects the competition between the relaxation of the nonhomogeneous spatial distributions of the radiation-induced reactants and their reactions. A variety of gamma and energetic electron experiments are available in the literature. Stochastic simulation methods have been used to model the observed short-time radiation chemical kinetics of water and the radiation chemistry of aqueous solutions of scavengers for the hydrated electron and the hydroxyl radical to provide fundamental information for use in the elucidation of more complex, complicated chemical, and biological systems found in real-world scenarios. 

Figure 6 RBE variation of neutron beams as a function of energy. The p(65) + Be beam of Louvain-la-Neuve is taken as reference (RBE = 1). The closed squares and circles correspond to six different visited neutron facilities. The open squares and circles correspond to beams produced at the variable-energy cyclotron of Louvain-la-Neuve. On the abscissa, the effective energy of the neutron beams is expressed by their half-value thickness (HVT 5/15) measured in specified conditions. Intestinal crypt regeneration in mice, after a single fraction irradiation, is taken as the biological system. The 95% confidence intervals are shown. A straight line is fitted through the points (squares) corresponding to neutron beams produced by the (p + Be) reaction. For comparison, the neutron beams produced by the (d + Be) reaction are represented by circles. (From Ref. 22.)...

Figure 23 Variation of RBE as a function of depth in the carbon-ion beam used for clinical applications at HIMAC, Chiba, Japan (carbon-12, 290 MeV/u, SOBP 60 mm). The biological system is the well-codified intestinal crypt regeneration in mice. The selected criterion is 20 regenerating crypts per circumference after a single fraction irradiation. RBE determinations were performed at the beginning, middle, and end of the SOBP and at the level of the initial plateau. The dose-effect relationship for cobalt-60 is indicated for comparison. An estimation of the LET is presented for each depth where biological determinations were made. (From Gueulette, unpublished.)...

The mechanisms of protection by these compounds have been evaluated for irradiated polymers. Many so-called protectors contain sulfhy-dryl groups and appear to operate by replacing or capturing the hydrogen atom lost by irradiating the macromolecule. It appears most reasonable to assume mat the same mechanism often occurs in biological systems. 

A more reliable means of providing a reference of -OH in a biological system maybe by means of irradiation with ionizing radiation (von Sonntag et al. 2000). The action of ionizing radiation on an aqueous medium gives rise to OH whose yield/dose relationship (G value) is known (Chap. 2). Apart from this, since biological media are concentrated solutions the formation of the indicator product, e.g., a phenol (ArOH), via the direct effect [expressions (69) and (70)] must in principle be taken into account as well. It can be shown that with k4i [probe]/ k42 [cellular components] above 10 4 the direct effect contributes less than 10%... 

Ewing D, Powers EL (1980) Oxygen-dependent sensitization of irradiated cells. In Meyn RE, Withers FIR (eds) Radiation biology in cancer research. Raven Press, New York, pp 143-168 Fahey RC, Newton GL (1983) Occurrence of low molecular weight thiols in biological systems. In A.Larson et al. (eds) In Functions of glutathione. Biochemical, Physiological. Raven Press, New York, pp 251-260... 

Photocycloaddition of C-P4-C. The photochemistry of coumarin and its derivatives has been the subject of numerous investigations, mainly as a consequence of its importance in biological systems [119-127], Irradiation of 7,7 -polymethylene-di(oxycoumarin)s at low concentrations yields only syn head-to-head and head-to-tail intramolecular cyclomers. Steric factors introduced in the corresponding molecule by methyl groups at the 4-positions of the coumarins led predominantly the syn head-to-tail cyclomer (Fig. 12). At higher concentrations, the amounts of intermolecular photoproducts increase as expected. 

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