Photochemical damage

Upon absorption of UV radiation from sunlight the bases can proceed through photochemical reactions that can lead to photodamage in the nucleic acids. Photochemical reactions do occur in the bases, with thymidine dimerization being a primary result, but at low rates. The bases are quite stable to photochemical damage, having efficient ways to dissipate the harmful electronic energy, as indicated by their ultrashort excited state lifetimes. It had been known for years that the excited states were short lived, and that fluorescence quantum yields are very low for all bases [4, 81, 82], Femtosecond laser spectroscopy has, in recent years, enabled a much... 

They are also potentially Interconvertible (5 ). Recent advances have allowed the Identification of IO2 in photochemical damage of biological systems by the use of quenchers W or D2O which enhances the lifetime of 02 (,]) Superoxide is detected in photc— oxidations by the use of the scavenging enzyme superoxlde dlsmutase (8). Important target molecules are proteins, lipids (especially cholesterol) and nucleic acids (, 9) but the... 

Fluorescent probes for microscopic evaluation of free intracellular Mg(II) should fulfil requirements such as adequate photochemical properties (excitation with laser-based instrumentation, high extinction coefficient and quantum yield, reduced interference from autofluorescence), low toxicity and low photochemical damage. Several Mg(II) microflu-orescent probes are in the market, for example, Mag-fura-2 (34a), Mag-indo-1 (35) and... 

That direct irradiation with ultraviolet light is damaging to single-cell organisms is well known. It also is known that the nucleic acids, DN A and RN A, are the important targets of photochemical damage, and this knowledge has stimulated... 

Fig. 7 An illustration of how molecules can enter cytosol after photochemical treatment (light first principle), (a) The photosensitizer (S) is endocytosed by the cells and when exposed to light the membranes of these vesicles will rupture and the contents (L) released into the cytosol, (b) Macromolecules that are administrated after the photochemical treatment will be endocytosed and end up in intact newly formed vesicles. These vesicles may then fuse with the photochemically damaged vesicles and the contents of the fused vesicles are released into the cytosol. The pictures at the bottom shows cells stained with fluorescently labelled dextran particles delivered before or after the photochemical treatment...

Figure 5.3 The spectral absorbance of macular pigment plotted with the blue light hazard function. (From Hammond, B.R. et al., Optom. Vis. Sci., 82(5), 387-404, 2005.) This function described the potential for photochemical damage to the retina resulting from exposure to light from about 400 to 500 nm as defined by the IESNA Photobiology Committee for ANSI (ANSI/ IESNA RP-27.1-05).

As an example, scheme i) gives a new transient Raman spectrum in which all the observed vibrational bands have the same rise time and the same enhancement profile. In scheme ii) all the new bands should have the same rise time but the relative band intensity of the new spectrum should change upon changing the probe laser frequency (if B and C have different optical absorption profiles.). Scheme iii) predicts changes in the relative intensity of the new bands with both the laser probe frequency as well as the time of delay between the photolysis and probe laser pulses. The difference between scheme iii) and iv) is that in iii) the bands of C and D could have different rise and decay times while in iv) they all should have similar rise times. Schemes iii) and v) are similar except that A in iii) disappears permanently upon laser exposure while in v) A regains its concentration and no permanent photochemical damage takes place. In scheme vi) the rise time of the vibrational bands of the (AB) transient (an excimer or an exciplex) should depend on the concentration of B. 

Nagai N., Zhao B. Q., Suzuki Y., Ihara H., Urano T., and Umemura K. (2002) Tissue-type plasminogen activator has paradoxical roles in focal cerebral ischemic injury by thrombotic middle cerebral artery occlusion with mild or severe photochemical damage in mice. J. Cereb. Blood Flow Metab. 22, 648-651. 

Photochemical damage occurs whenever fabrics are exposed to light of any strength or any type. Such damage, whether to fiber or dyes, is not easily recognizable during the course of daily maintenance since it occurs slowly. Once inflicted, however, the effect is often so severe that the fabric cannot be revitalized by conservation laboratory work. Therefore, fabrics must be stored in pitch darkness and their exposure to light restricted to study, conservation work, and limited exhibition only. 

The DNA bases involved in reproduction have short S excited state lifetimes of the order of one picosecond or less [13, 15, 19, 23, 73-75], It has been argued that this phenomenon serves to protect these bases against photochemical damage, because following excitation they do not cross to the reactive triplet state, but instead they rapidly internally convert to the electronic ground state [76], This may have been particularly significant under the conditions of the early earth when purines and pyrimidines presumably were assembled into the first macromolecular structures, producing RNA. 

With the exception of photodissociation to radical ions observed for retinol in polar solvents (144), cis-trans isomerization is the major photochemical transformation undergone by all forms of the free retinyl-polyene chromophore. [Unidentified photochemical damage has been reported to occur with very low quantum yields, e.g., 0.04 in the case of all-trans retinal (177).] We shall subsequently see that critical comparisons between the photochemical behavior of the biopigments and that of the opsin-free chromophore have led to the conclusion that the protein moiety plays a major role in governing the photochemical mechanism in rhodopsin (176). It is, therefore, natural that in parallel to spectroscopic and theoretical investigations, considerable attention has been devoted to the photoisomerization of model compounds, particularly to that of retinal isomers. 

Tucker, P. Kerr, N. Hersh, S. P. Photochemical Damage of Textiles presented at the 1980 Textile Preservation Symposium Textiles and Museum Lighting, Harpers Ferry Regional Textile Group, Washington, D.C., December 1980. 

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