Riboflavin photosensitization

A comparison of the photodynamic properties of H2(4-TRPyP) and its zinc metalated derivative, Zn(4-TRPyP) with the methylene blue and riboflavin photosensitizers also was carried out (240) using 2 -deoxyguanosine as a model compound (169, 241, 242). Riboflavin is a typical type 1 photosensitizer, while methylene blue exhibits a type If behavior. The selectivity measured by the ratio of the amount of photoproducts generated by type Il/type I mechanisms was 0.4 for riboflavin, and 2.3,3.6, and 5.6 for H2 TRPyP, methylene blue, andZn(4-TRPyP), respectively, showing that Zn(4-TRPyP) is the most specific type If photosensitizer of the series. 

H Kutsuki, A Enoki, and MH Gold. Riboflavin-Photosensitized Oxidative Degradation of a Variety of Lignin Model Compounds. Photochem and Photobiol 32 1-7, 1983. 

Bachem, A. (1956) Opthalmic ultraviolet action spectra, Am. J. Opthalmol., 41, 969-975. Baldursdottir, S.G., Kjpniksen, A.-L., Karlsen, J., Nystrpm, B., Roots, J., and Tpnnesen, H.H. (2003a) Riboflavin-photosensitized changes in aqueous solutions of alginate. Rheological studies, Biomacromol., 4, 429 -36. 

Unsaturated bonds in fatty acids can undergo riboflavin-photosensitized cis-trans isomerization. Lipid oxidation is also an important reaction which is catalysed by riboflavin and light and, together with the deamination of methionine to form methanal and dimethyl sulfide, it plays a major role in the development of light-induced oflf-flavour in milk. The demise of the home delivery of milk in glass bottles and the employment of alternative package materials, such as paper carton, reduced the incidence of sunlight-flavoured milk. 

Destruction of vitamins A, C, D and E induced by riboflavin-photosensitized oxidation has been reported. Vitamin A and its esters, along with carotenoids with pro-vitamin A activity, undergo ring opening upon sunlight exposure in the presence of riboflavin. Although an excellent antioxidant, ascorbic acid is rapidly photooxidized in the presence of riboflavin. Even a small decrease in riboflavin content in milk due to photosensitization can lead to virtual complete destruction of ascorbic acid. Fortunately, milk is not an important source of vitamin C in most diets. 

Riboflavin is heat-stable in the absence of light, but extremely photosensitive. It has a high degree of natural fluorescence when excited by UV light. This property can be used for detection and determination. Two coenzymes (Fig. 2), flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), are derived from riboflavin. 

Lu, C. Lin, W. Wang, W. Han, Z. Yao, S. Lin, N. (2000). Riboflavin-(VB2) photosensitized oxidation of 2 -deoxyguanosine-5 -mono-phosphate (dGMP) in aqueous solution A transient intermediates study. Physical Chemistry Chemical Physics, Vol.2, 0anuary 2000), pp.329-334, ISSN 1463-9076. 

Various dyes can be used as photosensitizers, including methylene blue, riboflavine, and hematoporphyrin derivative. The selection of the photosensitizer should be in favor of a compound that exclusively leads to Reaction (b), so that a clear interpretation of the results is possible. 

Figure 1 shows the graphs of the PCL that were recorded with riboflavin as the photosensitizer and luminol as the detector for free radicals [21], The course of the PCL reaction has two maxima at approximately 30 s and 3 min after the start of irradiation. It has been demonstrated by analysis of kinetics after addition of the reactants at varying times that the first maximum is riboflavin-dependent. Luminol is needed only for visualization of the superoxide radicals. 

The second maximum is riboflavin-independent (Fig. 1). In this case, luminol obviously plays a double role it is the chemiluminogenous detection compound for free radicals and photosensitizer as well. It is a remarkable characteristic of this system that the signal intensity decreases only very slowly, giving an opportunity for detection of nonenzymatic antioxidants. 

Figure 1 PCL graphs with riboflavin (R) as the photosensitizer and luminol (L) as the detector for free radicals. I = start of irradiation. (From Ref. 21.)...

Figure 2 Chemical mechanisms of superoxide-dependent PCL with riboflavin as photosensitizer. (From Ref. 22.)...

Infusion solutions of pyridoxine (274) hydrochloride were unaffected by ward lighting, but when riboflavine phosphate sodium was added the pyridoxine was completely decomposed in about 3 h. The photosensitized oxidation gave... 

Ritodrine (378) hydrochloride was also stable in infusion solutions containing glucose, but in the presence of riboflavin it photo-oxidized to 77-hydroxybenz-aldehyde [220]. See also the photosensitized reaction of pyridoxine. 

This occurs by a mechanism called static photosensitization, analogous to that followed by biologically acting photosensitizers like riboflavin. 

Larson et al. (1992) studied the photosensitizing ability of 2, 3, 4, 5 -tetraacetylriboflavin to various organic compounds. An aqueous solution containing aniline was subjected to a medium-pressure mercury arc lamp (X >290 nm). The investigators reported that 2, 3, 4, 5 -tetraacetylribofiavin was superior to another photosensitizer, namely riboflavin, in degrading aniline. Direct photolysis of aniline without any photosensitizer present resulted in a half-life of 23 h. In the presence of riboflavin and 2, 3, 4, 5 -tetraacetylribofiavin, the half-lives were 1 min and 45 sec, respectively. Photoproducts identified in both reactions were azobenzene, phenazine, and azoxybenzene. 

Larson. R.A.. Stackhouse. P.L., and Crowley. T.O. Riboflavin tetraacetate a potentially useful photosensitizing agent for the treatment of contaminated waters. Environ. Sci TechnoL, 26(9) 1792-1798, 1992. 

Riboflavin (CCLV) is photosensitive on irradiation in alkaline solution, it is converted into lumiflavin (CCLVI)302 and in neutral or acid solution, lumichrome (CCLVII) is produced.147 The photolysis of 9-(2 -hydroxyethyl)isoalloxazine (CCLVIII) is also a general aeid- and base-catalyzed reaction. 

Irradiation of olefins and dienes in the presence of oxygen and various sensitizers produces hydroperoxides and cyclic peroxides. Effective sensitizers include not only high-energy carbonyl triplets, but also low-energy organic dyes such as methylene blue, rose bengal, chlorophyl, and riboflavin. The same species that quench the phosphorescence of these complex molecules also quench their photosensitizing ability. 

From an analytical perspective, the single most important physicochemical characteristic of riboflavin is its photosensitivity (80-82). Exposure of this vitamin to ultraviolet and visible light results in irreversible photoreduction to lumiflavin and lumichrome and loss of vitamin activity. In addition, the coenzymes are subject to hydrolysis by endogenous phosphatases that are present in a number of foods. Since these enzymes are generally inactivated by thermal processing, they are a concern only in the analysis of fresh products. 

In the direct effect of ionizing radiation on DNA, radical cations are the primary products (Chap. 12). For this reason, their reactions are of considerable interest. Obviously, photoionization (e.g., at 193 nm) and laser multi-photon excitation leads to such species (e.g., Candeias and Steenken 1992b Malone et al. 1995 Chap. 2.2). Base radical cation electron pairs have been proposed to be the first observable intermediates with a lifetime of 10 ps for Ade and four times longer for the other nucleobases (Reuther et al. 2000). Radical cations are also assumed to be intermediates in the reactions of photosensitization reactions with qui-nones, benzophenone, phthalocyanine and riboflavin (Cadet et al. 1983a Decar-roz et al. 1987 Krishna et al. 1987 Ravanat et al. 1991, 1992 Buchko et al. 1993 Douki and Cadet 1999 Ma et al. 2000). Nucleobase radical cations may be produced by electrochemical oxidation (Nishimoto et al. 1992 Hatta et al. 2001) or with strongly oxidizing radicals (for a compilation of their reduction potentials see Chap. 5.3). Rate constants are compiled in Table 10.3. 

The riboflavin triplet reacts with dGMP acid by ET (k = 6.6 x 109 dm3 mol-1 s 1), and evidence for the formation of the (deprotonated) Gua radical cation has been obtained by laser flash photolysis (Lu et al. 2000). The photosensitized reactions of dGuo by TRP is thought to follow two pathways, the formation of Z has been attributed to an ET reaction (Type I), and the reaction of singlet dioxygen [ChCAg) Type II] leads to 4-OH-8-oxo-G and 8-oxo-G (Ravanat et al. 1998). The effect of D20 and azide on the 4-OH-8-oxo-G yields shows that this... 

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