Exogenous chromophores

III. IMAGING FOR CHEMICAL INFORMATION IN THE SKIN WITH EXOGENOUS CHROMOPHORES... 

ROS in skin can be detected by exogenous chromophores like dihydrorhodamine (DHR) (Figure 2.5). DHR is nonfluorescent until reaction with ROS when it becomes fluorescent rhodamine-123 (R123, = 535 nm). The reaction scheme is given in... 

Ongoing investigations underline that IR is able to induce significant ROS production (Schroeder and Krutmann, unpubhshed observations). The pho-tobiological effect of IR is closely related to the respective chromophore(s). As such a chromophore must be able to absorb within the IR band it is necessary that molecules other than those discussed for UV must be considered [45]. While the endogenous chromophores for IR remain to be identified, exogenous chromophores for IR are used for therapeutic purposes [46,47]. 

Here we discuss light absorption by various components of the cell and the effects caused by light absorption. Primary photoinduced cellular effects are produced by light absorption to induce transition between two electronic states (electronic or coupled electronic-vibrational [vibronic] transitions). Purely vibrational transitions (such as IR and Raman) are of significance only in stmctural identification and in conformational analysis. We first discuss the absorption by various constituent molecules and biopolymers. Subsequently we discuss the various photochemical and photophysical processes induced by light absorption. Then we discuss the effects produced from light absorption by an exogenous chromophore added to the cell. 

There are some important photochemical reactions that are produced by light absorption in chemicals introduced in a cell (or a tissue). These are exogenous chromophores that perform the fimction of photosensitization, i.e., sensitizing a photoprocess (Kochevar, 1995 Niemz, 1996). The mechanism for most photosensitization involves photoaddition... 

In TPM, fluorescence emission of a fluorophore (either intrinsic to the sample or an exogenously applied chromophore) is detected. A fluorophore is used to obtain specific chemical information in the skin, as discussed in more detail below. To excite the fluorophore, two-photon excitation is used, which is achieved by the simultaneous... 

The mechanism proposed for the pyruvate ferredoxin oxidoreductase is drastically different from that of the pyruvate dehydrogenase enzyme complexes, as radical intermediates are proposed, suggesting stepwise one-electron transfers. Furthermore, the preliminary work by Uyeda and Rabinowitz (211) on the mechanism of C. acidiurici pyruvate ferredoxin oxidoreductase indicates that there may be subtle differences in the mechanism of the oxidoreductase from different organisms. The key points and contrasts in the mechanism of these enzymes may be summarized as follows. Addition of pyruvate to a stoichiometric amount of enzyme leads to the formation of an equimolar amount of CO2. In H. halobium (213) a enzyme-mediated one-electron transfer to an exogenous electron acceptor occurs at this stage, with formation of a stable enzyme-bound radical intermediate, whereas the enzyme from C. acidiurici remains in the oxidized state. Addition of CoA leads to the subsequent formation of acetyl-CoA from both enzymes, which reduction (two electrons ) of an unknown chromophore in the... 

The chromophore is the light-active part of the protein and it is responsible for its spectral and photophysical properties. Two main families of FPs, namely GFP-like and flavin-binding FPs (FbFPs), have been developed over the last 15 years for genetically-encodable 2 production (Fig. 13). In the GFP-like family, the chromophore is part of the protein sequence in contrast, FbFPs incorporate the flavin from their cellular environment. Flavins are ubiquitous in cellular systems and do not need to be exogenously supplemented. 

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