Fluorescence, of porphyrins

Table II. Energetics amd rate constamts for electron tramsfer quenching of the fluorescence of porphyrins by alkyl halides in BuCl at 77 K.

Emission. The fluorescence properties of metalloporphyrin and porphyrin-like compounds depend on the character of central metal ion. Fluorescence of porphyrins is generally observed from the free-base or closed-shell metal complexes. The... 

Matsui et al. (2001) reported that the gastric mucosa in rats emits fluorescence of porphyrins at the onset of gastric lesions induced by diclofenac. HPLC analysis demonstrated that the fluorescent substances were mesoporphyrin and protoporphyrin. 

Methods for porphyrin fractionation are complex and time consuming and not available in every laboratory. For this reason, simple qualitative screening tests are often used to exclude the majority of specimens that do not require further investigation from the few that justify fractionation of the individual porphyrins. Screening tests in which extracts of urine or feces are examined visually for typical red-pink fluorescence of porphyrins lack sensitivity and should not be used. Methods based on spectrophoto-metric scanning of acidified urine or fecal extracts for the presence of the Soret band are recommended and yield semiquantitative information. Quantitative fluorometric methods are also available. ... 

Many small molecules of clinical interest can be detected by fluorescence. For example, porphyrins, which are intermediates in the metabolism of iron-containing heme prosthetic groups, can be detected and quantitated in urine by reversed-phase chromatography followed by fluorometric detection, exploiting the natural fluorescence of porphyrin compounds [18]. An example of reversed-phase chromatography for a mixture of carboxylated porphyrin precursors is demonstrated in Figure 21.2. The relative abundances of different porphyrin compounds can aid in diagnosing porphyria, which is a term that describes the collection of disorders caused by defects in the enzymes responsible for porphyrin metabolism. 

Fig. 51 Fluorescence intensities of porphyrins as a function of the concentration of the paraffin oil dipping solution mesoporphynn, coproporphynn, pentaporphynn...

Fig. 52 Fluorescence intensity of porphyrin chromatogram zones as a function of time after dipping in 50% liquid paraffin solution and storage in darkness. —...

When porphyrins dissolved in strong mineral acids or in organic solvents are illuminated by ultraviolet hght, they emit a strong red fluorescence. This fluorescence is so characteristic that it is often used to detect small amounts of free porphyrins. The double bonds joining the pyrrole rings in the porphyrins are responsible for the characteristic absorption and fluorescence of these compounds these double bonds are absent in the porphyrinogens. 

Nishitani et al. observed the tip-enhanced fluorescence of 8 nm thick meso-tetralds (3,5-di-tertiarybutyl-phenyl)porphyrin (H2TBPP) films on ITO with an Ag tip [38]. They reported that the fluorescence was enhanced by the locally confined electromagnetic field in the vicinity of the tip. The enhancement factor is evaluated to be larger than 2000. 

Two other aspects may be mentioned at the outset. First, fluorescent substances which accumulate preferentially in tumors (e.g., some porphyrins) may be used in diagnosis, since under appropriate irradiation (usually weak irradiation in the UV) the tumor tissue will be revealed by the fluorescence of the added substance. This is not a photodynamic effect, and is generally... 

Frechet and co-workers [32] studied the ability of the dendrimer shell to provide site isolation of the core porphyrin moiety, using benzene-terminated dendrimers Zn[G-n]4P (i.e. 6). From the cyclic voltammograms in CH2C12, the interfacial electron transfer rate between the porphyrin core and the electrode surface decreased with increasing dendrimer generation. However, small molecules like benzyl viologen could still penetrate the shell of 6 to access the porphyrin core as observed from the quenching of porphyrin fluorescence. Their results also revealed that the dendritic shell did not interfere electrochemically or photochemically with the porphyrin core moiety. 

Many metal-free porphyrins or porphyrins lacking a metal and therefore unable to ligate with His residues are nonetheless also bound by hemopexin, and changes in absorbance and fluorescence of both hemopexin and the ligand result. However, the affinity of binding... 

These types of arguments have been used by us for explanation of characteristic peculiarities in spectra of compound 10 NH-tautomers (Fig. 10) and were applied to understand the principal changes in the NH-tautomer visible absorption of investigated porphyrins with non-symmetrical substitution. It may be concluded that difficulties in the theoretical treatment of spectral peculiarities of porphyrins with ir-acceptor substituents ( ) are caused by the absence of detailed absorption and fluorescence excitation spectra of individual tautomers of these compounds. 

S2 - Sq fluorescence and radiationless transitions from the state of porphyrins have been studied in order to reveal photodynaunics of porphyrins. The S2 state fluorescence of zinc(II)-tetraphenylporphin is caused even by the excitation to the state. Two-photon absorption and optical-optical double resonance studies show that a stepwise two-photon absorption through the state is a main process populating the S2 state. 

The simplest covalently linked systems consist of porphyrin linked to electron acceptor or donor moiety with appropriate redox properties as outlined in Figure 1. Most of these studies have employed free base, zinc and magnesium tetrapyrroles because the first excited singlet state is relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. Additionally, these pigments have relatively high fluorescence quantum yields. These tetrapyrroles are typically linked to electron acceptors such as quinones, perylenes , fullerenes , acetylenic fragments (14, 15) and aromatic spacers and other tetrapyrroles (e.g. boxes and arrays). 

The TTF-porphyrin dyad 3 was described by the group of Odense.11 The fluorescence of 3 is significantly quenched by the photoinduced electron transfer process. Notably, the fluorescence intensity of dyad 3 increases largely after addition of Fe3 + that oxidizes TTF into TTF" +. Successive reduction of TTF" + is not reported. Nevertheless, it is anticipated that the fluorescence of dyad 3 can be reversibly modulated by redox reactions. In fact, the fluorescence of the supramolecule 4, formed between Zn-tetraphenylporphyrin and a pyridine-substituted TTF (TTF- ), can be reversibly tuned by sequential oxidation and reduction of the TTF moiety in 4.12 It should be noted in this context that the synthetically challenging system associating a porphyrin ring fused to four TTFs (5) was also reported.13... 

Nowadays, the preferred methodology is the separation and quantification of porphyrins by high-performance liquid chromatography (HPLC) with fluorescence detection. Thus, a method that detects the main isomers of specific porphyrins is recommended, as a high number of different isomers exist, especially in pathological conditions, and the relative abundance of the different isomers supports the distinction of the different porphyrias (Table 7.3.1). 

This work is also notable in that Cole et al. performed detailed aggregation studies of the heme MPPIX using UV-vis and fluorescence spectroscopies to detect the formation of 71-71 hetero-metalloporphyrin assemblies under assay conditions. By employing UV-vis absorbance spectroscopy, the aggregation of porphyrin and metalloporphyrin systems may be examined. The in vitro assay system used for hemozoin... 

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