Tetrapyrrole fluorescence

What is the role of energy or electron transfer in the quenching of tetrapyrrole fluorescence by carotenoids In other model studies the redox levels of a porphyrin-carotenoid dyad have been shown to influence the quenching mechanism to the extent that electron transfer from the carotenoid to the excited porphyrin was shown to occur (Hermant et al., 1993). However, in a series of carotenoid-porphyrin dyads in which the number of conjugated carbon-carbon double bonds in the carotenoid moiety was systematically increased from 7 to 11, quenching... 

According to Dher6 (24) the tripyrroles as well as the bilirubinoid tetrapyrroles fluoresce. They have either one fluorescence band or a diffuse fluorescence in the visible, and in the powdered form they fluoresce red. 

A few examples to render tetrapyrrolic compounds less phototoxic can be found in the hterature. In one approach, carotenoid structures were employed for the synthesis of some carotenoporphyrin derivatives [92-94]. Figure 8 shows two stuctures by way of example. Due to similar photophysical properties of the two structural components, the excited triplet state of the porphyrin is quenched by the carotenoid moiety, thus inhibiting the formation of singlet oxygen, while its fluorescence capabilities are still preserved. Biodistribution studies revealed enhanced uptake into tumour tissue [39,93,95]. However, microscopy studies have shown that such compounds are associated with connective tissues in the tumors rather than with cancerous cells indicating low specificities for mahgnant transformation [96]. 

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). 

Porphyrins consist of a tetrapyrrole ring with variable side chains. The physiological compounds are the reduced forms, the porphyrinogens that do not fluoresce. They react spontaneously with oxygen in the air to the strongly fluorescent porphyrins. 

Information regarding the solution conformation of 13 was derived from the pyropheophorbide ring current induced shifts in the resonance positions of the carotenoid and quinone moieties. These two species were found to be extended away from the tetrapyrrole, rather than folded back across it. The absorption spectrum of 13 was essentially identical to the sum of the spectra of model compounds. The pyropheophorbide fluorescence, however, was strongly quenched by the addition of the quinone. This implies the formation of a C-Phe -Q state via photoinitiated electron transfer from the pyropheophorbide singlet state, as was observed for C-P-Q triads (see Figure 4). Excitation of the molecule in dichloromethane solution at 207 K with a 590 nm laser pulse led to the observation of a carotenoid radical cation transient absorption. Thus, the C-Phe -Q " state can go on via an electron transfer step analogous to step 4 in Figure 4 to yield a final C -Phe-Q state. This state had a lifetime of 120 ns. The quantum yield at 207 K was 0.04. At ambient temperatures, the lifetime of the carotenoid radical cation dropped to about SO ns, and the quantum yield could not be determined accurately because of the convolution of the decay into the instrument response function. 

Figure 20. Absorption (upper curve) and corrected fluorescence excitation (lower curve) spectra for carotenopyropheophorbide 27 in toluene. The spectra have been normalized over the 6(X)-630 nm region. The excitation spectrum is virtually identical in shape to the absorption spectrum of methyl pyropheophorbide-a, and singlet-singlet energy transfer from the carotenoid to the tetrapyrrole is therefore minimal (<7%).

The fluorescent chlorophyll catabolites, such as pFCC (10), were observed not to accumulate during chlorophyll breakdown in senescent leaves (24). The indicated further transformation of the FCC chro-mophore to those of non-fluorescent chlorophyll catabolites (NCCs) was suggested to possibly be the result of a non-enzymic isomerization (56, 62). In analogy to the results of studies on the tautomerization chemistry of a range of hydro-porphinoids (91), the isomerization of the chromo-phore of FCCs into that of NCCs was judged to be rather favorable, thermodynamically. The complete de-conjugation of the four pyrrolic units, characteristic of the tetrapyrrolic NCCs, thus may occur in the course of natural chlorophyll breakdown under rather mild and, possibly, even without catalysis by (an) enzyme(s) (56). 

Transient absorption and fluorescence experiments on 1 and 4 and the model carotenoid pigments 3 and 6 yielded convincing results about the energy transfer pathway (Fig 1) (A.F. Moore et al, unpublished). In 1, the lifetime of Sj was measured by fluorescence upconversion to be 45 fs, whereas the Sj lifetime of model carotenoid 3 is 160fs.TheS, lifetime was 8 ps in both 1 and 3. Moreover, the S, rise time for tetrapyrrole 2 was found to be 62 fs by fluorescence upconversion. Therefore, only the carotenoid 83 state was quenched by the attached tetrapyrrole. The observation that the time constant associated with the decay of the energy donor(carotenoid 83) matches... 

Dyad 4 illustrates that both pathways can be active. In initial experiments on dyad 4 the attached tetrapyrrole was found to quench the carotenoid S3 state of6 from 95 to 28 fs and its S, level from 12 to 9 ps. The rise of the 8, level of tetrapyrrole 5 as measured by fluorescence upconversion required a major exponential component (74%) of 41 fs- and a minor component (26%) of4 ps-. While the match between the 9 ps decay of the carotenoid 8, and the 4 ps rise component of the tetrapyrrole 8, is only qualitative, these prelimenary experiments do provide evidence that both states can be energy donors. 

Figure B2.1.7 Transient hole-burned spectra obtained at room temperature with a tetrapyrrole-containing light-harvesting protein subunit, the a subunit of C-phycocyanin. Top fluorescence and absorption spectra of the sample superimposed with the spectrum of the 80 fs pump pulses used in the experiment, which were obtained from an amplifled CPM dye laser operating at 620 mn. Bottom absorption-difference spectra obtained at a series of probe time delays.

In a few eutrophic waters, absorbance due to pigments derived from algae or other microorganisms may become consequential. Chlorophylls and their degradation products are readily detectable because of their characteristic absorbance and fluorescence spectra. For example, chlorins (magnesium-free, tetrapyrrolic derivatives of chlorophyll) were detected in Mackenzie River (Canada) water at concentrations of up to 1(X) ng/L (Peake et al., 1972) and in other, more contaminated waters these classes of compounds may occur at higher levels. 

In contrast, the luminescence of metalloporphyrin dyes originates from the tetrapyrrolic macrocycle, but it is greatly affected by the nature of the central metal ion coordinated by the porphyrin ring and by axial ligands. Pt " - and Pd -porphyrins are particularly interesting because of their bright short-decay phosphorescence at room temperatures, and practically no fluorescence. They... 

---