Absorption spectra tetrapyrroles

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%).

Bilirubin is a red-brown tetrapyrrole, insoluble in water, dilute acids, and the common fat-solvents. It is soluble in hot chloroform and in alkalies, in which it dissolves to form salts. The solutions show no characteristic absorption spectrum. 

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.

Structure of products From its absorption properties, the photoproduct still appears to be a cyclic-conjugated tetrapyrrole. The nmr spectrum indicates a reduced ring-current. Since only two methine signals appear in both the educt and the photoproduct, the C-20 position lacks a hydrogen substituent in the product, too. The spectra are compatible with a 20-oxonia-chlorophyll (17). 

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