Pheophytins, properties

Polivka had investigated the co-adsorption of carotenoid and pheophytin (111) on the surface of TiC>2 electrode and the photophysical properties of pheophytin in this film. The results demonstrated that the fluorescence of 111 was efficiently reductive quenched by carotenoid in this co-assembled film, suggesting similar mechanisms to that in the natural photosynthetic systems. The radical anion of 111 formed during the electron transfer recovered to the neutral state quickly before the charge recombination between carotenoid cation and pheophytin anion took place. It is suspected that the electron injection from the pheophytin anion to the conduction band of Ti02 was responsible for this quick recovery. This result indicated that such a self-assembling strategy may be also considered for novel DSSC constructions [108]. 

Chlorophyll-Based Triads. The cyclic tetrapyrroles in natural photosynthetic reaction centers are chlorophylls and pheophytins, and therefore chlorins or other partially reduced porphyrin species. The artificial photosynthetic systems discussed so far are based on porphyrins. It was thus of interest to prepare a chlorophyll-based triad species and to investigate its properties. 

Fig. 4. (A) EPR spectra of TSF lla particles poised at -450 mV and after 90-s illumination at 295 or 220 K and measured at two different microwave powers. (B) shows effect of microwave power (P) on the amplitude of the photoinduced narrow (singlet) and doublet EPR signals at 7 K, Figure source Klimov, Dolan and Ke (1980) EPR properties of an intermediary electron acceptor (pheophytin) in photosystem II reaction centers at cryogenic temperatures. FEBS Lett 112 98,99 and Klimov, Dolan, Shaw and Ke (1980) Interaction between the intermediary electron acceptor (pheophytin) and a possible plastoquinone-lron complex in photosystem-ll reaction centers. Proc Nat Acad Sci, USA. 77 7228...

VV Klimov, E Dolan and B Ke (1980) EPR properties of an intermediary electron acceptor (pheophytin) in photosystem II reaction centers at cryogenic temperatures. FEBS Lett 112 97-100... 

T.M. Cotton and R. P. Vanduyne, Electrochemical Investigation of the Redox Properties of Bacteriochlorophyll and Bacterio-pheophytin in Aprotic Solvents, J. Am. Chem. Soc. 101, 7605 (1979)... 

Amphiphilic isotherm (65) analysis can be used for the determination of the interfacial structure. An amphiphilic molecule, which consists of two moieties with opposing properties such as a hydrophilic polar head and a hydrophobic hydrocarbon tad, should be used as an analytical tool located at the interface. Pheophytin a is a well-known surfactant molecule that contain a hydrophobic chain (phytol) and a hydrophilic head group. The value of p, less than 1.0, indicates that adsorbed molecules of n-octane are parallel to the interface between octane and water [23] (Fig. 11). Substitution of one adsorbed octane molecule requires about 4 to 5 adsorbed pheophytin... 

Transient absorption spectroscopy has been used to study isolated Photosystem 2 (PS2) reaction centres stabilised by the use of anaerobic conditions. In the absence of added artificial electron donors and acceptors, the light induced electron transfer properties of the reaction centre are restricted to the formation of the radical pair P680+Pheophytin and charge recombination pathways from this state [1]. This charge recombination has been observed to produce a 23% yield of a chlorophyll triplet state [1]. Attempts to reconstitute these particles with quinone have until now been limited to the observation of a steady state, quinone-mediated photoreduction of the cytochrome b-559 [2]. 

The spectral properties of the Ti-anion radicals of Chi and BChl and their respective magnesium-free analogs pheophytin (Pheo) and bacteriopheophytin (BPheo) are of considerable interest, since spectroscopic methods have detected the formation of the latter three radicals during the primary events of photosynthesis [49]. We have characterized the spectra of the anion radicals of Et-Pheo-a, Et-Chl-a, Et-BPheo-a and Et-BChl-a and have compared the calculated [33, 36, 37] and experimental spectra in Fig. 13. As shown in the figure, once again the absorption spectra display characteristic visible and Soret absorption regions, but the visible bands are generally much more structured than are those of either the neutral or cationic moieties. 

This is a specihc reaction catalyzed by the endogenous enzyme chlorophyllase (chlorophyll-chlorophyllide-hydrolase) (EC 3.1.1.14) giving rise to chlorophyHides. The basic macrocyclic structure is not modified, so that the chromophore properties are maintained and the color is unaltered. However, phytol (which makes the chlorophyll molecule liposoluble) is lost, resulting in a considerably increased polarity of the product. Pheophytin can also be the substrate of the enzyme chlorophyllase, with the reaction product of de-esterification being pheophorbide. Chemical de-esterification of phytol under acid or alkaline conditions is not a specific reaction, and is usually accompanied by oxidative side reactions [27]. 

If the aUomerization reaction takes place in a slightly alkaline medium and under inert atmosphere, there is only solvolysis of the isocyclic ring, and the resulting components are denominated esterified chlorin es and rhodin g-j, depending on whether they come from the a- or fc-series, respectively. Chlorophyll a yields Mg-phytol-chlorin while pheophytin a or pheophorbide a yields phytol-chlorin eg or free chlorin eg, respectively. The same series of transformations can be described for chlorophyll b. The structural difference between these compounds and the purpurins 7 is solely in the substituent of C-15, which, in place of an O, is an H, so that their coloration, polarity, and electron absorption properties are very similar. Other techniques, such as mass spectroscopy (MS), must be used for their differentiation [28]. 

In contrast, de-esterification of phytol in the chlorophyll molecule does not affect the chromophore stmcture, so the electron adsorption spectmm is unaltered. Thus, chlorophyUides and pheophorbides have the same spectroscopic properties as then-respective precursors, chlorophylls and pheophytins. The same is tme of some of the reactions that affect the isocyclic ring, such as epimerization and decarbometh-oxylation on C-13. ... 

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