Charged carotenoid species

According to a recommended strategy for this series on studies in natural products chemistry, the present review is emphasising recent work and current projects involving charged carotenoid species in the authors laboratory. However, the topic is treated in a broader context, referring to relevant studies by other groups. 

Whereas carotenoids in general are often considered by non-specialists as unstable compounds, they may easily be studied when particular precautions are made [1], This is also true for most subtypes with charged functional groups. The instability of carotenoid oxonium ions, radical cations, mono- and dications, however, represent special challenges. This review appears to be the first compilation on charged carotenoid species. 

As this paper is prepared for a special volume on bioactive natural products, a brief overview on bioactivity of carotenoids is included, divided into known functions of carotenoids and to their actions on biological systems. Charged carotenoid species are involved in the function of carotenoids in photosynthesis and may turn out to be relevant unstable intermediates in other biological contexts, including antioxidant action. 

Since caroviologens are rather fragile compounds, they can be protected from the environment by inclusion into polyanionic derivatives of (J-cyclodextrin in a rotaxane fashion 102 [8.156]. Also, in the design of molecular devices, it may be desirable to introduce some extent of redundancy in order to reduce the risk of device failure. This is the case in the tris-carotenoid macrobicycle 103 that represents a sort of triple-threated molecular cable whose crystal structure 104 has been determined. It forms a dinuclear Cu(i) complex 105 in which the bound ions introduce a positive charge at each of the species, a feature of potential interest for transmembrane inclusion [8.157]. 

Van Berkel and Zhou first tested (3-carotene with ESI positive in 1994 (van Berkel and Zhou, 1994). In this study, a doubly charged molecular ion of (3-carotene was observed as the primary species when triflur-oacetic acid was present in the solution. Van Breemen was the first to utilize ESI as an interface between HPLC and MS to analyze carotenoids (van Breemen, 1995). In this study, ESI operated in negative mode ionized xanthophylls (astaxanthin, (3-cryptoxanthin, and lutein), but did not ionize hydrocarbon carotenes (lycopene and (3-carotene). In contrast, ESI positive produced only [M" ] for all carotenoids in this study, and the addition of halogenated solvents to the post-column effluent greatly enhanced signal intensity (van Breemen, 1995). A later study by Guarantini et al. demonstrated the ability of ESI positive to produce both [M" "] and [M + H]" " for a number of xanthophylls, and these authors attributed the production of the two species to solvent system... 

One test of this proposed two-step charge recombination is provided by triad 8. In this molecule, the quinone moiety has been moved from the 5,15 relationship to the carotenoid as in 4 to the 5,10 relationship. Thus, the carotenoid and quinone species have been brought much closer together. This has been demonstrated by the NMR conformational studies of both molecules [27], If recombination of C -P-Q were to occur by a direct electron transfer as per step 5 in Figure 9, this change should lead to a large increase in the recombination rate. However, as seen in Table 2, triads 4 and 8 have identical charge separation lifetimes, within experimental error, as would be expected from the two-step mechanism. 

Thus, chlorophyll-based triad species can also successfully mimic the multistep strategy of natural photosynthesis to yield long-lived, energetic charge-separated states. Triad 13 and related molecules also mimic carotenoid photoprotection from singlet oxygen and carotenoid antenna function. These aspects of the molecules will be discussed in later sections. 

The long lifetime of the final C -P-P-Q species is the result of the large spatial separation between the carotenoid and quinone moieties and the fact that endergonic electron transfers would be needed to place the charges on... 

The absorption spectrum of 22 is nearly identical to the sum of the spectra of unlinked model compounds. The long wavelength band of the free base porphyrin is observed at 650nm in chloroform, whereas that of the zinc porphyrin is at 590nm. Excitation of a chloroform solution of 22 with a 15 ns pulse of 650 nm laser light leads to the formation of a carotenoid radical cation which can be detected by transient absorption spectroscopy (Figure 14). This ion arises from the charge separated state C -Pzh-P-Qa-Qb, and is formed with a quantum yield of 0.83. The lifetime of the species is 55/iS. 

The lifetimes of the final charge-separated states in the pentads are far longer than those observed for the simpler molecular devices discussed above, but are substantially shorter than would be expected for charge recombination via direct electron transfer from the benzoquinone to the carotenoid radical cation, based on results for related species. This may well signify that charge recombination occurs via a multistep pathway as was observed for some of the triad species discussed above. Additional studies will be necessary to elucidate the details of the various charge separation and charge recombination processes in the pentads. 

Radicals are species with an odd electron, and may or may not carry a formal charge. Thus, radicals of a carotenoid CAR are most simply obtained by adding or removing an electron to generate the radical anion and cation respectively, (CAR and CAR " ). For example a process involving aperoxyl radical (ROO ) can be written as ... 

A common feature of the triads and tetrads mentioned above is that in each case at least part of the competition against charge recombination at the first intermediate species was provided by electron donation from the carotenoid moiety to the porphyrin radical cation (this is sometimes known as... 

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