**6. Relating the Mechanisms of Photoprotection to the Photochemistry of Photosynthesis and Human Vision**

Continuing comparison of parallels between light absorption, light processing, and protection against excessive excitation in photosynthesis and human vision may be rewarding (Figure 2). Research on the photoprotection of photosynthesis has focused, e.g., on interaction of the light-absorbing chlorophylls with xanthophylls in the light-collecting system, with a lesser focus on lipid peroxidation-based messengers and signaling networks (Figure 2). On the other hand, research on human vision has identified synergies among factors involved in signaling networks, while the photo-physics of excited states of rhodopsin derivatives and their possible interaction with xanthophylls have not been fully elucidated (Figure 2). Possible practical applications based on insight into the photo-physics of visions will be addressed below.

**Figure 2.** Comparison of photosynthesis and vision with respect to the principal steps in the series of reactions from light absorption, by various chromophores, to transfer of excitation energy to singlet oxygen (1 O2\*) resulting in lipid peroxidation and formation of lipid-derived gene regulators. Light absorption occurs in chlorophyll (Chl)-antenna-protein complexes in photosynthesis, and in retinal-opsin-complexes in vision. Triplet excited chlorophyll (3 Chl\*) is known to act as the photosensitizer in photosynthesis, passing excitation energy to oxygen, thus forming singlet oxygen. The nature of the photosensitizer in vision (X) is currently under debate. In contrast to protein-bound Chl, opsin-bound retinal does not produce singlet oxygen. However, there is current debate that, once released from opsin, all-*trans* retinal absorbs another photon and may give rise to singlet oxygen formation (see, e.g., [78]). The boxes around the different phases of the reaction series serve to indicate where research focus has previously been placed.

The parallels between photosynthesis and human vision are remarkable—as would be expected from an evolutionary viewpoint considering not only the homology of these systems, but also common dictates from natural selection and adaptive advantage. Both systems must be able to collect photons of light (as carriers of information in vision, or carriers of energy in photosynthesis) via light-absorbing pigments or chromophores (retinal or chlorophyll), which is achieved by the chromophore's binding to a protein (opsin or chlorophyll-binding proteins, respectively). Figure 2 summarizes some of the parallels between human vision and photosynthesis.

Chromophore-binding proteins, such as the chlorophyll-binding proteins of photosynthesis, typically help increase the lifetime of a chromophore's excited state (produced by absorption of a photon of light) long enough to allow highly efficient transfer of excitation energy (e.g., into an electron transport chain in the case of photosynthesis). Increasing the chromophore's lifetime, however, comes at a cost under high light levels; if the chromophore's excited state builds up just for fractions of a second, a conversion (via intersystem crossing) occurs to a slightly lower (triplet) excited state of the chromophore able to pass on excitation energy to oxygen, thereby forming potentially highly destructive reactive oxygen species. Chromophores thus typically act as facilitators of photo-damage, or photosensitizers, although in the vision process, it is not opsin-bound retinal, but forms of retinal (all-*trans*-retinal or its derivatives) released from opsin that absorb another photon and may act as the singlet-oxygen-producing photosensitizer ([78–87]).

The accumulation of light-absorbing pigment protein complexes typically responds to long-term light availability. Plants accumulate more chlorophyll when grown in low light, and thereby become more susceptible to over-excitation and inactivation of photochemistry under high light exposure, than plants grown in high light [32]. Similarly, animals raised in low light environments accumulated more rhodopsin, and were more susceptible to high-light-induced vision loss (for an overview, see [78]). Based on these findings and the current understanding of the mechanisms of photo-damage, it is possible that long-term light environment may affect the risk for human eye disease. Future research should assess whether predominant exposure to very low light environments in typical home and office settings may increase the risk for eye disease compared to regular exposure to modest levels of natural sunlight outdoors, e.g., in mornings and afternoons or on overcast days (with typical light intensities around only 10 mol photons m<sup>í</sup><sup>2</sup> ·s<sup>í</sup><sup>1</sup> for indoors fluorescent lighting versus around 300 and up to 2000 mol photons m<sup>í</sup><sup>2</sup> ·s<sup>í</sup><sup>1</sup> for natural sunlight outdoors during overcast skies and full midday sunlight on clear days, respectively; B. Demmig-Adams, [79]).

It has been discussed that the human eye, just like photosynthesis, must maintain "a delicate balance between maximizing the absorption of photon for vision and retinal image quality while simultaneously minimizing the risk of photo-damage when exposed to bright light" [80]. Rhodopsin presence has been proposed to be required for photo-damage in the retina ([81]; see also [82,83]), and, as pointed out above, there has been recent discussion that all-*trans*-retinal, once released from rhodopsin during the vision cycle, or specific derivatives of all-*trans*-retinal, may accumulate in the retina and act as photosensitizer(s) ("X" in Figure 2; [78,81–88]). Lutein and zeaxanthin have been suggested to confer photoprotection against such an action [84]. It is currently unknown whether or not, or how, xanthophylls may facilitate de-excitation of any excited states of chromophores in human vision. In contrast, detailed information has been accumulated on chromophore de-excitation, and an involvement of xanthophylls, in photosynthesis (see below).

A chromophore's first excited state is typically a singlet-excited state (e.g., <sup>1</sup> Chl\*; Figure 2) electronically unable to pass excitation energy to oxygen in its ground (triplet) state. However, the chromophore's state reached after conversion via intersystem crossing is a triplet state (e.g., 3 Chl\*; Figure 2) that reacts readily with oxygen, resulting in the formation of highly reactive singlet-excited oxygen. Under high light exposure, a light-absorbing system like chlorophyll-binding proteins is thus extremely vulnerable to destruction by singlet oxygen. Multiple mechanisms have evolved to provide protection by safe de-excitation of "unwanted", excessive excited states at every step of the above cascade, *i.e.*, (1) de-excitation of the chlorophyll's singlet-excited state, (2) de-excitation of its triplet state, (3) de-excitation of singlet-excited oxygen, and, finally, (4) mechanisms that re-reduce oxidized lipids.

In photosynthesis, zeaxanthin (and to a lesser extent lutein) facilitate de-excitation of excessive singlet-excited chlorophyll [31,37]; efficient de-excitation of triplet-excited chlorophyll in light-harvesting complexes can also be catalyzed by lutein and zeaxanthin [89,90]. Singlet-excited oxygen (<sup>1</sup> O2\*) can be de-excited by vitamin E and various carotenoids (see [91]). Remarkably, a mixture of zeaxanthin, lutein, and a lutein derivative in the concentrations found in the human retina was shown to be more effective at singlet oxygen de-excitation than each xanthophyll alone [92]. Finally, zeaxanthin can apparently directly inhibit lipid peroxidation (L-OO• ) in plants [34,35] and may serve in a similar function in the human eye through the synergistic interaction with vitamin E described above.

The underlying photo-physical mechanism of de-excitation of singlet-excited chlorophyll facilitated by zeaxanthin has received much attention, and evidence has been provided (for reviews, see [32,93,94]) for several different mechanisms that may yet turn out to all contribute, *i.e.*, (1) direct transfer of excitation energy from singlet-excited chlorophyll to zeaxanthin (and perhaps lutein), followed by conversion of the excitation energy to harmless heat [95]; (2) reversible charge transfer between singlet-excited chlorophyll and zeaxanthin (transfer of an electron from chlorophyll to zeaxanthin and back) resulting in loss of the excitation energy as harmless heat [96]; (3) a zeaxanthin-induced change in the chlorophyll-binding light-harvesting proteins from a conformation that lengthens the lifetime of <sup>1</sup> Chl\* to a conformation allowing efficient return of <sup>1</sup> Chl\* directly to ground-state chlorophyll while releasing excitation energy as harmless heat [93,94]. Zeaxanthin's close structural isomer, lutein, differs in the energy levels of its excited states as well as in its exact structure (and perhaps its interaction with membranes and proteins).

The evolutionary conservation of xanthophyll association with light-absorbing systems in widely different organisms is as remarkable as the multitude of mechanisms of xanthophyll-facilitated photoprotection. More research is needed to elucidate whether different organisms employ different ones of these multiple mechanisms—or whether the whole suite of mechanisms may be conserved across species and light-processing systems.

#### **Acknowledgments**

We wish to thank Christopher M. Cohu for preparing the figures for this review.

#### **Conflict of Interest**

The authors declare no conflict of interest.

#### **References**

