Coloring Outside the Lines: Exploiting Pigment-Protein Synergy for Far-Red Absorption in Plant Light-Harvesting Complexes

Plants are designed to utilize visible light for photosynthesis. Expanding this light absorption toward the far-red could boost growth in low-light conditions and potentially increase crop productivity in dense canopies. A promising strategy is broadening the absorption of antenna complexes to the far-red. In this study, we investigated the capacity of the photosystem I antenna protein Lhca4 to incorporate far-red absorbing chlorophylls d and f and optimize their spectra. We demonstrate that these pigments can successfully bind to Lhca4, with the protein environment further red-shifting the chlorophyll d absorption, markedly extending the absorption range of this complex above 750 nm. Notably, chlorophyll d substitutes the canonical chlorophyll a red-forms, resulting in the most red-shifted emission observed in a plant light-harvesting complex. Using ultrafast spectroscopy, we show that the introduction of these novel chlorophylls does not interfere with the excited state decay or the energy equilibration processes within the complex. The results demonstrate the feasibility of engineering plant antennae to absorb deeper into the far-red region while preserving their functional and structural integrity, paving the way for innovative strategies to enhance photosynthesis.

Effects of peripheral substituents on epimerization kinetics of formylated chlorophylls

C132-[Formula: see text]-epimers of chlorophyll (Chl) molecules are important cofactors in the photosystem I reaction centers in oxygenic photosynthetic organisms; however, their production mechanism is still unclear. The reaction properties of Chl epimerization are helpful for a better understanding of the molecular mechanism of the in vivo formation of Chl C132-[Formula: see text]-epimers. We report herein the kinetic properties of the epimerization of formylated Chl molecules, Chl [Formula: see text] and Chl [Formula: see text], by use of triethylamine. Both Chl [Formula: see text] and Chl [Formula: see text] performed faster epimerization kinetics than Chl [Formula: see text], indicating that the electron-withdrawing ability of the formyl groups directly linked to the chlorin macrocycle is responsible for acceleration of the epimerization. Comparing the rate constants of the two mono-formylated Chl molecules indicated that the epimerization of Chl [Formula: see text] was faster than that of Chl [Formula: see text]. This difference is rationalized by invoking a combination of the inductive effects of the C3- and C7-substituents in Chls; the sums of Hammett [Formula: see text] parameters of the C3- and C7-substituents exhibited high correlations with the epimerization rate constants of Chls [Formula: see text], [Formula: see text], and [Formula: see text].

Electrostatic profiling of photosynthetic pigments: implications for directed spectral tuning

Photosynthetic pigment-protein complexes harvest solar energy with a high quantum efficiency. Protein scaffolds are known to tune the spectral properties of embedded pigments principally through structured electrostatic environments. Although the physical nature of electrostatic tuning is straightforward, the precise spatial principles of electrostatic preorganization remain poorly explored for different protein matrices and incompletely characterized with respect to the intrinsic properties of different photosynthetic pigments. In this work, we study the electronic structure features associated with the lowest excited state of a series of eight naturally occurring (bacterio)chlorophylls and pheophytins to describe the precise topological differences in electrostatic potentials and hence determine intrinsic differences in the expected mode and impact of electrostatic tuning. The difference electrostatic potentials between the ground and first excited states are used as fingerprints. Both the spatial profile and the propensity for spectral tuning are found to be unique for each pigment, indicating spatially and directionally distinct modes of electrostatic tuning. The results define a specific partitioning of the protein matrix around each pigment as an aid to identify regions with a maximal impact on spectral tuning and have direct implications for dimensionality reduction in protein design and engineering. Thus, a quantum mechanical basis is provided for understanding, predicting, and ultimately designing sequence-modified or pigment-exchanged biological systems, as suggested for selected examples of pigment-reconstituted proteins.

Harvesting Far-Red Light with Plant Antenna Complexes Incorporating Chlorophyll d

Increasing the absorption cross section of plants by introducing far-red absorbing chlorophylls (Chls) has been proposed as a strategy to boost crop yields. To make this strategy effective, these Chls should bind to the photosynthetic complexes without altering their functional architecture. To investigate if plant-specific antenna complexes can provide the protein scaffold to accommodate these Chls, we have reconstituted the main light-harvesting complex (LHC) of plants LHCII in vitro and in silico, with Chl d. The results demonstrate that LHCII can bind Chl d in a number of binding sites, shifting the maximum absorption ∼25 nm toward the red with respect to the wild-type complex (LHCII with Chl a and b) while maintaining the native LHC architecture. Ultrafast spectroscopic measurements show that the complex is functional in light harvesting and excitation energy transfer. Overall, we here demonstrate that it is possible to obtain plant LHCs with enhanced far-red absorption and intact functional properties.

Spectral Properties of Chlorophyll f in the B800 Cavity of Light-harvesting Complex 2 from the Purple Photosynthetic Bacterium Rhodoblastus acidophilus

The interactions of chlorophyll (Chl) and bacteriochlorophyll (BChl) pigments with the polypeptides in photosynthetic light-harvesting proteins are responsible for controlling the absorption energy of (B)Chls in protein matrixes. The binding pocket of B800 BChl a in LH2 proteins, which are peripheral light-harvesting proteins in purple photosynthetic bacteria, is useful for studying such structure-property relationships. We report the reconstitution of Chl f, which has the formyl group at the 2-position, in the B800 cavity of LH2 from the purple bacterium Rhodoblastus acidophilus. The Q_y absorption band of Chl f in the B800 cavity was shifted by 14 nm to longer wavelength compared to that of the corresponding five-coordinated monomer in acetone. This redshift was larger than that of Chl a and Chl b. Resonance Raman spectroscopy indicated hydrogen bonding between the 2-formyl group of Chl f and the LH2 polypeptide. These results suggest that this hydrogen bonding contributes to the Q_y redshift of Chl f. Furthermore, the Q_y redshift of Chl f in the B800 cavity was smaller than that of Chl d. This may have arisen from the different patterns of hydrogen bonding between Chl f and Chl d and/or from the steric hindrance of the 3-vinyl group in Chl f.

Energy Transfer Dynamics in Light-Harvesting Complex 2 Variants Containing Oxidized B800 Bacteriochlorophyll a

Excitation energy transfer (EET) in light-harvesting proteins is vital for photosynthetic activities. The pigment compositions and their organizations in these proteins are responsible for the EET functions. Thus, changing the pigment compositions in light-harvesting proteins contributes to a better understanding of EET mechanisms. In this study, we investigated the EET dynamics of two light-harvesting complex 2 (LH2) variants, in which nine B800 bacteriochlorophyll (BChl) a pigments were entirely or half converted to 3-acetyl chlorophyll (AcChl) a. The AcChl a pigments showed a Q_y band, which was blue-shifted by 107 nm from B800 BChl a in the two variants. EET from AcChl a to B850 BChl a was observed in both fully oxidized and half-oxidized LH2 variants, but the EET rates were lower than that from B800 to B850 BChl a. EET from AcChl a to the co-present B800 was barely detected in the half-oxidized LH2. The preferential EET from AcChl a to B850 instead of B800 was rationalized by little spectral overlap of AcChl a with B800 BChl a and the pigment geometry in the protein. The EET rate from B800 to B850 BChl a in the half-oxidized LH2 was analogous to that in native LH2, indicating that partial oxidation of B800 did not disturb the EET channel from the residual B800 to B850.

Excitation Energy Transfer from Bacteriochlorophyll b in the B800 Site to B850 Bacteriochlorophyll a in Light-Harvesting Complex 2

Control of the spectral overlap between energy donors and acceptors provides insight into excitation energy transfer (EET) mechanisms in photosynthetic light-harvesting proteins. Substitution of energy-donating B800 bacteriochlorophyll (BChl) a with other pigments in the light-harvesting complex 2 (LH2) of purple photosynthetic bacteria has been extensively performed; however, most studies on the B800 substitution have focused on the decrease in the spectral overlap integral with energy-accepting B850 BChl a by reconstitution of chlorophylls into the B800 site. Here, we reconstitute BChl b into the B800 site of the LH2 protein from Rhodoblastus acidophilus to increase the spectral overlap with B850 BChl a. BChl b in the B800 site had essentially the same hydrogen-bonding pattern as B800 BChl a, whereas it showed a red-shifted Q_y absorption band at 831 nm. The EET rate from BChl b to B850 BChl a in the reconstituted LH2 was similar to that of native LH2 despite the red shift of the Q_y band of the energy donor. These results demonstrate the importance of the contribution of the density of excitation states of the B850 circular assembly, which incorporates higher lying optically forbidden states, to intracomplex EET in LH2.

Reconstitution of 3-Acetyl Chlorophyll a into Light-Harvesting Complex 2 from the Purple Photosynthetic Bacterium Phaeospirillum molischianum

The manipulation of B800 bacteriochlorophyll (BChl) a in light-harvesting complex 2 (LH2) from the purple photosynthetic bacterium Phaeospirillum molischianum (molischianum-LH2) provides insight for understanding the energy transfer mechanism and the binding of cyclic tetrapyrroles in LH2 proteins since molischianum-LH2 is one of the two LH2 proteins whose atomic-resolution structures have been determined and is a representative of type-2 LH2 proteins. However, there is no report on the substitution of B800 BChl a in molischianum-LH2. We report the reconstitution of 3-acetyl chlorophyll (AcChl) a, which has a 17,18-dihydroporphyrin skeleton, to the B800 site in molischianum-LH2. The 3-acetyl group in AcChl a formed a hydrogen bond with β'-Thr23 in essentially the same manner as native B800 BChl a, but this hydrogen bond was weaker than that of B800 BChl a. This change can be rationalized by invoking a small distortion in the orientation of the 3-acetyl group in the B800 cavity by dehydrogenation in the B-ring from BChl a. The energy transfer from AcChl a in the B800 site to B850 BChl a was about 5-fold slower than that from native B800 BChl a by a decrease of the spectral overlap between energy-donating AcChl a and energy-accepting B850 BChl a.

A Dual Role for Ca2+ in Expanding the Spectral Diversity and Stability of Light-Harvesting 1 Reaction Center Photocomplexes of Purple Phototrophic Bacteria

The light-harvesting 1 reaction center (LH1-RC) complex in the purple sulfur bacterium Thiorhodovibrio ( Trv.) strain 970 cells exhibits its LH1 Q _y transition at 973 nm, the lowest-energy Q _y absorption among purple bacteria containing bacteriochlorophyll a (BChl a). Here we characterize the origin of this extremely red-shifted Q _y transition. Growth of Trv. strain 970 did not occur in cultures free of Ca²⁺, and elemental analysis of Ca²⁺-grown cells confirmed that purified Trv. strain 970 LH1-RC complexes contained Ca²⁺. The LH1 Q _y band of Trv. strain 970 was blue-shifted from 959 to 875 nm upon Ca²⁺ depletion, but the original spectral properties were restored upon Ca²⁺ reconstitution, which also occurs with the thermophilic purple bacterium Thermochromatium ( Tch.) tepidum. The amino acid sequences of the LH1 α- and β-polypeptides from Trv. strain 970 closely resemble those of Tch. tepidum; however, Ca²⁺ binding in the Trv. strain 970 LH1-RC occurred more selectively than in Tch. tepidum LH1-RC and with a reduced affinity. Ultraviolet resonance Raman analysis indicated that the number of hydrogen-bonding interactions between BChl a and LH1 proteins of Trv. strain 970 was significantly greater than for Tch. tepidum and that Ca²⁺ was indispensable for maintaining these bonds. Furthermore, perfusion-induced Fourier transform infrared analyses detected Ca²⁺-induced conformational changes in the binding site closely related to the unique spectral properties of Trv. strain 970. Collectively, our results reveal an ecological strategy employed by Trv. strain 970 of integrating Ca²⁺ into its LH1-RC complex to extend its light-harvesting capacity to regions of the near-infrared spectrum unused by other purple bacteria.

Selective oxidation of B800 bacteriochlorophyll a in photosynthetic light-harvesting protein LH2

Engineering chlorophyll (Chl) pigments that are bound to photosynthetic light-harvesting proteins is one promising strategy to regulate spectral coverage for photon capture and to improve the photosynthetic efficiency of these proteins. Conversion from the bacteriochlorophyll (BChl) skeleton (7,8,17,18-tetrahydroporphyrin) to the Chl skeleton (17,18-dihydroporphyrin) produces the most drastic change of the spectral range of absorption by light-harvesting proteins. We demonstrated in situ selective oxidation of B800 BChl a in light-harvesting protein LH2 from a purple bacterium Rhodoblastus acidophilus by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. The newly formed pigment, 3-acetyl Chl a, interacted with the LH2 polypeptides in the same manner as native B800. B850 BChl a was not oxidized in this reaction. CD spectroscopy indicated that the B850 orientation and the content of the α-helices were unchanged by the B800 oxidation. The nonameric circular arrangement of the oxidized LH2 protein was visualized by AFM; its diameter was almost the same as that of native LH2. The in situ oxidation of B800 BChl a in LH2 protein with no structural change will be useful not only for manipulation of the photofunctional properties of photosynthetic pigment-protein complexes but also for understanding the substitution of BChl to Chl pigments in the evolution from bacterial to oxygenic photosynthesis.

Engineering of B800 bacteriochlorophyll binding site specificity in the Rhodobacter sphaeroides LH2 antenna

The light-harvesting 2 complex (LH2) of the purple phototrophic bacterium Rhodobacter sphaeroides is a highly efficient, light-harvesting antenna that allows growth under a wide-range of light intensities. In order to expand the spectral range of this antenna complex, we first used a series of competition assays to measure the capacity of the non-native pigments 3-acetyl chlorophyll (Chl) a, Chl d, Chl f or bacteriochlorophyll (BChl) b to replace native BChl a in the B800 binding site of LH2. We then adjusted the B800 site and systematically assessed the binding of non-native pigments. We find that Arg_-10 of the LH2 β polypeptide plays a crucial role in binding specificity, by providing a hydrogen-bond to the 3-acetyl group of native and non-native pigments. Reconstituted LH2 complexes harbouring the series of (B)Chls were examined by transient absorption and steady-state fluorescence spectroscopies. Although slowed 10-fold to ~6 ps, energy transfer from Chl a to B850 BChl a remained highly efficient. We measured faster energy-transfer time constants for Chl d (3.5 ps) and Chl f (2.7 ps), which have red-shifted absorption maxima compared to Chl a. BChl b, red-shifted from the native BChl a, gave extremely rapid (≤0.1 ps) transfer. These results show that modified LH2 complexes, combined with engineered (B)Chl biosynthesis pathways in vivo, have potential for retaining high efficiency whilst acquiring increased spectral range.