Probing the effect of the binding site on the electrostatic behavior of a series of carotenoids reconstituted into the light-harvesting 1 complex from purple photosynthetic bacterium Rhodospirillum rubrum detected by stark spectroscopy

Intramolecular charge-transfer enhances energy transfer efficiency in carotenoid-reconstituted light-harvesting 1 complex of purple photosynthetic bacteria

In bacterial photosynthesis, the excitation energy transfer (EET) from carotenoids to bacteriochlorophyll a has a significant impact on the overall efficiency of the primary photosynthetic process. This efficiency can be enhanced when the involved carotenoid has intramolecular charge-transfer (ICT) character, as found in light-harvesting systems of marine alga and diatoms. Here, we provide insights into the significance of ICT excited states following the incorporation of a higher plant carotenoid, β-apo-8'-carotenal, into the carotenoidless light-harvesting 1 (LH1) complex of the purple photosynthetic bacterium Rhodospirillum rubrum strain G9+. β-apo-8'-carotenal generates the ICT excited state in the reconstituted LH1 complex, achieving an efficiency of EET of up to 79%, which exceeds that found in the wild-type LH1 complex.

5-HT2 Receptor Subfamily and the Halogen Bond Promise

The binding of C-4-halogenated 1-(4-X-2,5-dimethoxyphenyl)-2-aminopropane (DOX) serotonin agonist psychedelics at all three 5-HT₂ receptor subtypes is up to two orders of magnitude stronger for X = Cl, Br, or I (but not F) than when C-4 bears a hydrogen atom and more than expected from their hydrophobicities. Our docking and molecular dynamics simulations agree with the fact that increasing the polarizability of halogens results in halogen-oxygen distances to specific backbone C═O groups, and C-X···O angles, in ranges expected for halogen bonds (XBs), which could contribute to the high affinities observed. Good linear correlations are found for each receptor type, indicating that the binding pocket-ligand affinity is enhanced as the XB interaction becomes stronger (i.e., I ≈ Br > Cl > F). It is also striking to note how the linear equations unveil that the receptor's response on the strength of the XB interaction is quite similar among 5-HT_2A and 5-HT_2C, whereas the 5-HT_2B's sensitivity is less. The calculated dipole polarizabilities in the binding pocket of the receptors reflect the experimental affinity values, indicating that less-polarizable and harder binding sites are more prone to XB formation.

Cryo-EM structure of the Rhodospirillum rubrum RC-LH1 complex at 2.5 Å

The reaction centre light-harvesting 1 (RC–LH1) complex is the core functional component of bacterial photosynthesis. We determined the cryo-electron microscopy (cryo-EM) structure of the RC–LH1 complex from Rhodospirillum rubrum at 2.5 Å resolution, which reveals a unique monomeric bacteriochlorophyll with a phospholipid ligand in the gap between the RC and LH1 complexes. The LH1 complex comprises a circular array of 16 αβ-polypeptide subunits that completely surrounds the RC, with a preferential binding site for a quinone, designated Q_P, on the inner face of the encircling LH1 complex. Quinols, initially generated at the RC Q_B site, are proposed to transiently occupy the Q_P site prior to traversing the LH1 barrier and diffusing to the cytochrome bc₁ complex. Thus, the Q_P site, which is analogous to other such sites in recent cryo-EM structures of RC–LH1 complexes, likely reflects a general mechanism for exporting quinols from the RC–LH1 complex.

Tuning the Photophysical Features of Self-Assembling Photoactive Polypeptides for Light-Harvesting

The LH1 complex is the major light-harvesting antenna of purple photosynthetic bacteria. Its role is to capture photons, and then store them and transfer the excitation energy to the photosynthetic reaction center. The structure of LH1 is modular and it cooperatively self-assembles from the subunits composed of short transmembrane polypeptides that reversibly bind the photoactive cofactors: bacteriochlorophyll and carotenoid. LH1 assembly, the intra-complex interactions and the light-harvesting features of LH1 can be controlled in micellar media by varying the surfactant concentration and by adding carotenoid and/or a co-solvent. By exploiting this approach, we can manipulate the size of the assembly, the intensity of light absorption, and the energy and lifetime of its first excited singlet state. For instance, via the introduction of Ni-substituted bacteriochlorophyll into LH1, the lifetime of this electronic state of the antenna can be shortened by almost three orders of magnitude. On the other hand, via the exchange of carotenoid, light absorption in the visible range can be tuned. These results show how in a relatively simple self-assembling pigment-polypeptide system a sophisticated functional tuning can be achieved and thus they provide guidelines for the construction of bio-inspired photoactive nanodevices.

Strategies to enhance the excitation energy-transfer efficiency in a light-harvesting system using the intra-molecular charge transfer character of carotenoids

Fucoxanthin is a carotenoid that is mainly found in light-harvesting complexes from brown algae and diatoms. Due to the presence of a carbonyl group attached to polyene chains in polar environments, excitation produces an excited intra-molecular charge transfer. This intra-molecular charge transfer state plays a key role in the highly efficient (∼95%) energy-transfer from fucoxanthin to chlorophyll a in the light-harvesting complexes from brown algae. In purple bacterial light-harvesting systems the efficiency of excitation energy-transfer from carotenoids to bacteriochlorophylls depends on the extent of conjugation of the carotenoids. In this study we were successful, for the first time, in incorporating fucoxanthin into a light-harvesting complex 1 from the purple photosynthetic bacterium, Rhodospirillum rubrum G9+ (a carotenoidless strain). Femtosecond pump-probe spectroscopy was applied to this reconstituted light-harvesting complex in order to determine the efficiency of excitation energy-transfer from fucoxanthin to bacteriochlorophyll a when they are bound to the light-harvesting 1 apo-proteins.

End-to-end communication in a linear supermolecule with a BOPHY centre and N,N-dimethylanilino-based terminals

Long-range electrostatic interactions are sufficient to cause sequential ionization of the terminal groups in a BOPHY-based supermolecule.

Probing structure-function relationships in early events in photosynthesis using a chimeric photocomplex

The native core light-harvesting complex (LH1) from the thermophilic purple phototrophic bacterium Thermochromatium tepidum requires Ca²⁺ for its thermal stability and characteristic absorption maximum at 915 nm. To explore the role of specific amino acid residues of the LH1 polypeptides in Ca-binding behavior, we constructed a genetic system for heterologously expressing the Tch. tepidum LH1 complex in an engineered Rhodobacter sphaeroides mutant strain. This system contained a chimeric pufBALM gene cluster (pufBA from Tch. tepidum and pufLM from Rba. sphaeroides) and was subsequently deployed for introducing site-directed mutations on the LH1 polypeptides. All mutant strains were capable of phototrophic (anoxic/light) growth. The heterologously expressed Tch. tepidum wild-type LH1 complex was isolated in a reaction center (RC)-associated form and displayed the characteristic absorption properties of this thermophilic phototroph. Spheroidene (the major carotenoid in Rba. sphaeroides) was incorporated into the Tch. tepidum LH1 complex in place of its native spirilloxanthins with one carotenoid molecule present per αβ-subunit. The hybrid LH1-RC complexes expressed in Rba. sphaeroides were characterized using absorption, fluorescence excitation, and resonance Raman spectroscopy. Site-specific mutagenesis combined with spectroscopic measurements revealed that α-D49, β-L46, and a deletion at position 43 of the α-polypeptide play critical roles in Ca binding in the Tch. tepidum LH1 complex; in contrast, α-N50 does not participate in Ca²⁺ coordination. These findings build on recent structural data obtained from a high-resolution crystallographic structure of the membrane integrated Tch. tepidum LH1-RC complex and have unambiguously identified the location of Ca²⁺ within this key antenna complex.

The PufX quinone channel enables the light-harvesting 1 antenna to bind more carotenoids for light collection and photoprotection

Photosynthesis in some phototrophic bacteria requires the PufX component of the reaction centre–light‐harvesting 1–PufX (RC‐LH1‐PufX) complex, which creates a pore for quinone/quinol (Q/QH₂) exchange across the LH1 barrier surrounding the RC. However, photosynthetic bacteria such as Thermochromatium (T.) tepidum do not require PufX because there are fewer carotenoid binding sites, which creates multiple pores in the LH1 ring for Q/QH₂ exchange. We show that an αTrp_‐24→Phe alteration of the Rhodobacter (Rba.) sphaeroides LH1 antenna impairs carotenoid binding and allows photosynthetic growth in the absence of PufX. We propose that acquisition of PufX and confining Q/QH₂ traffic to a pore adjacent to the RC Q_B site is an evolutionary upgrade that allows increased LH1 carotenoid content for enhanced light absorption and photoprotection.

Ultra-broadband 2D electronic spectroscopy of carotenoid-bacteriochlorophyll interactions in the LH1 complex of a purple bacterium

We investigate the excitation energy transfer (EET) pathways in the photosynthetic light harvesting 1 (LH1) complex of purple bacterium Rhodospirillum rubrum with ultra-broadband two-dimensional electronic spectroscopy (2DES). We employ a 2DES apparatus in the partially collinear geometry, using a passive birefringent interferometer to generate the phase-locked pump pulse pair. This scheme easily lends itself to two-color operation, by coupling a sub-10 fs visible pulse with a sub-15-fs near-infrared pulse. This unique pulse combination allows us to simultaneously track with extremely high temporal resolution both the dynamics of the photoexcited carotenoid spirilloxanthin (Spx) in the visible range and the EET between the Spx and the B890 bacterio-chlorophyll (BChl), whose Qx and Qy transitions peak at 585 and 881 nm, respectively, in the near-infrared. Global analysis of the one-color and two-color 2DES maps unravels different relaxation mechanisms in the LH1 complex: (i) the initial events of the internal conversion process within the Spx, (ii) the parallel EET from the first bright state S2 of the Spx towards the Qx state of the B890, and (iii) the internal conversion from Qx to Qy within the B890.

The origin of the unusual Qy red shift in LH1-RC complexes from purple bacteria Thermochromatium tepidum as revealed by Stark absorption spectroscopy

Native LH1-RC of photosynthetic purple bacteria Thermochromatium (Tch.) tepidum, B915, has an ultra-red BChl a Qy absorption. Two blue-shifted complexes obtained by chemical modification, B893 and B882, have increasing full widths at half maximum (FWHM) and decreasing transition dipole oscillator strength. 77K Stark absorption spectroscopy studies were employed for the three complexes, trying to understand the origin of the 915 nm absorption. We found that Tr(∆α) and |∆μ| of both Qy and carotenoid (Car) bands are larger than for other purple bacterial LH complexes reported previously. Moreover, the red shifts of the Qy bands are associated with (1) increasing Tr(∆α) and |∆μ| of the Qy band, (2) the red shift of the Car Stark signal and (3) the increasing |∆μ| of the Car band. Based on the results and the crystal structure, a combined effect of exciton-charge transfer (CT) states mixing, and inhomogeneous narrowing of the BChl a site energy is proposed to be the origin of the 915 nm absorption. CT-exciton state mixing has long been found to be the origin of strong Stark signal in LH1 and special pair, and the more extent of the mixing in Tch. tepidum LH1 is mainly the consequence of the shorter BChl-BChl distances. The less flexible protein structure results in a smaller site energy disorder (inhomogeneous narrowing), which was demonstrated to be able to influence |∆μ| and absorption.

Stark absorption spectroscopy on the carotenoids bound to B800-820 and B800-850 type LH2 complexes from a purple photosynthetic bacterium, Phaeospirillum molischianum strain DSM120

Stark absorption spectroscopy was applied to clarify the structural differences between carotenoids bound to the B800-820 and B800-850 LH2 complexes from a purple photosynthetic bacterium Phaeospirillum (Phs.) molischianum DSM120. The former complex is produced when the bacteria are grown under stressed conditions of low temperature and dim light. These two LH2 complexes bind carotenoids with similar composition, 10% lycopene and 80% rhodopin, each with the same number of conjugated CC double bonds (n=11). Quantitative classical and semi-quantum chemical analyses of Stark absorption spectra recorded in the carotenoid absorption region reveal that the absolute values of the difference dipole moments |Δμ| have substantial differences (2 [D/f]) for carotenoids bound to either B800-820 or B800-850 complexes. The origin of this striking difference in the |Δμ| values was analyzed using the X-ray crystal structure of the B800-850 LH2 complex from Phs. molischianum DSM119. Semi-empirical molecular orbital calculations predict structural deformations of the major carotenoid, rhodopin, bound within the B800-820 complex. We propose that simultaneous rotations around neighboring CC and CC bonds account for the differences in the 2 [D/f] of the |Δμ| value. The plausible position of the rotation is postulated to be located around C21-C24 bonds of rhodopin.

Assembly of functional photosystem complexes in Rhodobacter sphaeroides incorporating carotenoids from the spirilloxanthin pathway

Carotenoids protect the photosynthetic apparatus against harmful radicals arising from the presence of both light and oxygen. They also act as accessory pigments for harvesting solar energy, and are required for stable assembly of many light-harvesting complexes. In the phototrophic bacterium Rhodobacter (Rba.) sphaeroides phytoene desaturase (CrtI) catalyses three sequential desaturations of the colourless carotenoid phytoene, extending the number of conjugated carbon–carbon double bonds, N, from three to nine and producing the yellow carotenoid neurosporene; subsequent modifications produce the yellow/red carotenoids spheroidene/spheroidenone (N = 10/11). Genomic crtI replacements were used to swap the native three-step Rba. sphaeroides CrtI for the four-step Pantoea agglomerans enzyme, which re-routed carotenoid biosynthesis and culminated in the production of 2,2′-diketo-spirilloxanthin under semi-aerobic conditions. The new carotenoid pathway was elucidated using a combination of HPLC and mass spectrometry. Premature termination of this new pathway by inactivating crtC or crtD produced strains with lycopene or rhodopin as major carotenoids. All of the spirilloxanthin series carotenoids are accepted by the assembly pathways for LH2 and RC–LH1–PufX complexes. The efficiency of carotenoid-to-bacteriochlorophyll energy transfer for 2,2′-diketo-spirilloxanthin (15 conjugated C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]>C bonds; N = 15) in LH2 complexes is low, at 35%. High energy transfer efficiencies were obtained for neurosporene (N = 9; 94%), spheroidene (N = 10; 96%) and spheroidenone (N = 11; 95%), whereas intermediate values were measured for lycopene (N = 11; 64%), rhodopin (N = 11; 62%) and spirilloxanthin (N = 13; 39%). The variety and stability of these novel Rba. sphaeroides antenna complexes make them useful experimental models for investigating the energy transfer dynamics of carotenoids in bacterial photosynthesis.

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The spirilloxanthin biosynthetic pathway has been constructed in Rba. sphaeroides.

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The new carotenoids are accepted by the photosystem assembly pathways.

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These pigments are efficiently integrated into LH2 and RC–LH1–PufX complexes.

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Carotenoid–BChl energy transfer drops with the number of conjugated CC bonds (N).

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The lowest efficiency, 35%, is for the N = 15 carotenoid 2,2′ diketospirilloxanthin.

Excitation energy trapping and dissipation by Ni-substituted bacteriochlorophyll a in reconstituted LH1 complexes from Rhodospirillum rubrum

Bacteriochlorophyll a with Ni(2+) replacing the central Mg(2+) ion was used as an ultrafast excitation energy dissipation center in reconstituted bacterial LH1 complexes. B870, a carotenoid-less LH1 complex, and B880, an LH1 complex containing spheroidene, were obtained via reconstitution from the subunits isolated from chromatophores of Rhodospirillum rubrum . Ni-substituted bacteriochlorophyll a added to the reconstitution mixture partially substituted the native pigment in both forms of LH1. The excited-state dynamics of the reconstituted LH1 complexes were probed by femtosecond pump-probe transient absorption spectroscopy in the visible and near-infrared spectral region. Spheroidene-binding B880 containing no excitation dissipation centers displayed complex dynamics in the time range of 0.1-10 ps, reflecting internal conversion and intersystem crossing in the carotenoid, exciton relaxation in BChl complement, and energy transfer from carotenoid to the latter. In B870, some aggregation-induced excitation energy quenching was present. The binding of Ni-BChl a to both B870 and B880 resulted in strong quenching of the excited states with main deexcitation lifetime of ca. 2 ps. The LH1 excited-state lifetime could be modeled with an intrinsic decay time constant in Ni-substituted bacteriochlorophyll a of 160 fs. The presence of carotenoid in LH1 did not influence the kinetics of energy trapping by Ni-BChl unless the carotenoid was directly excited, in which case the kinetics was limited by a slower carotenoid S1 to bacteriochlorophyll energy transfer.

Polarization-induced σ-holes and hydrogen bonding

The strong collinear polarizability of the A-H bond in A-H···B hydrogen bonds is shown to lead to an enhanced σ-hole on the donor hydrogen atom and hence to stronger hydrogen bonding. This effect helps to explain the directionality of hydrogen bonds, the well known cooperative effect in hydrogen bonding, and the occurrence of blue-shifting. The latter results when significant additional electron density is shifted into the A-H bonding region by the polarization effect. The shift in the A-H stretching frequency is shown to depend essentially linearly on the calculated atomic charge on the donor hydrogen for all donors in which A belongs to the same row of the periodic table. A further result of the polarization effect, which is also expected for other σ-hole bonds, is that the strength of the non-covalent interaction depends strongly on external electric fields.

Syntheses of C33-, C35-, and C39-peridinin and their spectral characteristics

Peridinin, a nor-carotenoid, exhibits an exceptionally high energy transfer efficiency to chlorophyll a in photosynthesis in the sea. This efficiency would be related to the unique structure of peridinin. To answer the question of why peridinin possesses the irregular C37 skeleton, we have achieved the synthesis of three peridinin derivatives. Their ultrafast time-resolved optical absorption and Stark spectra measurements have shown the presence of the characteristic intramolecular charge transfer state and the featured electrostatic properties of peridinin.

Syntheses of allene-modified derivatives of peridinin toward elucidation of the effective role of the allene function in high energy transfer efficiencies in photosynthesis

Peridinin is known as the main light-harvesting pigment in photosynthesis in the sea and exhibits exceptionally high energy transfer efficiencies to chlorophyll a. This energy transfer efficiency is thought to be related to the intricate structure of peridinin, which possesses allene and ylidenbutenolide functions in the polyene backbone. There are, however, no studies on the relationship between the structural features of peridinin and its super ability for energy transfer. We then focused on the subjects of why peridinin possesses a unique allene group and how the allene function plays a role in the exceptionally high energy transfer. Toward elucidation of the exact role of the allene function, we now describe the syntheses of three relatively unstable allene-modified derivatives of peridinin along with the results of the Stark spectroscopy of peridinin and the synthesized peridinin derivatives.