High efficiency light harvesting by carotenoids in the LH2 complex from photosynthetic bacteria: unique adaptation to growth under low-light conditions

Abstract

Rhodopin, rhodopinal, and their glucoside derivatives are carotenoids that accumulate in different amounts in the photosynthetic bacterium, Rhodoblastus (Rbl.) acidophilus strain 7050, depending on the intensity of the light under which the organism is grown. The different growth conditions also have a profound effect on the spectra of the bacteriochlorophyll (BChl) pigments that assemble in the major LH2 light-harvesting pigment-protein complex. Under high-light conditions the well-characterized B800-850 LH2 complex is formed and accumulates rhodopin and rhodopin glucoside as the primary carotenoids. Under low-light conditions, a variant LH2, denoted B800-820, is formed, and rhodopinal and rhodopinal glucoside are the most abundant carotenoids. The present investigation compares and contrasts the spectral properties and dynamics of the excited states of rhodopin and rhodopinal in solution. In addition, the systematic differences in pigment composition and structure of the chromophores in the LH2 complexes provide an opportunity to explore the effect of these factors on the rate and efficiency of carotenoid-to-BChl energy transfer. It is found that the enzymatic conversion of rhodopin to rhodopinal by Rbl. acidophilus 7050 grown under low-light conditions results in nearly 100% carotenoid-to-BChl energy transfer efficiency in the LH2 complex. This comparative analysis provides insight into how photosynthetic systems are able to adapt and survive under challenging environmental conditions.

Figure 1

Structures of (A) all-trans-rhodopin glucoside and 13-cis-rhodopinal glucoside and (B) one-third portion of the LH2 B800-820 ring complex from Rbl. acidophilus strain 7050 (PDB 1IJD) showing the protein-bound BChls (green) and carotenoids (purple).

Figure 2

Normalized steady-state absorption spectra of (A) rhodopin glucoside and (B) rhodopinal glucoside in carbon disulfide, benzyl alcohol, methanol, and acetonitrile recorded in 2 mm path length cuvettes at room temperature.

Figure 3

Normalized steady-state absorption spectra of the LH2 complexes from Rbl. acidophilus 10050, 7050 LL, and 7050 HL recorded in 2 mm path length cuvettes at room temperature.

Figure 4

HPLC chromatograms of the pigment extract from Rbl. acidophilus 7050 LH2 complexes prepared from cells grown under LL (top trace) and HL (bottom trace) conditions. Both chromatograms were detected at 502 nm. The major pigments were identified as follows: 1, rhodopinal glucoside; 2, rhodopinal; 3, rhodopin glucoside; 4, BChl a; 5, rhodopin; and 6, lycopene. The minor unlabeled peaks are primarily cis isomers of the major carotenoids.

Figure 5

Emission (blue), excitation (red), and 1-T (black) spectra of LH2 complexes obtained from Rbl. acidophilus 10050, 7050 LL, and 7050 HL. The green line shows the ratio of the normalized excitation and 1-T spectra and in the region of carotenoid absorption gives a quantitative measurement of the carotenoid-to-BChl energy transfer efficiency.

Figure 6

Reconstruction of the (A, C, E) 1-T and (B, D, F) fluorescence excitation spectra (black traces) of the LH2 complexes from Rbl. acidophilus 10050, 7050 LL, and 7050 HL. The 1-T spectra of purified rhodopin glucoside (orange traces) and rhodopinal glucoside (purple traces) were recorded in benzyl alcohol and summed to generate the reconstructed spectra (red traces). The BChl bands in the Soret region between 300 and 400 nm and in the Q_X region near 600 nm were modeled using Gaussian functions (green lines) for simplicity.

Figure 7

Transient absorption spectra of rhodopin glucoside and rhodopinal glucoside in carbon disulfide, benzyl alcohol, methanol, and acetonitrile recorded at room temperature using the indicated excitation wavelengths.

Figure 8

Evolution associated difference spectra (EADS) obtained from globally fitting the transient absorption data sets from rhodopin glucoside and rhodopinal glucoside in carbon disulfide, benzyl alcohol, methanol, and acetonitrile given in Figure 7.

Figure 9

Transient absorption spectra of LH2 complexes from Rbl. acidophilus 10050, 7050 LL, and 7050 HL recorded at room temperature using the indicated excitation wavelengths.

Figure 10

Evolution associated difference spectra (EADS) obtained from globally fitting the transient absorption data sets of LH2 complexes from Rbl. acidophilus 10050, 7050 LL, and 7050 HL given in Figure 9.

Figure 11

B3LYP/6-31G(d) calculated structures of (A) trans-rhodopin glucoside, (B) trans-rhodopinal glucoside, (C) trans-rhodopin, (D) trans-rhodopinal, (E) the S₁ relaxed excited state of trans-rhodopinal, (F) 13-cis-rhodopinal, and 13-cis-rhodopinal model. The structures given in panels C, D, E, and G are simplified, higher-symmetry analogues used in the MNDO-PSDCI, EOM-CCSD and CAS-SCF theoretical calculations, which retain the full π-system. The calculated vacuum dipole moments (in debyes (D)) and the dipole moment vectors (the length is not relevant) are shown underneath those structures that have a dipole moment. The dashed ellipse in panel D shows the primary repulsive atom–atom interaction responsible for making the cis configuration more stable than the trans configuration by ∼8 kJ/mol in nonpolar solvent (n-hexane) and ∼8.5 kJ/mol in polar solvent (acetonitrile).

Figure 12

Analysis of the excited state manifold responsible for the electronic absorption spectrum of rhodopinal in n-hexane (solid yellow spectra) based on MNDO-PSDCI theory. Four configurations (Figure 11) were investigated relative to experimental observations: trans-rhodopinal (upper left), 13-cis-rhodopinal (upper right), trans-rhodopinal in a corkscrew conformation (lower left), and 13-cis-rhodopinal in a corkscrew conformation (lower right). The corkscrew conformation was generated by adjusting the dihedral angles of the single bonds 10° from planar and the double bonds 5° from planar, with all dihedral distortions in the same direction. The cis configuration involves rotation of the double bond directly connected to the aldehyde group, identified using the torsional arrow in Figure 11D. The heights of the bars are proportional to the calculated oscillator strengths of the transitions, and the color reflects the ionic versus covalent character (Figure 13). The approximate symmetry is indicated for selected states, and 1 kilokayser (kK) = 1000 cm^–1.

Figure 13

Excited state ππ* level ordering for trans-rhodopin, trans-rhodopinal, and 13-cis-rhodopinal for excitations from the ground state for the equilibrium ground state conformation (S0 geom) and the relaxed first excited singlet conformation (S1 rlxd). The energies and oscillator strengths were calculated based on MNDO-PSDCI theory using a CI basis set of the 10 highest energy filled π orbitals and the 10 lowest energy unfilled π orbitals. The stationary states are represented by rectangles where the height is proportional to the oscillator strength, and the color reflects the ionic versus covalent character of the state (see inset). The symmetry labels are approximate. The values in parentheses index electronic states 8 and 11 discussed in the text.

Figure 14

Simulation of the S₁ to S_N spectra of trans-rhodopin (top), trans-rhodopinal (middle), and 13-cis-rhodopinal (bottom) based on MNDO-PSDCI theory (full single and double configuration interaction involving the 11 highest energy filled and 11 lowest energy unfilled π orbitals). All calculations assumed the S₁ relaxed excited state geometries from Figure 13. The horizontal axis is linear in energy, where 1 kilokayser (kK) = 1000 cm^–1, and the corresponding wavelength is marked as an inset in green.

Figure 15

Pathways of energy transfer in the LH2 complex. a, absorption; ta, transient absorption. Dashed arrows indicate radiationless processes.

Figure 16

Spectral overlap of the hypothetical fluorescence of the carotenoid donor (orange and purple traces) and absorption of the BChl acceptor (black trace). The absorption spectra of the carotenoids were reflected about their spectral origins to obtain approximations to the S₂ (1¹Bu⁺) → S₀ fluorescence spectra.