Carotenoid Charge Transfer States and Their Role in Energy Transfer Processes in LH1–RC Complexes from Aerobic Anoxygenic Phototrophs

Light-harvesting complexes ensure necessary flow of excitation energy into photosynthetic reaction centers. In the present work, transient absorption measurements were performed on LH1–RC complexes isolated from two aerobic anoxygenic phototrophs (AAPs), <i>Roseobacter</i> sp. COL2P containing the carotenoid spheroidenone, and <i>Erythrobacter</i> sp. NAP1 which contains the carotenoids zeaxanthin and bacteriorubixanthinal. We show that the spectroscopic data from the LH1–RC complex of <i>Roseobacter</i> sp. COL2P are very similar to those previously reported for <i>Rhodobacter sphaeroides</i>, including the transient absorption spectrum originating from the intramolecular charge-transfer (ICT) state of spheroidenone. Although the ICT state is also populated in LH1–RC complexes of <i>Erythrobacter</i> sp. NAP1, its appearance is probably related to the polarity of the bacteriorubixanthinal environment rather than to the specific configuration of the carotenoid, which we hypothesize is responsible for populating the ICT state of spheroidenone in LH1–RC of <i>Roseobacter</i> sp. COL2P. The population of the ICT state enables efficient S₁/ICT-to-bacteriochlorophyll (BChl) energy transfer which would otherwise be largely inhibited for spheroidenone and bacteriorubixanthinal due to their low energy S₁ states. In addition, the triplet states of these carotenoids appear well-tuned for efficient quenching of singlet oxygen or BChl-a triplets, which is of vital importance for oxygen-dependent organisms such as AAPs

Steady-state spectra of purified PS complexes from <i>G</i>. <i>phototrophica</i>.

<p>(A) Absorption spectra recorded at room temperature (red line) and at 77 K (blue line). (B) The thick line shows the LD (<i>LD</i> = <i>A</i>_H—<i>A</i>_V) spectra of the PS complex embedded in polyacrylamide gel. <i>A</i>_H and <i>A</i>_V correspond to absorbance of horizontally and vertically polarized light, respectively. For a flat, disk-like particle in a vertically compressed gel, the horizontal direction is parallel with the particle plane, vertical with particle normal. The thin line shows the reduced LD, <i>LD</i> / <i>Abs</i>., where <i>Abs</i>. is isotropic absorbance. (C) Circular dichroism spectrum of PS complexes in solution. All dichroic spectra were measured at room temperature. Abs, absorbance; CD, circular dichroism; LD, linear dichroism; mdeg, millidegree; PS, photosynthetic; RT, room temperature.</p

Raman Spectroscopy Adds Complementary Detail to the High-Resolution X-Ray Crystal Structure of Photosynthetic PsbP from <em>Spinacia oleracea</em>

<div><p>Raman microscopy permits structural analysis of protein crystals <em>in situ</em> in hanging drops, allowing for comparison with Raman measurements in solution. Nevertheless, the two methods sometimes reveal subtle differences in structure that are often ascribed to the water layer surrounding the protein. The novel method of drop-coating deposition Raman spectropscopy (DCDR) exploits an intermediate phase that, although nominally “dry,” has been shown to preserve protein structural features present in solution. The potential of this new approach to bridge the structural gap between proteins in solution and in crystals is explored here with extrinsic protein PsbP of photosystem II from <em>Spinacia oleracea</em>. In the high-resolution (1.98 Å) x-ray crystal structure of PsbP reported here, several segments of the protein chain are present but unresolved. Analysis of the three kinds of Raman spectra of PsbP suggests that most of the subtle differences can indeed be attributed to the water envelope, which is shown here to have a similar Raman intensity in glassy and crystal states. Using molecular dynamics simulations cross-validated by Raman solution data, two unresolved segments of the PsbP crystal structure were modeled as loops, and the amino terminus was inferred to contain an additional beta segment. The complete PsbP structure was compared with that of the PsbP-like protein CyanoP, which plays a more peripheral role in photosystem II function. The comparison suggests possible interaction surfaces of PsbP with higher-plant photosystem II. This work provides the first complete structural picture of this key protein, and it represents the first systematic comparison of Raman data from solution, glassy, and crystalline states of a protein.</p> </div

Pair alignment of spinach and tobacco PsbP sequences.

<p>Sequences are numbered starting with 1 at the first residue of the mature protein. Asterisks mark identities (149 of 186 residues, 78%). Residues present in the crystalline protein but unresolved in the electron density are bold (spinach) or underlined (tobacco); residues 1 to 9 of the tobacco structure were missing due to partial degradation <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0046694#pone.0046694-Ifuku4" target="_blank">[33]</a>.</p

Assignment of the Raman bands of spinach PsbP.

<p>Assignment of the Raman bands of spinach PsbP.</p

Modeling of spinach PsbP and cyanoP.

<p><b>Top</b>, root mean square fluctuation (y-axis, nm) of each C_α atom (x-axis, residue number) during the production phase of molecular dynamics simulations of spinach PsbP (red) and CyanoP (black). <b>Middle</b>, pair alignment of spinach PsbP and CyanoP sequences. Sequences begin with the first residue resolved at the N-terminus of the respective crystal structures. The two sequences were aligned and colored by Clustal X <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0046694#pone.0046694-Thompson1" target="_blank">[45]</a>. Identical residues are marked with an asterisk, those with high similarity with a colon, and those with lower similarity with one dot. The two red bars mark the large and small loops in PsbP, the black bar the small loop in CyanoP. <b>Bottom</b>, secondary structure (yellow) and water accessibility (blue) of spinach PsbP based on Procheck <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0046694#pone.0046694-Laskowski1" target="_blank">[46]</a>. Strands are represented as arrows, helices as folded tape, irregular regions as a line, gaps as a dash. Accessibility is shaded from white (fully accessible) to dark (fully buried).</p

Modeled structure of spinach PsbP.

<p>The structure is shown after 15 ns of molecular dynamics at 300 K. Secondary structure elements are indicated: strands, numbered blue; helices, lettered red; irregular, grey; modeled internal loops, yellow (longer loop, residues 90 to 107 between helix b and strand 6; shorter loop, residues 135 to 139 between strands 7 and 8). The Zn²⁺ ion (magenta sphere) is coordinated by Asp165 carboxylate and His144 imidazole (side chains in atomic colors with cyan carbons). The termini are labeled; the N-terminus is that of the crystal structure starting at residue 16.</p

Secondary structure content of PsbP.

<p>The amide I band was analyzed from Raman spectra acquired on protein samples in solution, glassy state (DCDR), and crystals. Spectra were deconvoluted using the pattern recognition least-squares method (LSA) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0046694#pone.0046694-Overman1" target="_blank">[38]</a> and two reference intensity profile methods (3-RIP and 4-RIP) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0046694#pone.0046694-Tuma1" target="_blank">[39]</a>. Secondary structure content is given as % of residues ± standard deviation calculated from the standard deviations for each respective reference set. All % values are based on the full sequence of 190 residues; the number of residues in each secondary structure type is given in parentheses. The 4-RIP method does not normalize to 100%. The categories α-ordered and α-disordered structures reflect helix mobility. In the model, the 15 native and 4 remaining His-tag residues were assigned as unordered, and added to the 48 residues observed in that conformation.</p

X-ray data collection and refinement statistics.

<p>Values in parentheses are for the highest-resolution shell.</p

Water vibration region of spinach PsbP Raman spectra.

<p>Spectra were acquired on samples of protein in DCDR (green) and crystal (red). The spectra are centered on the most intense Raman water band with maximum at ∼3400 cm⁻¹.</p