Hysteretic Pressure Dependence of Ca2+ Binding in LH1 Bacterial Membrane Chromoproteins

Comparative thermo- and piezostability study of photosynthetic core complexes containing bacteriochlorophyll a or b

The resilience of biological systems to fluctuating environmental conditions is a crucial evolutionary advantage. In this study, we examine the thermo- and piezo-stability of the LH1-RC pigment-protein complex, the simplest photosynthetic unit, in three species of phototropic purple bacteria, each containing only this core complex. Among these species, Blastochloris viridis and Blastochloris tepida utilize bacteriochlorophyll b as the main light-harvesting pigment, while Rhodospirillum rubrum relies on bacteriochlorophyll a. Through spectroscopic analyses, we observed limited reversibility in the effects of temperature and pressure, likely due to the malleability of pigment binding sites within the light-harvesting LH1 complex. In terms of thermal robustness, LH1 complexes in a detergent environment progressively dissociate into dimeric (B820) and monomeric (B777) subunits. However, in the native membrane, degradation primarily occurs directly into B777 without the intermediate formation of B820. Interestingly, while high-pressure compression of core complexes from Blastochloris viridis and Blastochloris tepida caused significant changes in compressibility around 1.3 kbar and the formation of B777 and B820 subunits upon decompression, no such compressibility changes or pressure-induced dissociation were observed in Rhodospirillum rubrum complexes, even at pressures as high as 11 kbar. This study reveals significant differences in the piezo- and thermal properties of phototrophs containing either BChl a or BChl b, underscoring the critical role of structural factors in understanding the temperature- and pressure-induced denaturation phenomena in photosynthetic complexes. Rhodospirillum rubrum, in particular, stands out as one of the most thermodynamically stable systems among phototrophic microorganisms, capable of withstanding temperatures up to 70 °C and pressures exceeding 11 kbar.

Supramolecular chirality in self-assembly of zinc protobacteriochlorophyll-d analogs possessing enantiomeric esterifying groups

Zinc 3-hydroxymethyl-pyroprotopheophorbides-a esterified with a chiral secondary alcohol at the 17-propionate residue were prepared as bacteriochlorophyll-d analogs. The synthetic zinc 31-hydroxy-131-oxo-porphyrins self-aggregated in an aqueous Triton X-100 micellar solution to give red-shifted and broadened Soret and Qy absorption bands in comparison with their monomeric bands. The intense, exciton-coupled circular dichroism spectra of their self-aggregates were dependent on the chirality of the esterifying groups. The observation indicated that the self-aggregates based on the J-type stacking of the porphyrin cores were sensitive to the peripheral 17-propionate residues. The supramolecular structures of the present J-aggregates as models of bacteriochlorophyll aggregates in natural chlorosomes were remotely regulated by the esterifying groups.

Dominant role of excitons in photosynthetic color-tuning and light-harvesting

Photosynthesis is a vital process that converts sunlight into energy for the Earth's ecosystems. Color adaptation is crucial for different photosynthetic organisms to thrive in their ecological niches. Although the presence of collective excitons in light-harvesting complexes is well known, the role of delocalized excited states in color tuning and excitation energy transfer remains unclear. This study evaluates the characteristics of photosynthetic excitons in sulfur and non-sulfur purple bacteria using advanced optical spectroscopic techniques at reduced temperatures. The exciton effects in these bacteriochlorophyll a-containing species are generally much stronger than in plant systems that rely on chlorophylls. Their exciton bandwidth varies based on multiple factors such as chromoprotein structure, surroundings of the pigments, carotenoid content, hydrogen bonding, and metal ion inclusion. The study nevertheless establishes a linear relationship between the exciton bandwidth and Qy singlet exciton absorption peak, which in case of LH1 core complexes from different species covers almost 130 nm. These findings provide important insights into bacterial color tuning and light-harvesting, which can inspire sustainable energy strategies and devices.

Evaluation of the relationship between color-tuning of photosynthetic excitons and thermodynamic stability of light-harvesting chromoproteins

Color-tuning is a critical survival mechanism for photosynthetic organisms. Calcium ions are believed to enhance both spectral tuning and thermostability in obligatory calcium-containing sulfur purple bacteria. This study examined the thermo- and piezo stability of the LH1-RC complexes from two calcium-containing sulfur purple bacteria notable for their extreme red-shifted spectra. The results generally show limited reversibility of both temperature and pressure effects related to the malleability of calcium-binding sites. While the pressure-induced decomposition product closely resembles the calcium-depleted form of the chromoproteins, the thermally induced products reveal monomeric B777 and dimeric B820 forms of bacteriochlorophyll a, similar to those seen in non-sulfur purple bacteria treated with detergent. The study further found nearly unison melting of the protein tertiary and secondary structures. Overall, our findings do not support a direct link between color adjustment and thermodynamic stability in light-harvesting chromoproteins.