Exciton Origin of Color-Tuning in Ca2+-Binding Photosynthetic Bacteria

Int J Mol Sci. 2021 Jul 8;22(14):7338. doi: 10.3390/ijms22147338.


Flexible color adaptation to available ecological niches is vital for the photosynthetic organisms to thrive. Hence, most purple bacteria living in the shade of green plants and algae apply bacteriochlorophyll a pigments to harvest near infra-red light around 850-875 nm. Exceptions are some Ca2+-containing species fit to utilize much redder quanta. The physical basis of such anomalous absorbance shift equivalent to ~5.5 kT at ambient temperature remains unsettled so far. Here, by applying several sophisticated spectroscopic techniques, we show that the Ca2+ ions bound to the structure of LH1 core light-harvesting pigment-protein complex significantly increase the couplings between the bacteriochlorophyll pigments. We thus establish the Ca-facilitated enhancement of exciton couplings as the main mechanism of the record spectral red-shift. The changes in specific interactions such as pigment-protein hydrogen bonding, although present, turned out to be secondary in this regard. Apart from solving the two-decade-old conundrum, these results complement the list of physical principles applicable for efficient spectral tuning of photo-sensitive molecular nano-systems, native or synthetic.

Keywords: Ca2+-binding bacteria; light-harvesting; molecular excitons; photosynthesis; spectral red-shift.

MeSH terms

  • Bacteria / chemistry*
  • Bacteria / metabolism
  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / metabolism
  • Bacteriochlorophylls / chemistry*
  • Bacteriochlorophylls / metabolism
  • Calcium / chemistry*
  • Calcium / metabolism
  • Light-Harvesting Protein Complexes / chemistry*
  • Light-Harvesting Protein Complexes / metabolism


  • Bacterial Proteins
  • Bacteriochlorophylls
  • Light-Harvesting Protein Complexes
  • Calcium