Absorption bands of bacteriochlorophyll types in purple bacteria and their response to illumination

• 1.

  1. Absorption spectra of various species of purple bacteria at 20° and −170° with a high spectral resolution, and spectra of light-induced absorbance changes, occurring upon illumination with light of a relatively low intensity, were measured in the near-infrared wavelength region between 750 and 950 mμ.

• 2.

  2. At least 5 spectroscopically different bacteriochlorophyll types, namely B^∗800, B 800, B 820, B 850 and B 890, were observed in Chromatium B^∗800, for which the maximum of the absorption band is at a somewhat lower wavelength than that for B 800, was also found in the Thiorhodaceae Rhodothece conspicua and Thiocapsa floridana; B 820 was also found in Rh. conspicua and in Rhodopseudomonas palustri. All species examined, except Rhodospirilium rubrum, contained B 850 in addition to B 890.

• 3.

  3. In all species, except R. rubrum and Rps. palustris, the absorption bands of either B 820 or B 850 and of B 890 shifted upon illumination towards a slightly longer wavelength. An analysis of the B 850 shift in Rhodopseudomonas spheroides indicated that allumination causes a shift of about 1.25 ± 0.20 mμ and that 25–28% of the B 850 molecules participate in this reaction. It was estimated that in this species the absorption bands of 10–15 molecules of B 850 were shifted per light quantum absorbed at 593 mμ. In Th. floridana and Rth. conspicua there occurred in addition a shift of a band at about 800 mμ towards a shorter wavelength. Lowering of the temperature also effected a shift of the bands of B 820, B 850 and B 890 towards a longer wavelength.

• 4.

  4. The light-induced spectral shifts are interpreted as being caused by conformational changes in the carrier molecules of the bacteriochlorophyll types.