Red Emission From LH2 at Low Temperature: Where Does it Come From?

At low temperature a new dynamic feature has been observed in LH2 at longer delays by Chachisvilis et al (1, 2). The stimulated emission/bleaching band broadens and splits into two bands in about 3 ps. The new band is located at about 870 nm and it continues to move further to the red and broadens on the tens of picoseconds time scale. Early 4.2 K steady state fluorescence measurements of a LH1-less mutant of Rb. sphaeroides showed an unusually large Stokes’ shift, which was explained by an emission from a minor long-wavelength component of B850 (3). On the other hand recent time resolved data were interpreted as a stimulated emission from the lowest exciton component of a disordered ring of B850 (4, 5), whereas the slower phases of the dynamics were assigned to the transfer among inhomogeneously distributed rings (5). The zero phonon hole action spectra have also been interpreted as to position the lowest exciton component of B850 at 870 nm (6). The effect is much less pronounced in LH1 suggesting that it may be related to the tendency of LH2 to form very large aggregates (7, 8) or to some LH2-specific impurity. In this work we try to identify the origin of this emission.