Inelastic neutron scattering study of light-induced dynamics of a photosynthetic membrane system

Inelastic neutron scattering was employed to study photoeffects on the molecular dynamics of membranes of the photosynthetic bacterium Rhodopseudomonas viridis. The main photoactive parts of this biomolecular system are the chlorophyll molecules whose dynamics were found to be affected under illumination by visible light in a twofold manner. First, vibrational modes are excited at energies of 12(2) and $88\left(21\right){\text{cm}}^{-1}$. Second, a partial “freezing” of rotational modes is observed at energies of 1.2(3) and $2.9\left(5\right){\text{cm}}^{-1}$. These results are attributed to a possible coupling between molecular motions and particular mechanisms in the photosynthetic process.

Figure 1

Conceptual design of the instrumental device developed for simultaneous light irradiation and neutron scattering experiments at low temperatures. M: neutron monochromator; A: neutron analyzer; D: neutron detector.

Figure 2

Left: spectral properties of the halogen lamp and the broadband filter. Right: schematic ground plan and vertical section of the sample cell.

Figure 3

Neutron time-of-flight spectra measured for a suspension of membranes of the photosynthetic bacterium Rhodopseudomonas viridis in ${\text{D}}_2\text{O}$ buffer at $T=57\text{K}$ with incoming neutrons of energy $10.3{\text{cm}}^{-1}$. The pulsed light-beam technique was applied with a pulse frequency $\nu =3\text{Hz}$ and with the use of a broadband filter (see Fig. 2). The total counting time amounted to 56 h.

Figure 4

Intensity difference of the “light” and “dark” energy spectra displayed in Fig. 3. Energy bands corresponding to light-induced excitations of vibrational modes and freezing of rotational modes are marked with full and broken arrows, respectively.

Figure 5

Rotation energy vs moment of inertia for selected ${\text{C}}_n{\text{H}}_m$ molecules as well as for bacteriochlorophyll b (${\text{chl}}_{1.2}$ and ${\text{chl}}_{2.9}$). The dashed line corresponds to the prediction of Eq. (2). The inset shows schematically the porphyrin ring of bacteriochlorophyll b (the phytol chain is summarized by ${\text{C}}_{20}{\text{H}}_{39}$).