Dependence of Hydration Status of Bacterial Light-Harvesting Complex 2 on Polyol Cosolvents
Low molecular weight (MW) polyols are organic as osmolytes influencing protein structure and activity. We have intended to investigate the effects of low MW polyols on the optical and excited-state properties of the light-harvesting complex 2 (LH2) isolated from photosynthetic bacterium Thermochromatium (Tch.) tepidum, a thermophile growing at 50 °C. Steady state spectroscopy demonstrated that, on increasing glycerol or sorbitol fractions up to 60% (polyol/water, v/v), the visible absorption of carotenoid (Crt) remained unchanged, while the near-infrared Qy absorption of bacteriochlorophyll a (BChl) at 800 nm (B800) and 850 nm (B850) varied slightly. Further increasing the fraction of glycerol (but not sorbitol) to 80% (v/v) induced distinct spectral changes of the near infrared absorption and fluorescence. Transient absorption spectroscopy revealed that, after the BChl-to-Crt triplet energy transfer processes, rather weak spectral signals of B800 and B850 remained and evolved in phase with triplet excited-state Crt (3Crt*), which are attributed to the B850 Qy-band shift induced by interacting with 3Crt*. The steady-state and transient spectroscopic responses of LH2s are found to correlate with the water activity varying upon changing polyol fraction or molecular weight, which are rationalized by the change of hydration status of LH2 C- and N-termini. With reference to the mesophilic purple bacterium Rhodobacter sphaeroides 2.4.1, Tch. tepidum adopts substantially more robust LH2 hydration against the osmotic effects of low MW polyols.