Dependence of the hydration status of bacterial light-harvesting complex 2 on polyol cosolventsf

Abstract

Low molecular weight (MW) polyols are organic osmolytes influencing protein structure and activity. We have intended to investigate the effects of low MW polyols on the optical and the excited-state properties of the light-harvesting complex 2 (LH2) isolated from the 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 carotenoids (Crts) remained unchanged, while the near infrared Q 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 changes of the near infrared absorption and fluorescence spectra. Transient absorption spectroscopy revealed that, following the fast processes of BChl-to-Crt triplet energy transfer, rather weak Q signals of B800 and B850 remained and evolved in phase with the kinetics of triplet excited state Crt (Crt*), which are attributed to the Q band shift as a result of Crt*-BChl interaction. The steady state and the transient spectral responses of the Q bands are found to correlate intimately with the water activity varying against polyol MW and mixing ratio, which are rationalized by the change of the hydration status of the C- and N-termini of LH2. Our results suggest that, with reference to the mesophilic purple bacterium Rhodobacter sphaeroides 2.4.1, Tch. tepidum adopts substantially more robust LH2 hydration against the osmotic effects from the low MW polyols.

Effects of low-molecular-weight polyols on the hydration status of the light-harvesting complex 2 from Rhodobacter sphaeroides 2.4.1

“…The lifetime is longer than 3 Crt* in Tch. tepidum LH2 at a similar condition in our previous result (data not shown) [24], because Crt with longer conjugation length has longer 3 Crt* lifetime (Crt with n C=C = 13 dominated in Tch. tepidum LH2) [25].…”

“…4a, b). This spectral feature could be described by the red-shifted B850 groundstate absorption spectrum OD B850 (λ + δλ) to minus the original B850 ground-state absorption spectrum OD B850 (λ) using the below equation [24]:…”

“…We have reported the effect of glycerol and sorbitol, as cosolvents, on the embedded pigment's excitation dynamics of LH2 from moderately thermophilic purple bacterium Tch. tepidum, in terms of the interference of hydration status of the whole pigment-protein complexes induced by glycerol and sorbitol [24]. In this study, we intend to examine the effects of glycerol and sorbitol on the hydration status of LH2s from Rba.…”

Cooperative Photoprotection by Multicompositional Carotenoids in the LH1 Antenna from a Mutant Strain of Rhodobacter sphaeroides

“…As demonstrated for Tch. tepidum LH2, 39,42 Spx in triplet excited state interacts tightly with the ring aggregate of BChl in ground state to form a triplet excimer-like species, which gives rise to a characteristic spectral signature to the longer-wavelength side of the LH2-Q y band. Such kind of "interactive" spectral features were also reported for the LHC II complexes of higher plant.…”

“…The TEP measurements were based on a nanosecond TA apparatus. , Briefly, an optical parametric oscillator driven by a Nd:YAG laser (Quanta-Ray Pro-Series, Spectra-Physics Lasers Inc.) delivered the excitation laser pulses (7 ns, 10 Hz). A continuous-wave white light source (LDLS-EQ-1500, Energetiq) provided the visible-to-near infrared probing light, which was sent to a monochromator (SP2500i, Princeton Instruments, USA) after interrogating with the photoexcited volume.…”