Light-Induced Conformational Changes in Photosynthetic Reaction Centers: Impact of Detergents and Lipids on the Electronic Structure of the Primary Electron Donor

Abstract

Light-induced hypsochromic shifts of the Q(y) absorption band of the bacteriochlorophyll dimer (P) from 865 to 850 nm were identified using continuous illumination of dark-adapted reaction centers (RCs) from Rhodobacter capsulatus when dispersed in the most commonly used detergent, the zwitterionic lauryl N-dimethylamine-N-oxide. Such a shift is known to be the consequence of the decreased degree of delocalization of P. A 2-fold acceleration of the recovery kinetics of P(+) was found in RCs that underwent light-induced structural changes compared to those where the P-band position did not change. The light-induced shift was irreversible except in the presence of a secondary electron donor. Prolonged (15 min) illumination resulted in a shift in the position of the P-band even in neutral or negatively charged detergents. In contrast, RCs reconstituted into liposomes made from lipids with different headgroup charges showed light-induced shifts only if shorter fatty acid chains were used. The light-induced conformational changes caused a prominent decrease of the redox potential of P ranging from 120 to 160 mV depending on the detergent compared to the potential of P in dark-adapted reaction centers. The measured light-induced potential decreases were 55 to 85 mV larger than those reported for reaction centers where the P-band position remained at 865 nm. The influence of structural factors, such as the delocalization of the electron hole on P(+), the involvement of Tyr M210, and the hydrophobic mismatch between the thickness of the hydrophobic belt of the detergent micelles or the lipid bilayer and the RC protein, on the spectral features and electron transfer kinetics is discussed.

Dehydration affects the electronic structure of the primary electron donor in bacterial photosynthetic reaction centers: evidence from visible-NIR and light-induced difference FTIR spectroscopy

“…19 The degree of delocalization of the electron hole on P + has already been proposed, among other structural factors, as playing a role in the stabilization of P + Q A − following photoexcitation on the minute time scale. 70 The notion that reorganization of the protein around P + plays a role in stabilizing charge separation on the long time scale is also in line with recent studies performed in mutant RCs, characterized by different H-bonding patterns of the P dimer. Interestingly, evidence of a lightinduced change of the local dielectric constant in the vicinity of the P dimer has been provided, 12 and proton release by one or more amino acid residues, mainly involving water molecules near P, has been proposed as the relaxation event which stabilizes the charge separated state on a long time scale.…”

“…8,10 The conclusion that the kinetics of P + Q A − recombination after a short (ns) light pulse are not affected by the asymmetry of charge distribution within the P + dimer does not exclude, however, that this parameter may influence the conformational RC rearrangements occurring upon continuous illumination on the second to minute time scale, and resulting in a dramatic stabilization of the charge separated state of the RC. 11,14,15,69,70 We have recently observed 16 that the response to dehydration of the P + Q A − recombination kinetics after a prolonged illumination (20 s) is markedly different from that observed after a short photoexcitation. 16,29 In particular, we found that in RC films dehydrated at r = 76%, continuous photoexcitation resulted in the appearance of an extremely slow phase of P + Q A − recombination, with a lifetime τ AP ≈ 8 s, accounting for about 50% of the decay.…”

Bacterial Reaction Centers Purified with Styrene Maleic Acid Copolymer Retain Native Membrane Functional Properties and Display Enhanced Stability

“…In the near-infrared region, it comprises three main bands that arise from the bacteriochlorin cofactors (Figure 1 B ), the relative intensities and wavelength maxima of which are known to be modulated somewhat by the detergent/lipid environment of the protein. 13 , 14 The SMA-solubilized RCs had an absorbance spectrum that was similar to that of RCs in native membranes or solubilized in either LDAO or DDM, showing that it is structurally intact in the SMA/lipid nanodiscs (Figure 1 B ). Small differences in intensity of the bands at 865 and 760 nm in the four preparations are a consequence of the sensitivity of cofactors at, or close to, the surface of the protein to its detergent/lipid environment.…”

Bacterial Reaction Centers Purified with Styrene Maleic Acid Copolymer Retain Native Membrane Functional Properties and Display Enhanced Stability

“…[12] However, such applications require the preparation of large amounts of protein that is structurally stable and optimally active. This presents challenges because the functional properties of the RC are known to be modulated when it is removed from the native membrane, [13][14][15] and its stability is known to be dependent on the detergent/lipid environment. [16] To determine the usefulness of the SMA copolymer as a new vehicle for housing integral membrane proteins such as RCs, the properties of SMA-purified RCs were systematically compared with those of RCs purified in the commonly used detergents N,N-dimethyldodecylamine N-oxide (LDAO) and n-dodecyl b-d-maltoside (DDM), as well as with those of RCs in intact native photosynthetic membranes.…”