Tuning the Photophysical Features of Self-Assembling Photoactive Polypeptides for Light-Harvesting

Abstract

The LH1 complex is the major light-harvesting antenna of purple photosynthetic bacteria. Its role is to capture photons, and then store them and transfer the excitation energy to the photosynthetic reaction center. The structure of LH1 is modular and it cooperatively self-assembles from the subunits composed of short transmembrane polypeptides that reversibly bind the photoactive cofactors: bacteriochlorophyll and carotenoid. LH1 assembly, the intra-complex interactions and the light-harvesting features of LH1 can be controlled in micellar media by varying the surfactant concentration and by adding carotenoid and/or a co-solvent. By exploiting this approach, we can manipulate the size of the assembly, the intensity of light absorption, and the energy and lifetime of its first excited singlet state. For instance, via the introduction of Ni-substituted bacteriochlorophyll into LH1, the lifetime of this electronic state of the antenna can be shortened by almost three orders of magnitude. On the other hand, via the exchange of carotenoid, light absorption in the visible range can be tuned. These results show how in a relatively simple self-assembling pigment-polypeptide system a sophisticated functional tuning can be achieved and thus they provide guidelines for the construction of bio-inspired photoactive nanodevices.

Evaluation of the relationship between color-tuning of photosynthetic excitons and thermodynamic stability of light-harvesting chromoproteins

“…While the latter structure evidently belongs to monomeric solubilized BChls units called B777, the 821-nm band resembles the spectrum of a detergent-solubilized B820 subunit from non-sulfur purple bacterium Rps. rubrum (Chang et al 1990, Pandit et al 2003, Fiedor and Scheer 2005, Michalik et al 2019. The B777 form that consists of BChl bound to either α or β polypeptide chain is typically obtained by further dissociation of B820 using an increased amount of detergent (Parkes-Loach et al 1988).…”