Expanding Covalent Attachment Sites of Nonnative Chromophores to Encompass the C-Terminal Hydrophilic Domain in Biohybrid Light-Harvesting Architectures

Increasing the solar spectral coverage of native photosynthetic antennas can be achieved using biohybrid light-harvesting (LH) structures comprised of native-like bacterial photosynthetic peptides and synthetic bacteriochlorins with strong near-infrared absorption. Four such biohybrids have been prepared wherein synthetic maleimido-bearing bacteriochlorin BC1-mal is covalently attached to a Cys residue substituted at either the +1, +5 or +11 position (relative to His-0) of the 48-residue β-peptide of Rb. sphaeroides LH1. In addition, a β-peptide with Phe substituted for Tyr at the +4 position along with +1Cys was used to examine possible quenching of the excited BC1 by the Tyr. The β-peptide analogs, as well as their peptide-BC1 conjugates when combined with native α-peptide, and bacteriochlorophyll a (BChl a) self-assemble to form αβ-dyads and therefrom LH1-type cyclic (αβ)_n oligomers. Static and time-resolved optical studies show that all of the oligomeric assemblies transfer excitation energy from the appended BC1 to the BChl a array (B875) with an average efficiency of 85 %.