Photophysical Properties of the Excited States of Bacteriochlorophyll <i>f</i> in Solvents and in Chlorosomes

Bacteriochlorophyll <i>f</i> (BChl <i>f</i>) is a photosynthetic pigment predicted nearly 40 years ago as a fourth potential member of the <i>Chlorobium</i> chlorophyll family (BChl <i>c</i>, <i>d</i>, and <i>e</i>). However, this pigment still has not been found in a naturally occurring organism. BChl <i>c</i>, <i>d</i>, and <i>e</i> are utilized by anoxygenic green photosynthetic bacteria for assembly of chlorosomeslarge light-harvesting complexes that allow those organisms to survive in habitats with extremely low light intensities. Recently, using genetic methods on two different strains of Chlorobaculum limnaeum that naturally produce BChl <i>e</i>, two research groups produced mutants that synthesize BChl <i>f</i> and assemble it into chlorosomes. In this study, we present detailed investigations on spectral and dynamic characteristics of singlet excited and triplet states of BChl <i>f</i> with the application of ultrafast time-resolved absorption and fluorescence spectroscopies. The studies were performed on isolated BChl <i>f</i> in various solvents, at different temperatures, and on BChl <i>f</i>-containing chlorosomes in order to uncover any unusual or unfavorable properties that stand behind the lack of appearance of this pigment in natural environments

Polymer–Chlorosome Nanocomposites Consisting of Non-Native Combinations of Self-Assembling Bacteriochlorophylls

Chlorosomes are one of the characteristic light-harvesting antennas from green sulfur bacteria. These complexes represent a unique paradigm: self-assembly of bacteriochlorophyll pigments within a lipid monolayer without the influence of protein. Because of their large size and reduced complexity, they have been targeted as models for the development of bioinspired light-harvesting arrays. We report the production of biohybrid light-harvesting nanocomposites mimicking chlorosomes, composed of amphiphilic diblock copolymer membrane bodies that incorporate thousands of natural self-assembling bacteriochlorophyll molecules derived from green sulfur bacteria. The driving force behind the assembly of these polymer–chlorosome nanocomposites is the transfer of the mixed raw materials from the organic to the aqueous phase. We incorporated up to five different self-assembling pigment types into single nanocomposites that mimic chlorosome morphology. We establish that the copolymer-BChl self-assembly process works smoothly even when non-native combinations of BChl homologues are included. Spectroscopic characterization revealed that the different types of self-assembling pigments participate in ultrafast energy transfer, expanding beyond single chromophore constraints of the natural chlorosome system. This study further demonstrates the utility of flexible short-chain, diblock copolymers for building scalable, tunable light-harvesting arrays for technological use and allows for an in vitro analysis of the flexibility of natural self-assembling chromophores in unique and controlled combinations