Comparison of the Energy Transfer Dynamics in Structural and Spectral Variants of the Light-Harvesting Complex 2 of Purple Bacteria

Abstract

Light-harvesting complex 2 (LH2) of purple bacteria is a well-known model to study the initial light-harvesting steps of photosynthesis. It is organized into two rings: eight or nine bacteriochlorophyll collectively absorb at 800 nm (the B800 ring), while sixteen or eighteen bacteriochlorophyll absorb at 850 nm (the B850 ring). LH2 is produced in several variants both within and across species. While the dynamics of some of these variants have been characterized, the energy transfer rates within many of these variants have not been measured. In this work, we utilize ultrafast transient absorption measurements to compare the energy transfer dynamics of three variants of LH2: LH2 R. sphaeroides, a nonamer, LH2 Ph. molischianum, an octamer, and LH3 Ph. molischianum, which has a blue-shifted absorption spectrum to 820 nm compared to the B850 ring of LH2 Ph. molischianum. These results provide insight into how variations between and within species impact energy flow in photosynthetic systems.