# Multiscale Simulation of Primary Charge Separation Mechanisms in an LH1-RC Complex

## TL;DR

This study reveals how charge separation in bacterial photosynthesis occurs in the LH1-RC complex, challenging traditional models by highlighting the role of the protein environment.

## Contribution

The first comprehensive analysis of the entire LH1-RC complex, identifying the protein scaffold's critical role in charge separation.

## Key findings

- Charge separation originates from the P/B BChl pair on the active branch, not the 'special pair' (P).
- The protein environment is pivotal in facilitating efficient charge separation.
- A low-lying charge-transfer state on the inactive branch is inefficient due to inconsistent directionality.

## Abstract

The light-harvesting complex II (LH2) of purple bacteria
captures
solar energy using bacteriochlorophyll (BChl) pigments. This energy
is then transferred to the LH1 complex and subsequently to the embedded
reaction center (RC). The initial separation of charges instigates
a series of subsequent processes, ultimately culminating in the synthesis
of adenosine triphosphate (ATP). While the excitation transfer process
within the LH2 complex has been thoroughly characterized, the atomistic
mechanism of charge separation in the RC remains unresolved. In this
study, we employed a combination of classical molecular dynamics (MD),
ab initio quantum mechanics/molecular mechanics (QM/MM) MD, and time-dependent
density functional theory (TD-DFT) to delineate the excitation funnel
within the LH1 ring of Thermochromatium tepidum, which is instrumental in facilitating charge separation in the
RC. The analysis of the excitation profile indicates that the process
of charge separation does not originate from the so-called “special
pair” (P), but rather from the adjacent P/B BChl pair located
on the active branch. The protein environment has been determined
to play a pivotal role in this process. A competing low-lying charge-transfer
state on the inactive branch exhibits inconsistent directionality,
thereby rendering it an inefficient route. This study presents the
first comprehensive analysis of an entire LH1-RC complex, and the
findings challenge traditional models, highlighting the role of the
protein scaffold as a crucial factor for charge separation in bacterial
photosynthesis.