Hidden Effects of Anharmonic Bath on the Excitation Energy Transfer in the Light Harvesting 2 Complex of Purple Bacteria.

Abstract

Environmental effects in the excitation energy transfer (EET) processes have been predominantly modeled by baths of harmonic oscillators; however, the key features of anharmonicities that are overlooked in such descriptions remain poorly understood. Through detailed comparison of the dynamics of a realistic molecular bath with its best effective harmonic representation, here, we uncover new qualitative details of such effects accompanying EET in the light harvesting 2 (LH2) complex of purple bacteria. Results of mixed quantum-classical simulations demonstrate that the harmonic oscillator bath model can effectively reproduce major features of the EET dynamics of those for anharmonic bath with proper choice of initial conditions, confirming the validity of many spectroscopic data modeled by the harmonic oscillator bath. However, it falls short of representing the nonequilibrium nuclear dynamics, even at the qualitative level. In particular, we find that vibrational energy flow mechanisms due to anharmonic couplings between pigment vibrational modes cannot be captured by the harmonic oscillator bath. The importance of this observation in relation to biological mechanisms of heat dissipation are discussed. We also identify important signatures of anharmonicities in detailed features of bath spectral densities. These results demonstrate hidden roles and features of anharmonic effects that can be perceived through accurate representation of potential energy surfaces, which contribute to the robust and resilient functionality of natural light-harvesting complexes.