Examining the origins of observed terahertz modes from an optically pumped atomistic model protein in aqueous solution

TL;DR

This study investigates the microscopic origins of terahertz vibrational modes in proteins, demonstrating how optical excitations can trigger specific THz modes through atomistic simulations and information theory analysis.

Contribution

It introduces a molecular dynamics approach with optical excitations and applies information theory to reveal multiscale responses in proteins, advancing understanding of THz modes in biological systems.

Findings

THz modes emerge significantly from collective behavior

Optical excitations induce multiscale responses in proteins

Results suggest new experimental and simulation directions

Abstract

The microscopic origins of terahertz (THz) vibrational modes in biological systems are an active and open area of current research. Recent experiments [Physical Review X 8, 031061 (2018)] have revealed the presence of a pronounced mode at $\sim$0.3 THz in fluorophore-decorated bovine serum albumin (BSA) protein in aqueous solution under nonequilibrium conditions induced by optical pumping. This result was heuristically interpreted as a collective elastic fluctuation originating from the activation of a low-frequency phonon mode. In this work, we show that the sub-THz spectroscopic response emerges in a statistically significant manner (> 2$\sigma$) from such collective behavior, illustrating how specific THz vibrational modes can be triggered through optical excitations and other charge reorganization processes. We revisit the theoretical analysis with proof-of-concept molecular dynamics that introduce optical excitations into the simulations. Using information theory techniques, we show that these excitations can induce a multiscale response involving the two optically excited chromophores (tryptophans), other amino acids in the protein, ions, and water. Our results motivate new experiments and fully nonequilibrium simulations to probe these phenomena, as well as the refinement of atomistic models of Fr\"ohlich condensates that are fundamentally determined by nonlinear interactions in biology.