# Mechanical Modulation of S0–S1 and S0–T1 Energy Gaps of 11-cis and All-trans Retinal Schiff Bases

## TL;DR

This paper studies how mechanical forces affect the energy gaps of retinal Schiff bases, which are important in capturing sunlight in proteins.

## Contribution

A new first-order formalism and algorithm are introduced to analyze the mechanical response of retinal chromophores.

## Key findings

- Retinal Schiff base shows significant mechanical response to external forces.
- Optimal forces involve low-frequency coordinates, with differences between S1/T1 states and isomers.
- Mechanical modulation of bond length alternation is not possible.

## Abstract

The
retinal Schiff base is a chromophore of significant biological
relevance, as it is responsible for capturing sunlight in rhodopsins,
which are photoactive proteins found in various living organisms.
Additionally, this chromophore is subjected to various mechanical
forces in different proteins, which alter its structure and, consequently,
its properties. To thoroughly understand the mechanical response limits
of the retinal excitation energy, a simple first-order formalism has
been developed to quantify the chromophore’s optimal mechanical
response to applied external forces (on the order of tens of pN).
Additionally, the response to larger forces is analyzed by using an
algorithm to explore the potential energy surfaces. It can be concluded
that the retinal Schiff base exhibits a significant mechanical response
and that the optimal forces and displacements involve certain coordinates
typically of low frequency, showing differences between the S1 and T1 states, as well as between the 11-cis and all-trans isomers. Additionally,
the possibility of mechanically modulating the bond length alternation
using mechanical forces is ruled out.