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

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.