Structural Analysis of the Oxyluciferin Emitting Species in Aqueous and Luciferase Media

Understanding the biological environment that surrounds the fireflies’ bioluminescent chromophore is crucial to unravel the key factors that lead to the fine tuning of the emitted radiation. In this work we used the recently published OLU Force Field (OLUFF) to sample the conformational space of the four possible oxyluciferin emitters in both the ground and first electronically excited states in aqueous media and inside the luciferase enzyme to describe the solvation and interaction with the protein. It is shown that, in an aqueous solution, the degree of solvation agrees with the magnitude of the atomic charges on the electronegative heteroatoms for both biological environments. However, in the enzymatic environment, the hydrogen bonding pattern with solvent molecules changes because it competes with direct hydrogen bonds with the protein and the adenosine monophosphate co-factor. These interactions can also be facilitated in some cases via hydrogen bond bridging through water molecules.