The Protein Environment Restricts the Intramolecular Charge Transfer Character of the Luciferine/Luciferase Complex

The elecronic characterization of the luciferine/luciferase complex is fundamental to tune its photophysical properties and develop more efficient devices based on this luminiscent system. Here, we apply molecular dynamics simulations, hybrid quantum mechanics/molecular mechanics (QM/MM) calculations and transition density analysis to compute the absorption and emission spectra of luciferine/luciferase and analyze the nature of the relevant electronic state and its behaviour with the intramolecular and intermolecular degrees of freedom. It is found that the torsional motion of the chromophore is hampered by the presence of enzyme, reducing the intramolecular charge transfer nature of the absorbing and emitting state. In addition, such a reduced charge transfer character does not correlate in a strong way with any individual intramolecular or intermolecular degree of freedom because it is likely influenced by a combination of them.