Prediction of First Hyperpolarizability of Fluorescent Proteins

de Wergifosse, Marc [UCL] et al.

Physicists have been trying to explain naturally occurring phenomena by the so-called "Laws of Physics" for ages. Nowadays they have gotten to the point that they can predict a vast number of situations or events. For the last decennia biophysicists have been trying to make up a complementary set of laws of biophysics in order to be able to predict biomolecular interactions, optical, physical and other properties of biomolecules and much more. Here, we need to take into account that biomolecular systems are much more complex than small molecules. Because of the size of biomolecules, the calculation of their properties requires larger computational resources than that of small organic molecules. So, scientists have already developed Molecular Mechanics (MM) and Molecular Dynamics (MD) algorithms for modeling biomolecular dynamics and interactions1,2, while the computation of the linear and nonlinear optical properties of biomolecules - to our knowledge - still requires quantum chemical treatments. Nevertheless, these calculations can only retain those molecular fragments responsible for the major part of the optical responses while the effects of the surrounding can be incorporated using electrostatic schemes3. Our research contributes to the prediction of the first hyperpolarizability (β), a second order nonlinear optical property, of a set of fluorescent proteins (FPs), specifically concentrated on the included chromophores. Usually we can assume that small organic chromophores move freely in solution and they can only interact with each other and with the solvent. The chromophores of FPs on the other hand are located inside a so-called beta-barrel, protected from the environment by the 3D structure of the rest of the protein, and are kept in position by interactions with neighboring atoms inside the protein (Fig. 1). This produces a lot more possible interactions for the chromophore than in the case of small organic molecules in solution, and hence many more factors to take into account in the computations.