CHARMM-DYES: Parameterization of Fluorescent Dyes for Use with the CHARMM Forcefield

We present CHARMM-compatible forcefield parameters for a series of fluorescent dyes from the Alexa, Atto and Cy families, commonly used in F¨orster resonance energy transfer (FRET) experiments. These dyes are routinely used in experiments to resolve the dynamics of proteins and nucleic acids at the nanoscale. However, little is known about the accuracy of the theoretical approximations used in determining the dynamics from the spectroscopic data. Molecular dynamics simulations can provide valuable insights into these dynamics at an atomistic level, but this requires accurate parameters for the dyes. The complex structure of the dyes, and the importance of this in determining their spectroscopic properties, means that parameters generated by analogy to existing parameters do not give meaningful results. Through validation relative to quantum chemical calculation and experiment, the new parameters are shown to significantly outperform those that can be generated automatically, giving better agreement in both the charge distributions and structural properties. These improvements, in particular with regards to orientation of the dipole moments on the dyes, are vital for accurate simulation of FRET processes.