Electrostatic Spectral Tuning Maps for Biological Chromophores

The mechanism by
which the absorption wavelength of a molecule
is modified by a protein is known as spectral tuning. Spectral tuning
is often achieved by electrostatic interactions that stabilize/destabilize
or modify the shape of the excited and ground-state potential energy
surfaces of the chromophore. We present a protocol for the construction
of three-dimensional “electrostatic spectral tuning maps”
that describe how vertical excitation energies in a chromophore are
influenced by nearby charges. The maps are built by moving a charge
on the van der Waals surface of the chromophore and calculating the
change in its excitation energy. The maps are useful guides for protein
engineering of color variants, for interpreting spectra of chromophores
that act as probes of their environment, and as starting points for
further quantum mechanical/molecular mechanical studies. The maps
are semiquantitative and can approximate the magnitude of the spectral
shift induced by a point charge at a given position with respect to
the chromophore. We generate and discuss electrostatic spectral tuning
maps for model chromophores of photoreceptor proteins, fluorescent
proteins, and aromatic amino acids. Such maps may be extended to other
properties such as oscillator strengths, absolute energies (stability),
ionization energies, and electron affinities.