Coherences of Bacteriochlorophyll a Uncovered Using 3D-Electronic Spectroscopy

Mapping
the multidimensional energy landscape of photosynthetic
systems is crucial for understanding their high efficiencies. Multidimensional
coherent spectroscopy is well suited to this task but has difficulty
distinguishing between vibrational and electronic degrees of freedom.
In pigment–protein complexes, energy differences between vibrations
within a single electronic manifold are similar to differences between
electronic states, leading to ambiguous assignments of spectral features
and diverging physical interpretations. An important control experiment
is that of the pigment monomer, but previous attempts using multidimensional
coherent spectroscopy lacked the sensitivity to capture the relevant
spectroscopic signatures. Here we apply a variety of methods to rapidly
acquire 3D electronic–vibrational spectra in seconds, leading
to a mapping of the vibrational states of Bacteriochlorophyll a (BChla) in solution. Using this information,
we can distinguish features of proteins containing BChla from the monomer subunit and show that many of the previously reported
contentious spectral signatures are vibrations of individual pigments.