  Over the last decades, photoreceptive proteins were extensively studied with
biophysical methods to gain a fundamental understanding of their working
mechanisms and further guide the development of optogenetic tools from them.
Time-resolved infrared (IR) spectroscopy is one of the key methods to access
their functional non-equilibrium processes with high temporal resolution, but
has the major drawback that experimental data is usually highly complex.
Linking the spectral response to specific molecular events is a major obstacle.
Here, we investigate a cyanobacteriochrome (CBCR) photoreceptor with a combined
approach of transient absorption spectroscopy in the Visible and IR spectral
regions. We obtain kinetic information in both spectral regions by analysis
with two different fitting methods, global multiexponential fitting and
lifetime analysis. We investigate the ground state dynamics that follow
photoexcitation in both directions of the bi-stable photocycle (Pr* and Pg*) in
the nanosecond and microsecond time regime. We find two ground state
intermediates associated with the decay of Pr* and four with Pg* and report the
macroscopic time constants of their interconversions. One of these processes is
assigned to a structural change in the protein backbone. At later delay times,
the spectroscopic responses are stretched out in time, an effect that can be
better described by the lifetime analysis than the global fit. We ascribe it to
the intrinsic ruggedness of the free energy landscape of proteins.
