  Azido-modified alanine residues (AlaN$_3$) are environment-sensitive,
minimally invasive infrared probes for the site-specific investigation of
protein structure and dynamics. Here, the capability of the label is
investigated to query whether or not a ligand is bound to the active site of
Lysozyme and how the spectroscopy and dynamics change upon ligand binding. The
results demonstrate specific differences for center frequencies of the
asymmetric azide stretch vibration, the long time decay and the static offset
of the frequency fluctuation correlation function - all of which are
experimental observables - between the ligand-free and the ligand-bound,
N$_3$-labelled protein. Changes in dynamics can also be mapped onto changes in
the local and through-space coupling between residues by virtue of dynamical
cross-correlation maps. This makes the azide label a versatile and structurally
sensitive probe to report on the dynamics of proteins in a variety of
environments and for a range of different applications.
