  The spectroscopic response of and structural dynamics around all
azido-modified alanine residues (AlaN$_3$) in Lysozyme is characterized. It is
found that AlaN$_3$ is a positionally sensitive probe for the local dynamics,
covering a frequency range of $\sim 15$ cm$^{-1}$ for the center frequency of
the line shape. This is consistent with findings from selective replacements of
amino acids in PDZ2 which reported a frequency span of $\sim 10$ cm$^{-1}$ for
replacements of Val, Ala, or Glu by azidohomoalanine (AHA). For the frequency
fluctuation correlation functions (FFCFs) the long-time decay constants
$\tau_2$ range from $\sim 1$ to $\sim 10$ ps which compares with experimentally
measured correlation times of 3 ps. Attaching azide to alanine residues can
yield dynamics that decays to zero on the few ps time scale (i.e. static
component $\Delta_0 \sim 0$ ps$^{-1}$) or to a remaining, static contribution
of $\sim 0.5$ ps$^{-1}$ (corresponding to 2.5 cm$^{-1}$), depending on the
local environment on the 10 ps time scale. The magnitude of the static
component correlates qualitatively with the degree of hydration of the
spectroscopic probe. Although attaching azide to alanine residues is found to
be structurally minimally invasive with respect to the overall protein
structure, analysis of the local hydrophobicity indicates that the hydration
around the modification site differs for modified and unmodified alanine
residues, respectively.
