  The infrared spectroscopy and dynamics of -CO labels in wild type and mutant
insulin monomer and dimer are characterized from molecular dynamics simulations
using validated force fields. It is found that the spectroscopy of monomeric
and dimeric forms in the region of the amide-I vibration differs for residues
B24-B26 and D24-D26, which are involved in dimerization of the hormone. Also,
the spectroscopic signatures change for mutations at position B24 from
phenylalanine - which is conserved in many organisms and known to play a
central role in insulin aggregation - to alanine or glycine. Using three
different methods to determine the frequency trajectories - solving the nuclear
Schr\"odinger equation on an effective 1-dimensional potential energy curve,
instantaneous normal modes, and using parametrized frequency maps - lead to the
same overall conclusions. The spectroscopic response of monomeric WT and mutant
insulin differs from that of their respective dimers and the spectroscopy of
the two monomers in the dimer is also not identical. For the WT and F24A and
F24G monomers spectroscopic shifts are found to be $\sim 20$ cm$^{-1}$ for
residues (B24 to B26) located at the dimerization interface. Although the
crystal structure of the dimer is that of a symmetric homodimer, dynamically
the two monomers are not equivalent on the nanosecond time scale. Together with
earlier work on the thermodynamic stability of the WT and the same mutants it
is concluded that combining computational and experimental infrared
spectroscopy provides a potentially powerful way to characterize the
aggregation state and dimerization energy of modified insulins.
