Aggregates from Perylene Bisimide Oligopeptides as a Test Case for Giant Vibrational Circular Dichroism

Vibrational
circular dichroism (VCD) spectroscopy has become an
excellent tool to study biological nanostructures and biomimetic materials
in their functional environment and is thus complementary to otherwise
employed diffraction, imaging, and spectroscopy methods. However,
it is still difficult to relate the observed exceptionally large VCD
signals to specific structural elements. Here, we systematically studied
the VCD signatures of structurally well-defined and thoroughly characterized
nanofibrils from oligopeptide-substituted perylene bisimides that
comprise single parallel β-sheets. These nanofibrils show a
giant VCD signal in the absence of β-sheet stacking and a negative
VCD couplet despite their right-handed helicity. The giant VCD signal
was very sensitive to subtle changes in the molecular structure as
well as 13C-labeling, which caused a strong disruption
of the exciton system as confirmed by two-dimensional infrared spectroscopy.
Simulations based on the commonly applied transition dipole coupling
model qualitatively reproduced the IR spectra but failed to account
for the observed giant VCD or the strong isotope effect. Because our
model system and isotope labeling imposes stringent structural constraints
of the observed spectroscopic features, our results challenge current
assumptions regarding the structural parameters determining VCD sign
and intensity. The investigated system may, hence, serve as a benchmark
for more sophisticated models with better predictive power for the
investigation of protein aggregates in biomedical context or novel
oligopeptide-based nanomaterials.