Extracting Infrared Spectra of Protein Secondary Structures Using a Library of Protein Spectra and the Ramachandran Plot

Infrared (IR) spectra from 1200 to
1800 cm–1 of
the pure α-helix and β-sheet secondary structures have
been extracted using a covariant least-squares procedure which relates
a library of 40 infrared (IR) solution protein spectra from the work
of Dong, Carpenter, and Caughey and amino acid fractions of the proteins
based on assignments by STRIDE (secondary structure identification) of Eisenhaber and Argos. The excitonic splitting
of the β-sheet structures is determined for this library of
solution proteins. The method is extended to find a set of spectral
basis functions that analyze IR spectra of protein samples for α-helix
and β-sheet content. A rigorous error analysis including covariance,
the correlations between the input library spectra, was used to justify
the results and avoid less meaningful results. The utility of the
results on α-helix and β-sheet regions is demonstrated
by detecting protein changes due to cancer in imaging Fourier transform
IR (FTIR) spectra of liver tissue slices. This work ends with a method
to extract IR spectra of less prominent torsional angle distributions.