Following Torii and Tasumi , the calculated spectra can be structurally analyzed using so-called doorway state calculations. This analysis is based on singular value decomposition (SVD) of the ensemble of N normal modes within a particular frequency range, which decomposes this ensemble into N orthogonal left and right singular vectors. These SVD components are sorted by their singular values, which reflect the relative contributions of various orthogonally decomposed components to the ensemble of normal modes within a particular frequency range. The first component is used as the doorway state, which is a characteristic of the modes in this range. As, of all doorway states, it contains the most common features within the range due to the singular-value sorting of the doorway states. We divide the amide I frequency range (1600-1700 cm -1 ) into the same six doorway ranges used by Chung & Tokmakoff , where the range between 1640 and 1680 cm -1 is divided into 10 -1 intervals (labelled as R2, R3, R4, R5 in the order of increasing frequency), when the range below 1640 -1 (R1) and above 1680 -1 (R6) form two groups. The splitting of the frequency range is guided by the density of eigenstates and the desired frequency resolution, so that the number of eigenstates in each region is appropriate for resolving the frequency-structure relationship. For actin, in these six ranges, the first doorway states contain 42-70% of the squared transition dipole moments of the total of three bright doorway states. We pictorially represent the common features of a given range (see Figure ) with the first column (which represents the first doorway state) of the transformation matrix given by the dot product of the eigenvectors and the left singular vectors of the SVD. This matrix can also be used to determine the relative contributions of different types of secondary structures by multiplying the squares of the first column of the matrix by binary vectors that list the presence of secondary structures as a function of the sequence index .