Application of the three-dimensional doorway-state theory to analyses of the amide-I infrared bands of globular proteins

The three-dimensional doorway-state theory is applied to globular proteins to analyze the structure-spectrum correlation of the amide-I infrared (ir) bands in detail. The following three examples are presented. (1) The doorway states of the frequency region around 1630 cm−1 of flavodoxin and carboxypeptidase A are calculated. It is shown that the ir intensity in this frequency region largely originates from the peptide groups in the central zones of β sheets. (2) Calculations on α-lactalbumin and lysozyme clarify the vibrational motions giving rise to the difference in the amide-I band envelopes of these two proteins. (3) Contributions of the A- and E1-modelike vibrational motions of α helices to the amide-I band envelopes are analyzed. It is shown that the E1-modelike motions have large contributions to the ir intensities of the bands in the 1640–1630 cm−1 region, solving the question as to the origin of such bands observed for highly helical proteins. It is also demonstrated that the frequency splitting between the A- and E1-modelike motions depends strongly on the helix length. These three examples reveal the importance of the detailed analyses of vibrational dynamics that gives rise to characteristic amide-I band envelopes.