Proteins are very complex entities that can undergo complex changes after being exposed to an external perturbation. Homopolypeptides have been used extensively to assign IR bands to secondary structures (Susi 1969). ttey are used because of their ability to adopt regular-canonical conformations without containing tertiary structure. Polyproline is a particular example since its inability to form hydrogen bonds with other peptide bonds produces the so-called polyproline I and polyproline II (PPII) helices. Between them, the PPII helix, left-handed with dihedral angles closer to ^-sheet than to a-helix is a special secondary structure. Temperature produces a single transition in this helix that can be studied by 2D-IR spectroscopy. Since no tertiary structure is involved, the transition in a D20 buffer goes from the PPII helix to an unordered conformation without residual structure as can be seen in Fig. 4.5. tte perturbation produces a change in intensity of the peak together with a shift in band position. In Fig. 4.6 the 2D-IR maps are shown, tte synchronous spectrum represents two autopeaks centred around 1,624 and 1,642 cm-1, corresponding to the PPII bands that vary in intensity. As expected from the simulation corresponding to changes in bandwidth, the asynchronous map gives not only noise. Accordingly, the temperature-induced unfolding of the PPII helix corresponds to a protein with a perturbation involving changes in the intensity of two components with variations in bandwidth.
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