ABSTRACT
Most of the conventional linear spectroscopic methods, though they have been proven to be extremely useful for studying structural and dynamical properties of complex molecules in condensed phases, provide highly averaged information. Therefore, novel spectroscopic techniques capable of providing much higher information content have been sought and tested incessantly. In the research community of NMR spectroscopy, such efforts led to developing a variety of 2D NMR techniques such as NOESY (nuclear Overhauser enhancement spectroscopy) and COSY (correlation spectroscopy) methods among many others, and they have been extensively used to study structural and dynamical properties of proteins in solutions.1, 2
Although the 2D optical spectroscopy that has been considered to be an optical analog of 2D NMR does not provide atomic resolution structures of complex molecules, optical domain multidimensional spectroscopy has certain advantages because of the dramatic gain in time resolution (~ subpicosecond scale) possible and the ability to directly observe and quantify the couplings between quantum states involved in molecular dynamical processes.19 An elementary and highly simplified schematic diagram in Figure 1.1 demonstrates that time-resolved 2D vibrational spectroscopic technique can provide detailed information on the 3D structure of a given complex molecule, that is, proteins.20 A pair of vibrational chromophores, for example, amide I local modes in polypeptide backbone, are coupled to each other via hydrogen-bonding interaction, which results in cross-peaks in the 2D amide I infrared (IR) spectrum. As a molecule undergoes a structural transition along a certain reaction (folding or unfolding) coordinate, where hydrogen-bond breaking occurs, the cross-peaks will disappear in time.21 Consequently, the transient 2D vibrational spectroscopy will provide information on the local conformational change of the target molecule in this case.
