Department of Chemistry and Center for Multidimensional Spectroscopy, Korea University, Seoul 136-701, Korea, and Multidimensional Spectroscopy Laboratory, Korea Basic Science Institute, Seoul 136-713, Korea
Received October 16, 2007Introduction
Two-dimensional (2D) optical spectroscopy utilizing multiple ultrafast coherent laser pulses in the infrared or UV-vis frequency range has been used to study protein structure and dynamics, hydrogen-bonding dynamics, femtosecond solvation dynamics, solute-solvent complexation, excitation migration process in photosynthetic light harvesting complexes, and coherence transfers in electronically coupled multichromophore systems. Due to a dramatic advent of laser technology, femtosecond laser systems operating in infrared and visible frequency ranges have been commercially available so that we have seen a wide range of applications utilizing such ultrafast nonlinear optical spectroscopic techniques.
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, can only provide highly averaged information. Therefore, novel spectroscopic techniques with much higher information content have been sought and tested continuously. In 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 solution.1,2
Although the optical analogues of 2D NMR do 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 because of the ability to directly observe and quantify the couplings between quantum states involved in molecular dynamical processes. An elementary and highly simplified schematic diagram in Figure 1 demonstrates that the 2D vibrational spectroscopic technique can provide detailed information on the 3D structure of a given complex molecule, i.e., proteins. A given pair of vibrational chromophores, e.g., amide I local modes, are coupled to each other via a hydrogen-bonding interaction, which results in cross peaks. As the molecule undergoes a structural transition along the reaction coordinate, which leads to a hydrogen-bond breaking, the cross peaks will disappear in time. Consequently, the transient 2D vibrational spectroscopy will provide information on the local conformational change of the target molecule in this case.* Telephone: +82-2-3290-3133. Fax: +82-2-3290-3121.