Two-dimensional terahertz/IR spectroscopy: a unique probe of ultrafast hydrogen-bond dynamics of liquid water and model systems

氢键是理解水化学和几乎所有生物化学的关键。当然，氢键决定了纯水的结构，并赋予了纯水许多奇异的特性，比如它在4摄氏度时的最大密度，以及它在冻结时的膨胀。氨基酸残基之间的氢键和协同效应决定了肽和蛋白质的结构。最后，同样重要的是，蛋白质和酶结合袋的表面经常被水溶解，有强烈的迹象表明，蛋白质改变了周围水的结构，而水改变了蛋白质的动力学。蛋白质结合水似乎具有许多类似于结晶水或玻璃水的性质，甚至经常出现在x射线晶体结构中。因此，研究过冷水，特别是玻璃水的氢键结构和动力学是非常重要的。为了分离出氢键动力学和协同性的影响，研究肽模型系统的结构和动力学也是必要的。氢键动力学有很多种形式。平衡动力学范围从周期为~0.2-0.5 ps的氢键弯曲和拉伸模式到慢至8 ps的扩散平移和旋转弛豫模式。非平衡动力学（例如在oh -拉伸模式激发后）涉及在~1-2 ps的时间尺度上氢键的断裂。这些过程很难用一种技术来研究。红外和电介质技术受限于频率范围。拉曼散射技术受微弱信号的影响，对水中的旋转运动不敏感。在此，我们提出了很少使用的太赫兹场诱导二次谐波产生（TFISH）的超快光谱技术的发展。众所周知，它能在水中产生大量信号，涵盖了从旋转扩散到氢键弯曲和伸展的整个范围。此外，它可以扩展为多维光谱（2D-TFISH），可用于测量非平衡动力学。这将给我们提供一个独特的机会来直接研究非平衡弛豫动力学（如oh -拉伸和弯曲模式的弛豫）对氢键的影响。该技术将用于过冷水（体积或局限在二氧化硅纳米孔中），以确定在~220K下液-液相变的存在，并首次研究相变对水的结构和动力学的影响。它们还将用于肽模型系统，如n-甲基乙酰胺和类似物，以研究协同性和非平衡弛豫动力学。

Hydrogen bonds are the key to understanding aqueous chemistry and nearly all of biochemistry. Of course, hydrogen bonds determine the structure of pure water and give it many of its bizarre properties such as its density maximum at 4 degrees C and its expansion on freezing. Hydrogen bonds between amino-acid residues and cooperative effects determine the structure of peptides and proteins. Finally yet importantly, the surfaces of proteins and enzyme binding pockets are often solvated by water and there are strong indications that the protein changes the structure of the surrounding water while the water changes the dynamics of the protein. Protein-bound water appears to have many properties like that of crystalline or perhaps glassy water and often even shows up in X-ray crystal structures. It is therefore vital to study the hydrogen-bond structure and dynamics of supercooled water and especially glassy water, which can be made through nano-confinement. It is also essential to study the structure and dynamics of peptide model systems in order to be able to isolate the effects of hydrogen-bond dynamics and cooperativity.Hydrogen-bond dynamics takes on many forms. Equilibrium dynamics ranges from hydrogen-bond bend and stretch modes with periods of ~0.2-0.5 ps to diffusive translational and rotational relaxation as slow as 8 ps. Non-equilibrium dynamics (such as after the excitation of an OH-stretch mode) involve the breaking of hydrogen bonds on a ~1-2 ps timescale. These processes are difficult to study with one technique. Infrared and dielectric techniques suffer from limited frequency ranges. Raman scattering techniques suffer from weak signals and turn out not to be sensitive to rotational motion in water.Here we propose the development of the little-used ultrafast spectroscopy technique of terahertz-field-induced second-harmonic generation (TFISH). It is known to yield large signals in water and covers the entire range from rotational diffusion to hydrogen-bond bends and stretches. Moreover, it lends itself to be expanded into a multi-dimensional spectroscopy (2D-TFISH) that can be used to measure non-equilibrium dynamics. This would give us the unique opportunity to study the effects of non-equilibrium relaxation dynamics (such as the relaxation of OH-stretch and bend modes) on hydrogen bonds directly. The techniques will be used on supercooled water (bulk or confined in silica nanopores) to determine the presence of a liquid-liquid phase transition at ~220K and for the first time the effects of the phase transition on the structure and dynamics of water. They will also be used on peptide model systems such as N-methylacetamide and analogues to study cooperativity and non-equilibrium relaxation dynamics.