溶液中的化学反应的动力学可以由Hendrik Kramers在上个世纪40年代建立的一个理论得到最好的描述，该理论将爱因斯坦关于布朗运动的研究与速率理论联系了起来。
此前，人们一直没有可能测定Kramers的理论所预测的有关小分子的参数。现在，Hoi Sung Chung 和 William Eaton对在蛋白折叠过程中由单个分子所发射的光子进行了监测，发现内部摩擦对Kramers扩散系数有很大贡献。
他们所测出的蛋白折叠的“过渡路径时间”，是在任何一个系统中对Kramers扩散系数和“自由能势垒高度”的首次定性。（生物谷Bioon.com）
生物谷推荐的英文摘要
Nature doi:10.1038/nature12649
Single-molecule fluorescence probes dynamics of barrier crossing
Hoi Sung Chung& William A. Eaton
Kramers developed the theory on how chemical reaction rates are influenced by the viscosity of the medium1， 2. At the viscosity of water， the kinetics of unimolecular reactions are described by diffusion of a Brownian particle over a free-energy barrier separating reactants and products. For reactions in solution this famous theory extended Eyring’s transition state theory， and is widely applied in physics， chemistry and biology， including to reactions as complex as protein folding3， 4. Because the diffusion coefficient of Kramers’ theory is determined by the dynamics in the sparsely populated region of the barrier top， its properties have not been directly measured for any molecular system. Here we show that the Kramers diffusion coefficient and free-energy barrier can be characterized by measuring the temperature- and viscosity-dependence of the transition path time for protein folding. The transition path is the small fraction of an equilibrium trajectory for a single molecule when the free-energy barrier separating two states is actually crossed. Its duration， the transition path time， can now be determined from photon trajectories for single protein molecules undergoing folding/unfolding transitions5. Our finding of a long transition path time with an unusually small solvent viscosity dependence suggests that internal friction as well as solvent friction determine the Kramers diffusion coefficient for α-helical proteins， as opposed to a breakdown of his theory， which occurs for many small-molecule reactions2. It is noteworthy that the new and fundamental information concerning Kramers’ theory and the dynamics of barrier crossings obtained here come from experiments on a protein rather than a much simpler chemical or physical system.
