溶液中的化學(xué)反應(yīng)的動力學(xué)可以由Hendrik Kramers在上個世紀(jì)40年代建立的一個理論得到最好的描述,，該理論將愛因斯坦關(guān)于布朗運動的研究與速率理論聯(lián)系了起來。
此前,，人們一直沒有可能測定Kramers的理論所預(yù)測的有關(guān)小分子的參數(shù)?，F(xiàn)在，Hoi Sung Chung 和 William Eaton對在蛋白折疊過程中由單個分子所發(fā)射的光子進行了監(jiān)測,，發(fā)現(xiàn)內(nèi)部摩擦對Kramers擴散系數(shù)有很大貢獻,。
他們所測出的蛋白折疊的“過渡路徑時間”，是在任何一個系統(tǒng)中對Kramers擴散系數(shù)和“自由能勢壘高度”的首次定性,。（生物谷Bioon.com）
生物谷推薦的英文摘要
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.
