Center for Biophysics and Computational Biology, University of Illinois, Urbana, Illinois 61801, USA.
J Chem Phys. 2009 Nov 21;131(19):195101. doi: 10.1063/1.3262489.
Protein folding barriers can be so low that a substantial protein population diffusing in the transition state region can be detected. The very fast kinetic phase contributed by transition state transit is the molecular phase. We detect the molecular phase of the beta-sheet protein FiP35 from 60 to 83 degrees C by T-jump relaxation experiments. The molecular phase actually slows down slightly with increasing temperature. Thus the friction that controls the prefactor in Kramers' transition state model does not scale with solvent viscosity. Instead, we postulate that an increase in the energy landscape roughness as the hydrophobic effect strengthens with increasing temperature explains the slowing of the molecular phase. We measured that the duration tau(m) of the molecular phase depends slightly on the size of the T-jump, in agreement with this explanation. The tau(m) measured here provides the best current estimate for the transit time from folded to unfolded state of a single protein molecule. We confirm this by directly comparing relaxation and single molecule signals computed by using Langevin trajectory models on a realistic FiP35 free energy surface.
蛋白质折叠的障碍可能非常低，以至于可以检测到大量在过渡态区域扩散的蛋白质。由过渡态转变贡献的非常快速的动力学相是分子相。我们通过 T-jump 弛豫实验检测到 FiP35 β-折叠蛋白在 60 到 83 摄氏度之间的分子相。随着温度的升高，分子相实际上会稍微减慢。因此，控制 Kramers 过渡态模型中前因子的摩擦力与溶剂粘度不成比例。相反，我们假设随着温度升高疏水性增强，能量景观粗糙度的增加解释了分子相的减慢。我们测量到分子相的持续时间 tau(m) 稍微取决于 T-jump 的大小，这与该解释一致。此处测量的 tau(m) 提供了当前对单个蛋白质分子从折叠状态到未折叠状态的转变时间的最佳估计。我们通过直接比较弛豫和单分子信号来证实这一点，这些信号是使用基于 FiP35 自由能表面的 Langevin 轨迹模型计算得出的。
