Biophysics and Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, USA.
J Phys Chem B. 2012 Nov 26;116(46):13618-23. doi: 10.1021/jp305279w. Epub 2012 Nov 12.
Low-lying excited states that correspond to rare conformations or transiently bound species have been hypothesized to play an important role for amyloid nucleation. Despite their hypothesized importance in amyloid formation, transiently occupied states have proved difficult to detect directly. To experimentally characterize these invisible states, we performed a series of Carr-Purcell-Meiboom-Gill (CPMG)-based relaxation dispersion NMR experiments for the amyloidogenic Aβ(1-40) peptide implicated in Alzheimer's disease. Significant relaxation dispersion of the resonances corresponding to the side-chain amides of Q15 and N27 was detected before the onset of aggregation. The resonances corresponding to the peptide backbone did not show detectable relaxation dispersion, suggesting an exchange rate that is not within the practical limit of detection. This finding is consistent with the proposed "dock and lock" mechanism based on molecular dynamics simulations in which the Aβ(1-40) monomer transiently binds to the Aβ(1-40) oligomer by non-native contacts with the side chains before being incorporated into the fiber through native contacts with the peptide backbone.
低能激发态对应于罕见的构象或瞬态结合物种，据推测在淀粉样核形成中发挥重要作用。尽管它们在淀粉样形成中具有假设的重要性，但瞬态占据态已被证明难以直接检测到。为了实验表征这些不可见的状态，我们对与阿尔茨海默病相关的淀粉样蛋白形成 Aβ（1-40）肽进行了一系列基于 Carr-Purcell-Meiboom-Gill（CPMG）的弛豫分散 NMR 实验。在聚集开始之前，检测到对应于 Q15 和 N27 侧链酰胺的共振的显著弛豫分散。肽骨架对应的共振没有检测到可检测的弛豫分散，表明交换率不在实际检测限范围内。这一发现与基于分子动力学模拟的“对接和锁定”机制一致，其中 Aβ（1-40）单体通过与侧链的非天然接触暂时与 Aβ（1-40）寡聚体结合，然后通过与肽骨架的天然接触被纳入纤维。
