Mass Mapping of Macromolecular Assemblies

扫描透射电子显微镜（STEM）提供了一个通用的方法来确定分子质量，从而在大的蛋白质组装的亚基的安排。大分子被吸附到一个薄的支持膜和纳米尺寸的电子探针扫描整个样品，而收集的弹性散射信号。 所得到的数字图像强度与样本的局部质量密度成比例。图像记录在低电子剂量计数单脉冲从一个环形探测器定位后的标本。STEM可以直接应用于异质群体中的单个大分子，因此不会受到平均伪影的影响。 质量测定通常可以基于仅1000个分子的分析达到1%或2%的精度，这对应于约1毫微微克的材料，尽管在实践中目前需要几纳克来制备合适的样品。

Scanning transmission electron microscopy (STEM) provides a versatile method for determining the molecular mass and hence the arrangement of subunits in large protein assemblies. Macromolecules are adsorbed onto a thin support film and a nanometer-sized electron probe is scanned across the specimen while the elastic scattering signal is collected. The resulting digital image intensity is proportional to the local mass density of the specimen. Images are recorded at low electron dose by counting single pulses from an annular detector positioned after the specimen. STEM can be applied directly to individual macromolecules in a heterogeneous population and is not therefore subject to averaging artifacts. Mass determination can often be made to an accuracy of 1 or 2% based on analysis of only 1000 molecules, which corresponds to about 1 femtogram of material, although in practice several nanograms are currently required to prepare a suitable specimen.