megadaltonscale

Megadalton-scale refers to macromolecular assemblies with masses on the order of one million daltons (1 MDa) or more. In practical terms, this describes complexes with roughly 10^6 to 10^7 Da, including large ribosomes, multi-subunit protein complexes, and viral capsids. For example, bacterial 70S ribosomes weigh about 2.5–3 MDa, and eukaryotic ribosomes are around 4–5 MDa. Many viral capsids and large proteasome assemblies also reside in this range, with some giant viruses reaching tens of megadaltons.

Measurement and analysis at this scale uses specialized techniques. Native mass spectrometry and charge-detection mass spectrometry can measure intact megadalton complexes under gentle ionization, though increasing heterogeneity and size pose challenges. Analytical ultracentrifugation (AUC) provides sedimentation-based mass estimates; SEC with multi-angle light scattering (SEC-MALS) yields molar mass and size information; small-angle X-ray scattering (SAXS) and cryo-electron microscopy (cryo-EM) offer shape and structural context. Mass determination by cryo-EM often relies on density and subunit stoichiometry, while native MS can reveal subunit composition and stoichiometry in some cases.

Context and significance: The megadalton scale encompasses molecular machines such as ribosomes, spliceosomes, proteasomes, and viral assemblies, which are central to biology and medicine. Challenges include heterogeneous composition, dynamic subassemblies, and sensitivity to buffer conditions. Sample preparation commonly uses volatile buffers like ammonium acetate to preserve non-covalent interactions during ionization.