Principal component analysis of the SARS-CoV-2 spike structure. (a) Eigenvalues of principal components (PCs). The first three PCs are significant. (b) Scatter plot of the contribution of the first three PCs to each particle image together with the projection of the open and closed class maps, shown as red points. The difference between the projections of the two maps is mostly aligned along principal component 1 (PC1). (c) Side view of the first two PCs shown as mean ± 2 × std, where std is the square root of the eigenvalue. Coloring indicates the z-depth of the structure, and is added to assist visualization. Supplementary Figs. S4 and S5 contain additional views of these structures. The scale bar is 5 nm in length.