Fig. 1. Graph displaying the abundance of each K-Ras homodimer normalized to the total ion current (TIC) for the native ESI mass spectra (Fig. S4†) for WT (blue), G12C (red), G12V (green), and G12S (yellow) bound to two GDP or two GppNHp (1 : 1 ratio) at a protein concentration of 20 μM (solid bars) or 80 μM (hashed bars). GppNHp is a non-hydrolyzable analogue of GTP and represents the active state ligand of K-Ras. The theoretical and measured monoisotopic masses for each observed homodimer are reported in Table S1.† .