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. Author manuscript; available in PMC: 2021 May 6.
Published in final edited form as: J Am Soc Mass Spectrom. 2020 Apr 21;31(5):1140–1150. doi: 10.1021/jasms.0c00066

Figure 4.

Figure 4.

Difference plots for the UVPD backbone cleavage distributions of (a) apo SNase and Ca(II)-bound SNase (summed holo and apo fragment ions) and (b) apo SNase and Pr(III)-bound SNase (summed holo and apo ions) (8+). Negative values represent regions of the protein backbone for which backbone cleavages of the metal complexes are suppressed relative to the apoprotein. The segmented bar beneath the residue numbers displays secondary structural elements: yellow (turn), blue (beta strand), orange (α helix), and black (unassigned). The binding sites for Ca(II) and Pr(III) are demarcated by pink bars. (c, d) The values from the difference plots are superimposed on the crystal structure (PDB: 3BDC) as heat maps to show areas of enhancement (red) and suppression (blue) of fragmentation upon metal binding. Selected residues are labeled. The binding site for SNase is expanded and labeled in greater detail in Figure S4.