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<p><span style="font-family: Helvetica, Arial, sans-serif; font-size: 14px; background-color: rgb(244, 249, 253);">Protein crystallization remains a serious bottleneck to structure determination by X-ray diffraction methods. Compounds acting as &ldquo;molecular glue&rdquo; provide a promising strategy to overcome this bottleneck. Such molecules interact via noncovalent bonds with two or more protein surfaces to promote lattice formation. Here, we report a 1.5 Å resolution crystal structure of lysine-rich cytochrome&nbsp;</span><span style="font-style: italic; font-family: Helvetica, Arial, sans-serif; font-size: 14px;">c</span><span style="font-family: Helvetica, Arial, sans-serif; font-size: 14px; background-color: rgb(244, 249, 253);">&nbsp;complexed with&nbsp;</span><span style="font-style: italic; font-family: Helvetica, Arial, sans-serif; font-size: 14px;">p</span><span style="font-family: Helvetica, Arial, sans-serif; font-size: 14px; background-color: rgb(244, 249, 253);">-phosphonatomethyl-calix[4]arene (</span><b style="font-family: Helvetica, Arial, sans-serif; font-size: 14px;">pmclx</b><span style="vertical-align: -0.4em; font-size: 0.8em; line-height: 0.8em; font-family: Helvetica, Arial, sans-serif;"><b>4</b></span><span style="font-family: Helvetica, Arial, sans-serif; font-size: 14px; background-color: rgb(244, 249, 253);">). Evidence for complex formation in solution was provided by NMR studies. Similar to&nbsp;</span><span style="font-style: italic; font-family: Helvetica, Arial, sans-serif; font-size: 14px;">p</span><span style="font-family: Helvetica, Arial, sans-serif; font-size: 14px; background-color: rgb(244, 249, 253);">-sulfonato-calix[4]arene (</span><b style="font-family: Helvetica, Arial, sans-serif; font-size: 14px;">sclx</b><span style="vertical-align: -0.4em; font-size: 0.8em; line-height: 0.8em; font-family: Helvetica, Arial, sans-serif;"><b>4</b></span><span style="font-family: Helvetica, Arial, sans-serif; font-size: 14px; background-color: rgb(244, 249, 253);">), the cavity of&nbsp;</span><b style="font-family: Helvetica, Arial, sans-serif; font-size: 14px;">pmclx</b><span style="vertical-align: -0.4em; font-size: 0.8em; line-height: 0.8em; font-family: Helvetica, Arial, sans-serif;"><b>4</b></span><span style="font-family: Helvetica, Arial, sans-serif; font-size: 14px; background-color: rgb(244, 249, 253);">&nbsp;entrapped a single lysine side chain. Interesting features of protein recognition by the phosphonate substituents were identified in the crystal structure. A new calixarene binding site was identified at Lys54. The electron density at this site indicated two distinct calixarene conformers, suggesting a degree of ligand mobility. The role of&nbsp;</span><b style="font-family: Helvetica, Arial, sans-serif; font-size: 14px;">pmclx</b><span style="vertical-align: -0.4em; font-size: 0.8em; line-height: 0.8em; font-family: Helvetica, Arial, sans-serif;"><b>4</b></span><span style="font-family: Helvetica, Arial, sans-serif; font-size: 14px; background-color: rgb(244, 249, 253);">&nbsp;in protein crystal packing (molecular glue and patchy particle model) as well as differences in protein-binding with respect to&nbsp;</span><b style="font-family: Helvetica, Arial, sans-serif; font-size: 14px;">sclx</b><span style="vertical-align: -0.4em; font-size: 0.8em; line-height: 0.8em; font-family: Helvetica, Arial, sans-serif;"><b>4</b></span><span style="font-family: Helvetica, Arial, sans-serif; font-size: 14px; background-color: rgb(244, 249, 253);">&nbsp;are discussed.</span></p>