The Underestimated Halogen Bonds Forming with Protein Side Chains in Drug Discovery and Design

Halogen bonds (XBs) have been attracting increasing attention in biological systems, especially in drug discovery and design, for their advantages of both improving drug-target binding affinity and tuning ADME/T properties. After a comprehensive literature survey in drug discovery and design, we found that most of the studies on XBs between ligands and proteins have focused on the protein backbone. Meanwhile, we also noticed that the proportion of side-chain XBs to overall XBs decreases as structural resolution becomes lower and lower. We postulated that protein side chains are more flexible in comparison with backbone structures, leading to more unclear electron density and lower resolution of the side chains. As the classic force field used to refine protein structures from diffraction data cannot handle XBs correctly, some of the interactions are lost during the refinement. On the contrary, there is no change in the corresponding ratio of hydrogen bonds (HBs) during structural resolution because HBs can be handled well with the classic force field. Further analysis revealed that Thr and Gln account for a large part of the decreasing XB trend, which could be partly attributed to the misidentified N, C, or O atoms. In addition, the lost XBs might be recovered after the atoms are reassigned, e.g., by flipping Thr side chains. In summary, formation of XBs with protein side chains is underestimated, and more attention should be paid to the potential formation of XBs between organohalogens and protein side chains during X-ray crystallography studies.