The binding affinity of human pediatric respiratory syncytial virus Phosphoprotein's C-terminal tail to nucleocapsid can be improved by a rationally designed halogen-bonded system.

Human respiratory syncytial virus (hRSV) is a common contagious virus that causes infections of pediatric pneumonia and specifically impacts infants and small children. The hRSV phosphoprotein is a key component of the viral RNA polymerase, which can interact with nucleocapsid and other partners through its C-terminal tail (CTT) to promote the formation of viral transcriptase complex, where the Phe241 is a key anchor residue. Based on the crystal template-modeled complex structure of hRSV nucleocapsid with a peptidic segment derived from the phosphoprotein's CTT, we successfully introduced a rationally designed halogen-bonded system to the complex interface by substituting para (p)-position of the side-chain phenyl moiety of CTT Phe241 residue with a halogen atom X (X = F, Cl, Br or I). The halogen-bonded system consists of a halogen bond (X-bond) between nucleocapsid Ser131 residue and CTT Phe241 residue as well as a hydrogen bond (H-bond) between nucleocapsid Ser131 residue and nucleocapsid Glu128 residue; the X-bond and H-bond share a common hydroxyl group of nucleocapsid Ser131 residue. High-level theoretical calculations suggested that bromine Br is the best choice that can render strong potency for the X-bond and can confer high affinity to the nucleocapsid-CTT binding. Affinity analysis revealed that the p-brominated CTT ([p]bCTT) exhibited 6.3-fold affinity improvement relative to its nonhalogenated counterpart. In contrast, the Br-substitutions at ortho (o)- and meta (m)-positions, which resulted in two negative controls of o-brominated [o]bCTT and m-brominated [m]bCTT, respectively, were unable to form effective X-bond with nucleocapsid according to theoretical investigation and did not improve the binding affinity essentially relative to native CTT.