## 2 Results and Discussion
### Computational Studies  
The computational analysis of synthesized and tested compounds follows the previously generated protocol [46]. Briefly, the hemagglutinin 3D structure was generated by homology modeling using the exact sequence of the assayed viral strains [46]. The Receptor Binding Site, responsible for recognizing the sialic acid and highlighted by the SiteMap analysis on the HA surface, was selected to perform the docking calculations. Structure-based studies exploited the SP-peptide protocol available in Glide to explore the large conformational flexibility of the studied ligands. As already described, the 100 final poses were clustered to obtain each ligand's best representative binding geometry. As all compounds showed activity toward A/Parma/24/09 H1N1, we focused our study on this viral strain's hemagglutinin. Additionally, we report the discussion about the most interesting compounds (**4**, **9**, **16**, and **18**) binding to the RBS of A/Parma/24/09 H1N1. Docking score values for the four new ligands in comparison with previously studied tetrapeptides are reported in Table 4. The Glide calculated score is similar for all ligands and in line with the HI activity in Table 2. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Pep**} & \multirow{2}{*}{**Sequence**} & **IC\({}_{50}\)b ** (nM)** \\ \cline{3-3}  & & **A/Parma/24/09 H1N1** \\ \hline
**4** & S(N-Me)LDC & 4.6 \(\pm\) 2.3 \\
**9** & SK(N-Me)HS & 3.2 \(\pm\) 1.6 \\
**16** & SNKHS & 1.2 \(\pm\) 0.6 \\
**18** & SKHINRS & 3.3 \(\pm\)1.7 \\ \hline \hline \end{tabular}  
* [PERSON] et al. (2017) [23]. All peptides are amidated and acetylated. b IC\({}_{50}\) corresponds to the peptide concentration that inhibits virus-induced hemagglutinin by 50%.  
\end{table} 
Table 2: _Cont._ \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Pep** & **Sequence** & **EC\({}_{50}\)a ** (\(\mu\)M)** & **SI b ** \\ \hline
**1 c** & SLDC & 4.6 \(\pm\) 0.05 \(\times\) 10\({}^{-6}\) & \textgreater{}5.4 \(\times\) 10\({}^{6}\) \\
**2 c** & SKHS & 4.8 \(\pm\) 0.12 \(\times\) 10\({}^{-8}\) & \textgreater{}5.2 \(\times\) 10\({}^{8}\) \\
**4** & S(N-Me)LDC & 8.5 \(\pm\) 0.04 \(\times\) 10\({}^{-5}\) & \textgreater{}2.9 \(\times\) 10\({}^{5}\) \\
**9** & SK(N-Me)HS & 1.425 \(\pm\) 0.02 \(\times\) 10\({}^{-4}\) & \textgreater{}1.78 \(\times\) 10\({}^{3}\) \\
**16** & SNKHS & 4.7 \(\pm\) 0.02 \(\times\) 10\({}^{-4}\) & \textgreater{}5.3 \(\times\) 10\({}^{4}\) \\
**18** & SKHINRS & 5 \(\pm\) 0.04 \(\times\) 10\({}^{-5}\) & \textgreater{}5 \(\times\) 10\({}^{5}\) \\ \hline \hline \end{tabular}  
\end{table} 
Table 3: Sequence and Nt activity of peptides and peptoids. Compounds **16** and **18** are SKHS peptiods where the side chains of Lys2 and Ser4 residues, respectively, are moved on the peptide nitrogen atom. The docked binding mode follows the same profile we observed previously for the lead tetrapeptide SKHS (Figure S17): the ligands occupy the water-exposed RBS in an extended conformation, with Ser4 forming a network of hydrogen bonds with Gln210, Glu211, and Gly212. Differently from SKHS, the lysine side chain is able to form an ionic contact with Asp174 in both cases (Figure 4). Figure 4: (Panels (**A**,**C**) Docked poses of peptiods **16** and **18** represented as sticks (orange and purple C atoms, respectively) in the RBS of A/Parma/24/09 H1N1 (deep-cyan cartoon). Just the HA residues that interact with the ligands are shown as lines for clarity’s sake. H-bonds are represented as yellow dashed lines. (Panels (**B**,**D**)) Corresponding 2D ligand interaction diagrams; in these diagrams, residues close to the ligand are colored according to their properties (orange, negatively charged; blue, positively charged; green, hydrophobic; cyan, polar); H-bonds are shown as magenta arrows; and a gray shadow encircles atoms that are exposed to the solvent.