2.2. Linear peptide synthesis
The sequences we used for the design of linear peptides of the RBD, S2, and NP of SARS-CoV-2 were obtained from GenBank under accession numbers NC_045512.2 and MN908947.3. For epitope screening of RBD and NP, we designed panels of linear peptides spanning the entire sequence of the RBD and NP protein of SARS-CoV-2 (each peptide contains 20 amino acid residues with 5 residues overlapping with the adjacent peptides). For epitope screening of S2, only three non-overlapping individual peptides, FE-18, FI-22, and CK-16, with lengths ranging from 16-22 aa, were used. All the peptides were synthesized by Pepmic company (China). Peptide synthesis was performed using a standard solid-phase Fmoc method. Peptides were purified to homogeneity (purity more than 90%) by HPLC and identified by laser desorption mass spectrometry. Peptides were used individually or as pooled sets. Three peptides were combined to form one pooled peptide set. To obtain a stock solution, lyophilized individual peptides were dissolved in deionized water or Dimethyl Sulfoxide 3% (DMSO, Sigma Aldrich, Germany) according to the manufacturer’s instruction. Amino acid sequences of peptide sets are shown in table 1.