The invention concerns a structural peptide, identified by antibodies directed against a polypeptide, comprising the 2-45 amino acid sequence of the N-terminal end of the Core or nucleocapsid (p21) protein of the hepatitis C virus (HCV), excluding any protein or peptide compound comprising or consisting of the N-terminal end. The invention is characterized in that it comprises a tertiary structure consisting of at least a first peptide fragment having a secondary structure in helix, a second peptide fragment having secondary structure in helix and a third peptide bond fragment linking the two helices, these two helices being substantially perpendicular to each other in space. This peptide can be used for detecting antibodies directed against the p21 protein of HCV, for detecting all of part of the RNA of HCV and for preparing a diagnostic, prophylactic or therapeutic composition for detecting, preventing or treating an HCV infection.