Immunogenic and protective peptide sequences are of prime importance in the search for an anti-malarial vaccine. The MSP-1 conserved and semi-conserved sequences have been shown to contain red blood cell (RBC) membrane high affinity binding peptides (HABP). HABP 1513 sequence ((42)GYSLFQKEKMVLNEGTSGTA(61)), from this protein's N-terminal, has been shown to possess a T-epitope; however, it did not induce a humoral immune response or complete protection when evaluated in Aotus monkeys. Analogue peptides with critical binding residues replaced by amino acids with similar mass but different charge were synthesised and tested for immunogenicity and protectivity in monkey. NMR studies correlated structural behaviour with biological function. Non-immunogenic and non-protective 1513 native peptide presented a helical fragment between residues L-4 and E-14. C-terminal, 5-residue-shorter, non-immunogenic, non-protective peptide 17894 contained an alpha-helix from Q(6) to L-12 residues. Immunogenic and protective peptide 13946 presented a shorter alpha-helix between K-7 to N-13 residues. These data suggest that changing certain residues permits better peptide fit within the MHC class II-peptide-TCR complex, thus activating the immune system and inducing a protective immune response. (C) 2004 Elsevier Inc. All rights reserved.