The immunologically dominant central portion of the circumsporozoite (CS) surface protein on the malaria parasite Plasmodium falciparum contains a large number of tandemly repeated NPNA tetrapeptide motifs. The preferred secondary structure of this repeat unit in aqueous solution has been investigated with the aid of the secondary structure-inducing amino acid (S)-alpha-methylproline (p(Me)). H-1-Nuclear magnetic resonance (NMR) and circular dichroism (CD) spectroscopy have been used to probe the structures of synthetic peptides containing one to three tetrapeptide NP(Me)NA units. The far-UV CD spectra of these peptides show more intense negative bands at 215 nm than do similar peptides based on the NPNA motif. This and the temperature dependence of the peptide amide chemical shifts, the pattern of NOE connectivities, and the magnitude of (3)J coupling constants, derived from one- and two-dimensional NMR spectra of Ac(NP(Me)NA)(3)-OH, provide strong evidence for stable turnlike structures. From NOE distance and dihedral angle restraints, structures consistent with the NMR parameters were calculated.