A model for the three-dimensional structure of the repeating Asn-Pro-Asn-Ala tetrapeptide of the immunodominant circumsporozoite protein of Plasmodium falciparum has been developed. A trial structure in the form of a type I beta turn with asparagine side chains hydrogen-bonded to the backbone peptide linkages was used as a starting point. A repeating oligomer of this trial structure was modeled using energy minimization and molecular dynamics computer simulations in conjunction with image boundary conditions. The most stable structure generated is a right-handed 12(38) helix, which is unlike any previously identified protein secondary structure. The helix has 12 residues per turn, corresponding to an angle of twist of 120 degrees per tetrapeptide unit, and a pitch of 4.95 A, corresponding to a rise of 1.65 A per tetrapeptide unit. It is highly stabilized by extensive hydrogen bonding, with each tetrapeptide unit acting as an acceptor for five hydrogen bonds and as a donor for five hydrogen bonds to residues in adjacent turns as well as having four weak internal hydrogen bonds. A number of nearly isoenergetic variations on the most stable structure that still retained the basic 12(38) helical motif were also discovered. The implications of these structures for vaccine development are discussed.