title: The preferred conformation of the tripeptide Ala-Phe-Ala in water is an inverse gamma-turn: implications for protein folding and drug design.
abstract: Recent studies have provided evidence that peptides as short as tripeptides do adopt preferred conformations. Here we report that the tripeptide Ala-Phe-Ala (AFA) in aqueous solution preferentially forms an inverse gamma-turn. Circular dichroism (CD) indicated the presence of a predominant turn structure, and Fourier transform infrared (FTIR) bands suggested the presence of a gamma-turn forming a bifurcated H-bond with the solvent molecules. The high-resolution structure was obtained by a combined use of NMR spectroscopy and calculations. On the basis of 30 unambiguous ROESY-derived distance restraints (including the Halpha-NH NOE between Ala(1) and Ala(3) and a hydrogen bond between the CO group of Ala(1) and the NH group of Ala(3)), calculations clearly demonstrated the presence of an inverse gamma-turn centered on Phe(2). From NOE data, we estimated a mole fraction for the gamma-turn of 0.65. Since for AFA an extended beta-strand was also reported [Eker, F., Griebenow, K., Cao, X., Nafie, L. A., and Schweitzer-Stenner, R. (2004) Proc. Natl. Acad. Sci. U.S.A. 101, 10054-10059], we investigated the possibility that gamma-turn and beta-strand may represent two major conformations. By using a best-fit procedure that calculated experimental NOEs as weighted averages of the effects originating from both structures, we were able to calculate with good accuracy the backbone NOEs at 280 K in terms of the two limiting conformers, yielding a mole fraction for the gamma-turn and beta-strand conformations of 0.60 and 0.40, respectively, in good agreement with those found by NOE data. The implication of the existence of a preferred conformation by a small structural element is discussed in the context of the nucleation of protein folding events and the design of small peptide and peptidomimetic drugs.
authors.affiliation: Istituto di Chimica Biomolecolare del CNR, Comprensorio Olivetti, Edificio A, Via Campi Flegrei 34, I-80078 Pozzuoli (Napoli), Italy. amotta@icmib.na.cnr.it
authors.forename: Andrea
authors.initials: A
authors.lastname: Motta
authors.forename: Meital
authors.initials: M
authors.lastname: Reches
authors.forename: Lucia
authors.initials: L
authors.lastname: Pappalardo
authors.forename: Giuseppina
authors.initials: G
authors.lastname: Andreotti
authors.forename: Ehud
authors.initials: E
authors.lastname: Gazit
mesh_terms: D000409:Alanine / Q000031:analogs & derivatives*; D002942:Circular Dichroism; D015195:Drug Design; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D018716:Molecular Mimicry; D009842:Oligopeptides / Q000737:chemistry*; D017510:Protein Folding; D012997:Solvents / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared; D014867:Water / Q000737:chemistry*
chemical_list: C507335:Ala-Phe-Ala; D009842:Oligopeptides; D012997:Solvents; D014867:Water; D000409:Alanine