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The alanine-rich XAO peptide adopts a heterogeneous population, including turn-like and polyproline II conformations

The solution structure of the hepta-alanine polypeptide Ac-X2A7O2-NH2 (XAO) has been a matter of controversy in the current literature. On one side of the argument is a claim that the peptide adopts a mostly polyproline II (PPII) structure, with a <20% population of β conformations at room temperature [Shi Z, Olson CA, Rose GA, Baldwin RL, Kallenbach NR (2002) Proc Natl Acad Sci USA 99:9190–9195], whereas the other side of the argument insists that the peptide exists as an ensemble of conformations, including multiple β-turn structures [Makowska J, Rodziewicz-Motowidlo S, Baginska K, Vila JA, Liwo A, Chmurzynski L, Scheraga HA (2006) Proc Natl Acad Sci USA 103:1744–1749]. We have used an excitonic coupling model to simulate the amide I band of the FTIR, vibrational circular dichroism, and isotropic and anisotropic Raman spectra of XAO, where, for each residue, the backbone dihedral angle φ was constrained by using the reported 3JCαHNH values and a modified Karplus relation. The best reproduction of the experimental data could only be achieved by assuming an ensemble of conformations, which contains various β-turn conformations (≈26%), in addition to β-strand (≈23%) and PPII (≈50%) conformations. PPII is the dominant conformation in segments not involved in turn formations. Most of the residues were found to sample the bridge region connecting the PPII and right-handed helix troughs in the Ramachandran plot, which is part of the very heterogeneous ensemble of conformations generally termed type IV β-turn.