Circular dichroism eigenspectra of polyproline II and β-strand conformers of trialanine in water: Singular value decomposition analysis.

Despite that a number of experimental and theoretical investigations have been carried out to determine the structure of trialanine in water, the reported populations of polyproline II (PPII) and β-strand conformers vary and were found to be dependent on which spectroscopic method was used. Such discrepancies are due to limitations of different spectroscopic methods used. Here, the temperature- and pH-dependent circular dichroism (CD) and NMR experiments have been carried out to develop a self-consistent singular value decomposition procedure. The temperature-dependent CD spectra indicate the presence of two conformers, but due to the two peptide bonds in a trialanine, one should take into consideration of four different conformers to fully interpret the NMR results. From the pH-dependent NMR coupling constant measurements, the conformation of zwitterionic trialanine is little different from that of cationic one. The strong pH dependency of CD spectrum is likely due to charge transfer transitions between carboxylate and nearby peptide groups or internal field effects not to pH-dependent conformational change. To simultaneously analyze the temperature-dependent CD and NMR data, a self-consistent procedure was used to newly determine the reference NMR coupling constants required to estimate one of the peptide dihedral angles. From the estimated enthalpy and entropy changes associated with the transition from enthalpically favorable PPII conformer to entropically favorable β-strand conformer, the relative populations of the four possible conformers of trialanine were determined and compared with the previous experimental findings. We anticipate that the present experimental results and interpretation procedure would be of use in determining the solution structures of small oligopeptides in the future.