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Effects of the Terminal Aromatic Residues on Polyproline Conformation: Thermodynamic and Kinetic Studies B

Lin, Yu-Ju, Chu, Li-Kang, Horng, Jia-Cherng
The Journal of physical chemistry 2015 v.119 no.52 pp. 15796-15806
1-propanol, amino acids, aqueous solutions, circular dichroism spectroscopy, enthalpy, entropy, hydrophobicity, peptides, physical chemistry
In a peptide or protein, the sequence of aromatic residue–proline or proline–aromatic residue shows a high propensity in forming cis prolyl bonds due to aromatic–proline interactions. In this work, we designed and prepared the polyproline peptides with aromatic amino acids (F, Y, W) incorporated into their N-terminal or C-terminal end to investigate the effects of a terminal aromatic residue on polyproline conformation and the transition kinetics of polyproline I (PPI) to polyproline II (PPII) helices. Circular dichroism measurements reveal that the N-terminal aromatic–proline interaction imposes a more pronounced consequence on the forming propensity of PPI conformation than does the C-terminal aromatic–proline interaction in n-propanol. The propensity of forming PPI is correlated with the strength of aromatic–proline interactions in the order of Tyr-Pro > Trp-Pro > Phe-Pro. In aqueous solution, kinetic studies indicate that aromatic-substitution effects are nondirectional and indistinct on the PPI → PPII conversion rates, suggesting that aromatic–proline interactions may not be an important factor in this process. In addition, the temperature-dependent kinetics shows that the hydrophobicity of aromatic side chain may play a critical role affecting the activation enthalpy and entropy of the conversion of PPI to PPII, providing new insights into the folding of polyproline helices.