Conformational transition of Poly-L-proline studied by IR, NMR, and SAXS

Abstract

Poly-l-proline (PLP) takes two extreme local conformations, right-handed helix (Helix-I) and left-handed helix (Helix-II) and two overall conformations at a steady-state, PLP-I and –II, each of which is constructed by sequences of Helix-I and –II, respectively. The study with IR and proton NMR spectroscopy has verified that PLP-I dissolved in glacial acetic acid is gradually converted into PLP-II (forward-mutarotation) and PLP-II dissolved in a mixture of glacial acetic acid and n-propanol is gradually converted into PLP-I (reverse-mutarotation), and has roughly evaluated the rate of the conformational change: It takes ca. 48 hrs. and ca. 40–50 h at 25 °C, respectively, for the forward- and reverse-mutarotation process to reach the middle stage. The analysis of the SAXS data has shown that the conformation of PLP at the middle stage of the forward mutarotation process is represented by joining alternatively rodlike structure of Helix-I and –II, which has equal 180 ± 90 proline residues per a helical rod, whereas the overall conformation of PLP-II dissolved in CD3COOD is represented by an intact helical rod or an interrupted helical rod having a few breaking-points. These data suggest that the conformational transition of PLP would proceed in a zipper-like pattern.