本研究利用超低頻拉曼光譜技術研究不同相態的聚脯胺酸 (Poly-L-proline) 和不同末端取代基的脯胺酸鏈如P12、RP11、P11R、KP11和P11K，搭配理論計算的結果，指認脯胺酸鏈分子間與分子內的振動模態。吾人測量PPI和PPII型聚脯胺酸的晶體拉曼光譜，指認PPI型的CO從平行於螺旋軸的方向轉成垂直於螺旋軸的彎曲振動模式，此模態應是脯胺酸鏈進行順反異構化反應之途徑軸。PPII的吡咯烷間剪式振動在74 cm-1及CO鍵間扭轉運動在104 cm-1。聚脯胺酸、P12、RP11、P11R、KP11及P11K的固態粉末拉曼光譜及溶在水中的液態拉曼光譜和溶在正丙醇中的液態拉曼光譜中，由水溶液低頻拉曼光譜指認分子與水的氫鍵作用力在40 – 85 cm-1，RP11和KP11的譜峰強度最強、P11R和P11K次之以及P12最弱，愈極性的分子與水的交互作用力愈好因此強度愈強。藉由拉曼光譜可以區分PPI和PPII構形，當脯胺酸鏈溶在水中為PPII構形，拉曼的特徵峰為310 cm-1，指認為NC面外彎曲振動模式，而分子溶在正丙醇中為PPI構形，拉曼的特徵峰為365、660以及960 cm-1，分別指認為CO面內彎曲振動、末端COO-的CO面外彎曲振動和環形變。吾人以365 cm-1的譜峰強度作為估算PPI構形百分比之依據，當R和K接在N末端時在正丙醇中此譜帶強度強於接在C末端，表示RP11和KP11的PPI/PPII數量比例比P11R和P11K的高。因為當N端接上帶正電荷的R和K時，此時末端電荷與螺旋偶極作用力可穩定PPI構形，因此脯胺酸鏈趨近於PPI。當C端接上R和K時，此時R和K的正電荷會補償COO-的負電荷，因此會趨近於PPII。我們利用超低頻拉曼光譜技術分析了脯胺酸鏈彼此間以及與溶劑間的分子間交互作用力，證實高濃度的正丙醇水溶液的PPI/PPII數量比排序為RP11和KP11最高，P12次之，P11R和P11K則最低。

Using low-wavenumber Raman spectroscopy technique we study the vibrational structures of poly-L-proline and proline oligomers P12 with different terminal positively charged amino acid residues in solution and in solid phase. Combining with the theoretical calculations using density functional theory-D3 on the crystal structures, we assign the 130 cm-1 band in PPI conformer of poly-L-proline to be the carbonyl group ring-opening motion which possibly serves as the reaction coordinate of conversion between the PPI and PPII forms. The bands at 74, 104, and 129 cm-1 in PPII are assigned to scissoring motion of pyrrolidine rings, twist of carbonyl group, and twist of pyrrolidine rings, respectively. The proline oligomers form hydrogen bonding with water to display the hydrogen-bond phonon modes at 40–85 cm-1 both in solid and in aqueous solution. The positively charged N-terminus proline oligomers RP11 and KP11 have the most intense H-bond band than the positively charged C-terminus, P11R and P11K; P12 is the weakest indicating that with the charged amino acid substitution, proline oligomers become more hydrophilic. The N-terminus prolines have better interaction with water. In the Raman spectra, the PPII exhibits a distinct band at 310 cm-1 assigned to the NC out-of-plane bending and the PPI conformer with bands at 365, 660, and 960 cm-1 assigned to CO in-plane bending, CO bending of terminal COO-, and ring deformation, respectively. These bands with reasonable intensity can be used to differentiate these two structures. In detecting the isomerization reaction, the band intensity at 365 cm-1 is used to estimate the amount of PPI existing in various percentages of n-propanol/water solution. In pure n-propanol, the 365 cm-1 band intensity of RP11 and KP11 is greater than that of P11R and P11K because the charge-dipole interaction in the positively charged N-terminus proline oligomers increases the stability of PPI; thus, the PPI/PPII population ratio is the greater. The PPI/PPII ratio remained at certain values when the amount of water was increased to 20%. This is because in the positively charged N-terminus proline oligomers, the energy of PPI (propanol) lies just slightly above the PPII (water). A small fraction of proline oligomers remains in PPI conformation at equilibrium. However, in P11R and P11K most PPI has been converted to PPII when 20% water is added to the n-propanol solution.

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