Investigation of the cis–trans structures and isomerization of oligoprolines by using Raman spectroscopy and density functional theory calculations: solute–solvent interactions and effects of terminal positively charged amino acid residues. Issue 57 (17th September 2020)

• Record Type: Journal Article
• Authors: Huang, Mei-Chun; Chen, Wei-Hao; Huang, Chen-Wei; Huang, Kuei-Yen; Horng, Jia-Cherng; Hayashi, Michitoshi; Chen, I.-Chia
• Is Part Of: RSC advances. Volume 10:Issue 57(2020)
• Page Start: 34493
• Page End: 34500
• Publication Date: 2020-09-17
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/d0ra05746k
• Languages: English
• ISSNs: 2046-2069

Abstract

Abstract : The low-wavenumber Raman spectra in combination with theoretical calculations via solid-state density functional theory (DFT)-D3 are displayed. The vibrational structures and interaction with solvent of poly-l -proline and the oligoproline P12 series are identified. Abstract : Using low-wavenumber Raman spectroscopy in combination with theoretical calculations via solid-state density functional theory (DFT)-D3, we studied the vibrational structures and interaction with solvent of poly-l -proline and the oligoproline P12 series. The P12 series includes P12, the positively charged amino acid residue (arginine and lysine) N-terminus proline oligomers RP11 and KP11, and the C-terminus P11R and P11K. We assigned the spring-type phonon mode to 74–76 cm −1 bands for the PPI and PPII conformers and the carbonyl group ring-opening mode 122 cm −1 in the PPI conformer of poly-l -proline. Amide I and II were assigned based on the results of mode analysis for O, N, and C atom displacements. The broad band feature of the H-bond transverse mode in the Raman spectra indicates that the positively charged proline oligomers PPII form H-bonds with water in the solid phase, whereas P12 is relatively more hydrophobic. In propanol, the PPI conformer of the P12 series forms less H-bond network with the solvent. The PPII conformer exhibits a distinct Raman band at 310 cm −1, whereas the PPI has bands at 365, 660, and 960 cm −1 with reasonable intensity that can be used to quantitatively determine these two conformational forms. The 365 cm −1 mode comprising the motion of a CO group turning to the helix axis was used to monitor the isomerization reaction PPI ↔ PPII. In pure propanol, RP11 and KP11 were found to have mostly PPI present, but P11R and P11K preferred PPII. After adding 20% water, the PPI in P11R and P11K was completely converted to PPII, whereas a small fraction of PPI remained in RP11 and KP11. The substituted positively charged amino acid affected the balance of the PPI/PPII population ratio.