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Continous gradient temperature Raman Spectroscopy identifies flexible sites in proline and alanine peptides
- Schmidt, W. F., Kim, M. S., Nguyen, J. K., Qin, J., Chao, K., Broadhurst, C. L., Shelton, D. R.
- Vibrational Spectroscopy 2015 v.80 pp. 59-65
- Raman spectroscopy, alanine, amides, chemical bonding, chemical structure, differential scanning calorimetry, dipeptides, energy, phase transition, proline, temperature profiles, thermal stability
- Continuous gradient temperature Raman spectroscopy (GTRS) applies the temperature gradients utilized in differential scanning calorimetry (DSC) to Raman spectroscopy, providing a straightforward technique to identify molecular rearrangements that occur near phase transitions. Herein we apply GTRS and DSC to the solid dipeptides Ala-Pro, Pro-Ala, and the mixture Ala-Pro/Pro-Ala 2:1. A simple change in residue order resulted in dramatic changes in thermal stability and properties. Characteristic Pro vibrations were observed at ∼75 °C higher temperature in Pro-Ala than Ala-Pro. The appearance/disappearance of characteristic vibrational modes with increasing temperature showed that a double peak in the Ala-Pro major phase transition (174–184 °C) was due to a gauche to anti 165° rotation of H3C-C*-NH3 about C*. CH2 rocking and wagging frequencies present in Pro-Ala were not observed in Ala-Pro. For Ala-Pro, the Ala +NH3, and Pro COO− sites were flexible whereas the Pro ring moiety was not; since the O=C-N (-C)2 amide bond is planar the C-N-C moiety keeps the Pro ring rigid. For Pro-Ala, CH2 sites in the Pro ring were flexible and the O=C-NH amide bond is perpendicular to the Pro ring. Since the mass of the Pro ring is significantly larger than the mass of the flexible Ala +NH3 moiety, Pro-Ala absorbs more thermal energy, corresponding to a higher phase transition temperature (240–260 °C). Ala-Pro, Pro-Ala, and Ala-Pro/Pro-Ala 2:1 exhibited α-helix, β-sheet, α-helix secondary structure conformations, respectively.