Protein Dynamical Transition Does Not Require Protein Structure

Terahertz time domain spectroscopy shows that the protein dynamical transition, the rapid increase in protein dynamics occurring at $\sim 200\mathrm{K}$, needs neither tertiary nor secondary structure. Further, short chain alanine studies find a dynamical transition down to penta-alanine, with no transition observed for di-alanine or tri-alanine. These results reveal the temperature dependence arises strictly from the side-chain interaction with the solvent. The lack of a transition for shorter chain peptides may indicate a qualitative change in this interaction occurs at a specific peptide chain length.

Figure 1

The frequency dependent absorption coefficient $\alpha$ (a) and index of refraction $n$ (b) for N-HEWL and D-HEWL solutions at several different temperatures.

Figure 2

The temperature dependent THz absorption coefficient $\alpha$ (a) and index of refraction $n$ (b) for N-HEWL (unfilled symbols) and D-HEWL (filled symbols) and neat GdmHCl solutions (gray symbols). Symbol notation: ■: 0.51 THz; ▲: 0.70 THz; ●: 0.98 THz.

Figure 3

The temperature dependent absorption coefficient $\alpha$ for random coil poly lysine.

Figure 4

The temperature dependent absorption coefficient for A2 (a), A3 (b), A5 (c), A7–A10 (d) and PA (e) at frequency 1.00 THz (■) and 1.21 THz (▲).

Figure 5

Temperature dependent calculated $⟨x^2⟩$ for a peptide in random coil and $\alpha$-helix conformations: (a) fully hydrated, and (b) dehydrated.