THz Spectroscopy of Small Polypeptides


K. Siegrist, D. F. Plusquellic

Optical Technology Division, Physics Laboratory

National Institute of Standards and Technology

Gaithersburg  MD  20899 USA


In order to establish a basis for understanding the complex dynamics of protein folding processes, much current work focuses on investigations of small polypeptide chains.  THz radiation is well-suited for experimentally probing the non-local low-frequency modes which are relevant to motions along folding pathways.  Accurate computational prediction in this regime is challenging, however, since the vibrational state energies are small, and potential wells may be shallow, anharmonic, and strongly influenced by the molecular environment.  THz studies of simple peptides can provide a needed benchmark for theoretical investigations of molecular conformation and dynamics.  In this work, THz spectra of a number of small peptides are measured  from 2 cm-1 to 100 cm-1 at temperatures of 4.2K to 300K.  The crystalline forms of trialanine, in particular, have structures that are sufficiently well-documented to allow interpretation of the THz spectra measured and thus provide insight into important processes affecting this difficult-to-model spectral region.  The hemihydrate crystalline tripeptide, which contains 2 water molecules and 4 trialanine molecules per unit cell, was dehydrated while preserving the antiparallel beta sheet structure. This dehydrated structure was found to give a very different THz spectrum from that of the hydrated crystal, indicating that the presence or absence of  hydrogen-bonded water substantially impacts these very low frequency modes. In contrast, the FTIR spectra of the hydrated and dehydrated peptide are indistinguishable, and serve to identify both structures as the antiparallel beta sheet form.  Finally, in studies of several samples, including trialanine and biotin, performed at 4.2K and 300K, THz spectra were found to be strongly temperature dependent. The absorption lineshapes broaden asymmetrically with increasing temperature, from widths of 1 - 5 cm-1 at 4.2 K, to widths of 3 - 10 cm-1 at 300 K.  Large increases in the vibrational partition functions (1 to >108 in biotin) due to the increased available thermal energy provide access to the anharmonic regions of the potential energy surfaces.  Calculations show that lineshape models which include vibrational anharmonicity can account for the observed asymmetrical lineshapes, and the increases in linewidths with temperature.  Together, these results suggest that inclusion of environmental and anharmonic effects may be necessary to accurately capture the dynamics of more complex, biologically interesting systems. Efforts are currently directed at computational modeling to provide better understanding of the processes influencing the lowest frequency modes, as well as continuing THz studies to further investigate effects due to intermolecular forces.


K. Siegrist

Mail stop 8441

B227  Bldg 216

Phone 301-975-8134 

Fax 301-869-5700

Advisor: D. Plusquellic

Sigma Xi member: no

Category:  Biology