High-Resolution Terahertz Spectroscopy of Crystalline Trialanine:  Extreme Sensitivity to β-Sheet Structure and Cocrystallized Water

Abstract

High-resolution terahertz absorption spectra (0.06-3 THz) have been obtained at 4.2 K for three crystalline forms of trialanine [H2+-(Ala)3-O-]. The crystal structures differ in their beta-sheet forms (parallel vs antiparallel) and in their water composition (hydrated vs dehydrated antiparallel beta-sheet). The spectra are nearly vibrationally resolved, with little absorption below 1 THz. In sharp contrast to observations made in the mid-IR region, the spectral patterns of all three forms are qualitatively different, illustrating the extreme sensitivity to changes in the intermolecular hydrogen-bonding networks that stabilize peptide crystals. Predictions obtained from a classical force field model (CHARMM) and density functional theory (DFT/PW91) for periodic solids are compared with the X-ray structural data and the terahertz absorption spectra. In general, the results for the parallel beta-sheet are in better agreement with experiment than those for the antiparallel beta-sheet. For all three structures, however, most hydrogen bond distances are underestimated at both levels of theory, and the predicted absorption features are significantly red-shifted for the two antiparallel beta-sheet structures. Moreover, the nuclear motions predicted at the two levels of theory are qualitatively different. These results indicate that the PW91 functional is not sufficient to treat the weak intersheet hydrogen bonding present in the different beta-sheet forms and strongly suggest the need for improved force field models that include three-atom hydrogen-bonding terms for periodic solids.

Optical measurements of long-range protein vibrations

“…The sharp orientation-dependent resonances may originate from either intramolecular motions or crystal lattice vibrations, phonons. Typical molecular crystals (such as sucrose) with basis group molecular weights of B100 Da have lattice phonons in the 10-100 cm À 1 range [29][30][31] . The lattice phonon frequency will scale inversely with the square root of the mass of the lattice basis, suggesting the lattice phonons for the 14-kDa molecular weight CEWL basis crystals are over an order of magnitude smaller, with frequencies less than 10 cm À 1 .…”

An extended dynamical hydration shell around proteins

“…Dielectric spectroscopy extends the time scale from microseconds down to 0.1 ns (5). Experiments have been extended to the THz range in films and crystals, probing motions on the picosecond time scale (6,7). Hydrated protein powders probed by inelastic neutron scattering (0.1-100 ps) or solid-state NMR (nanoseconds) reveal that slower protein time scales and faster solvent time scales indeed show correlated dynamics (8).…”

Analytical Terahertz Spectroscopy

“…23 Since the THz frequency matches the low-frequency vibration and the vibration modes of weak interactions, such as hydrogen bonds, THz-TDS constitutes a unique tool for studying various molecules and materials whose molecular and/or crystal structures are stabilized by these weak interactions. The steric structures of many biological molecules are stabilized by hydrogen bonds, and THz-TDS studies have been undertaken on proteins, 26 polypeptides, 7,27,28 and DNA. 29,30 Figure 3(a) shows the absorption spectra of myoglobin powder obtained with THz (solid line) and Fourier-transform infrared spectroscopy (dotted line).…”

“…28,40,41 Recently, we proposed a way to distinguish intramolecular modes selectively from intermolecular modes by incorporating target molecules in nano-sized spaces in porous materials to reduce the effect of the intermolecular interactions. 65 We measured the THz spectra of low molecular weights of organic acids, namely maleic and fumaric acids, with THz-TDS and compared the differences between the spectral features of the bulk crystal form and the molecules incorporated in the nano-sized pores of mesoporous silicate.…”