High-quality, temperature-dependent terahertz spectroscopy of single crystalline L-alanine: Experiment and density-functional theory

THz spectroscopy on the amino acids L-serine and L-cysteine

We present a detailed study on the temperature-dependent THz spectra of the polycrystalline amino acids, L-serine and L-cysteine, for wavenumbers from 20 to 120 cm-1 and temperatures from 4 to 300 K. Even though the structure of these two amino acids is very similar, with a sulfur atom in the side chain of cysteine instead of an oxygen atom in serine, the excitation spectra are drastically different. Obviously, the vibrational dynamics strongly depend on the ability of cysteine to form sulfur-hydrogen bonds. In addition, cysteine undergoes an order-disorder type phase transition close to 80 K, documented by additional specific heat experiments, with accompanying anomalies in the THz results. On increasing temperatures, well-defined vibrational excitations exhibit significant shifts in the eigenfrequencies with concomitant line-broadening yielding partly overlapping modes. Interestingly, several modes completely lose all their dipolar strength and are unobservable under ambient conditions. Comparing the recent results to the published work utilizing THz, Raman, and neutron-scattering techniques, as well as with ab initio simulations, we aim at a consistent analysis of the results ascribing certain eigenfrequencies to distinct collective lattice modes. We document that THz spectra can be used to fine-tune the parameters of model calculations and as fingerprint properties of certain amino acids. In addition, we analyzed the low-temperature heat capacity of both the compounds and detected strong excess contributions compared to the canonical Debye behavior of crystalline solids, indicating soft excitations and a strongly enhanced phonon-density of states at low frequencies.

Determination of Vibrational Modes of l-Alanine Single Crystals by a Combination of Terahertz Spectroscopy Measurements and Density Functional Calculations

Density-functional theory may be used to predict both the frequency and the dipole moment of the fundamental oscillations of molecular crystals. Suitably polarized photons at those frequencies excite such oscillations. Thus, in principle, terahertz spectroscopy may confirm the calculated fundamental modes of amino acids. However, reports to date have multiple shortcomings: (a) material of uncertain purity and morphology and diluted in a binder material is employed; (b) consequently, vibrations along all crystal axes are excited simultaneously; (c) data are restricted to room temperature, where resonances are broad and the background dominant; and (d) comparison with theory has been unsatisfactory (in part because the theory assumes zero temperature). Here, we overcome all four obstacles, in reporting detailed low-temperature polarized THz spectra of single-crystal l-alanine, assigning vibrational modes using density-functional theory, and comparing the calculated dipole moment vector direction to the electric field polarization of the measured spectra. Our direct and detailed comparison of theory with experiment corrects previous mode assignments for l-alanine, and reveals unreported modes, previously obscured by closely spaced spectral absorptions. The fundamental modes are thereby determined.