Terahertz (THz) spectroscopic investigations of condensed-phase biological samples are reviewed ranging from the simple crystalline forms of amino acids, carbohydrates and polypeptides to the more complex aqueous forms of small proteins, DNA and RNA. Vibrationally resolved studies of crystalline samples have revealed the exquisite sensitivity of THz modes to crystalline order, temperature, conformational form, peptide sequence and local solvate environment and have given unprecedented measures of the binding force constants and anharmonic character of the force fields, properties necessary to improve predictability but not readily obtainable using any other method. These studies have provided benchmark vibrational data on extended periodic structures for direct comparisons with classical (CHARMm) and quantum chemical (density functional theory) theories. For the larger amorphous and/or aqueous phase samples, the THz modes form a continuum-like absorption that arises because of the full accessibility to conformational space and/or the rapid time scale for inter-conversion in these environments. Despite severe absorption by liquid water, detailed investigations have uncovered the photo- and hydration-induced conformational flexibility of proteins, the solvent shell depth of the water/biomolecule boundary layers and the solvent reorientation dynamics occurring in these interfacial layers that occur on sub-picosecond time scales. As such, THz spectroscopy has enhanced and extended the accessibility to intermolecular forces, length- and timescales important in biological structure and activity.

Broadband vibrational sum frequency spectroscopy (VSFS) has been used to examine the surface structure of alkane liquid/vapor interfaces. The alkanes range in length from n-nonane (C(9)H(20)) to n-heptadecane (C(17)H(36)), and all liquids except heptadecane are studied at temperatures well above their bulk (and surface) freezing temperatures. Intensities of vibrational bands in the CH stretching region acquired under different polarization conditions show systematic, chain length dependent changes. Data provide clear evidence of methyl group segregation at the liquid/vapor interface, but two different models of alkane chain structure can predict chain length dependent changes in band intensities. Each model leads to a different interpretation of the extent to which different chain segments contribute to the anisotropic interfacial region. One model postulates that changes in vibrational band intensities arise solely from a reduced surface coverage of methyl groups as alkane chain length increases. The additional methylene groups at the surface must be randomly distributed and make no net contribution to the observed VSF spectra. The second model considers a simple statistical distribution of methyl and methylene groups populating a three dimensional, interfacial lattice. This statistical picture implies that the VSF signal arises from a region extending several functional groups into the bulk liquid, and that the growing fraction of methylene groups in longer chain alkanes bears responsibility for the observed spectral changes. The data and resulting interpretations provide clear benchmarks for emerging theories of molecular structure and organization at liquid surfaces, especially for liquids lacking strong polar ordering.

Vibrational spectra in the CH stretching region have been acquired from the n-hexadecane, 1-chlorohexadecane, and 1-bromohexadecane liquid/vapor interfaces using broadband vibrational sum-frequency spectroscopy. Different polarization conditions of the incident and generated fields are used to assess surface structure based on functional group alignment along the surface normal (S sum S vis P IR ) and within the plane of the surface (S sum P vis S IR ). The n-hexadecane liquid/vapor interface evinces a surprising degree of conformational order, given that X-ray scattering studies imply an absence of any surface organization for alkanes above their surface freezing temperature. Replacing a terminal hydrogen of n-hexadecane with either a chlorine or bromine substituent significantly changes the out-of-plane surface structure, but the in-plane surface structure remains qualitatively unchanged. These effects are attributed to dipole-dipole interactions between the halogenated ends of the C 16 alkanes and are indicative of a higher surface activity of the halogenated ends, compared to the unsubstituted ends of the molecule.

Dimethyl sulfoxide (DMSO) is a small molecule with polar, aprotic and amphiphilic properties. It serves as a solvent for many polar and nonpolar molecules and continues to be one of the most used solvents (vehicle) in medical applications and scientific research. To better understand the cellular effects of DMSO within the concentration range commonly used as a vehicle (0.1–1.5%, v/v) for cellular treatments, we applied Attenuated Total Reflectance (ATR) Fourier Transform Infrared (FT-IR) spectroscopy to DMSO treated and untreated epithelial colon cancer cells. Both unsupervised (Principal Component Analysis-PCA) and supervised (Linear Discriminant Analysis-LDA) pattern recognition/modelling algorithms applied to the IR data revealed total segregation and prominent differences between DMSO treated and untreated cells at whole, lipid and nucleic acid regions. Several of these data were supported by other independent techniques. Further IR data analyses of macromolecular profile indicated comprehensive alterations especially in proteins and nucleic acids. Protein secondary structure analysis showed predominance of β-sheet over α-helix in DMSO treated cells. We also observed for the first time, a reduction in nucleic acid level upon DMSO treatment accompanied by the formation of Z-DNA. Molecular docking and binding free energy studies indicated a stabilization of Z-DNA in the presence of DMSO. This alternate DNA form may be related with the specific actions of DMSO on gene expression, differentiation, and epigenetic alterations. Using analytical tools combined with molecular and cellular biology techniques, our data indicate that even at very low concentrations, DMSO induces a number of changes in all macromolecules, which may affect experimental outcomes where DMSO is used as a solvent.