Applications of Terahertz Spectroscopy in Biosystems

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.     

Ultrafast chelation dynamics of a manganese tricarbonyl derivative in solid polyacrylonitrile

Ultraviolet pump, infrared probe transient absorption studies of the chelatable compound 1, Mn{eta(5)-C(5)H(4)C(O)C(SCH(3))3}(CO)3, dispersed in room temperature, spin-coated polyacryclonitrile (PAN) films (approximately 25 microm thick on a CaF2 surface) are reported for the first time. Irradiation of 1 at 289 nm induces CO loss with high yield and generates the Mn-S bound chelate within 160 ps. There is no evidence for undesirable matrix cage CO recombination or secondary competing solvation pathways for this system, which may serve as the basis for future solid-state photoswitches.     

Applications of the phase conjugate configuration in nonlinear spectroscopy

Various light generation processes, phase matching conditions and efficiencies are examined for the phase conjugate four-wave mixing configuration in the case of one-photon electronic resonances involving both nondegenerate and degenerate wave conditions. The effects of thermal gratings on the dispersion of the nonlinear susceptibility are also considered. These analyses are used to characterize the useful properties of each generated wave.     

Time-resolved infrared absorption study of cyclopentadienyl manganese tricarbonyl derivatives: Chelation of pendant sulfides in acetonitrile

The chelation dynamics of [Mn{eta5-C5H4C(O)R}(CO)3] complexes 1 (R = CH2(SCH3)), 2 (R = CH(SCH3)2), and 3 (R = C(SCH3)3) in room-temperature acetonitrile solution have been investigated on the picosecond time scale by UV-pump IR-probe transient absorption spectroscopy. Similar to the previously observed behavior in n-heptane solution, irradiation of 3 in acetonitrile at 289 nm induces CO loss to exclusively yield a Mn-S chelated dicarbonyl product. Unlike the behavior of 1 and 2 in n-hexane and n-heptane solutions, UV excitation of either 1 or 2 in acetonitrile solution induces CO loss to also exclusively yield the chelated products, with no evidence of a competing solvation pathway. All three complexes exhibit ultrafast chelation in <13 ps. Faster vibrational cooling in acetonitrile vs alkane solutions suggests stronger solute-solvent interaction, perhaps via hydrogen bonding. Ring-opening resulting from continuous irradiation of the pendant sulfide's chelates, [Mn{eta5-C5H4C(O)CH(SCH3)2-kappaS}(CO)2] (4) and [Mn{eta5-C5H4C(O)C(SCH3)3-kappaS}(CO)2] (5), is also discussed.     

Terahertz, infrared and Raman vibrational assignments of [FeFe]-hydrogenase model compounds

Using Raman, terahertz (THz), and mid-infrared (IR) spectroscopies, the vibrational spectra of two chromophore models of hydrogen-producing [FeFe]-hydrogenase, Fe₂(μ-S₂C₃H₆)(CO)₆ and Fe₂(μ-S₂C₂H₄)(CO)₆, have been assigned. The combination of absorption and scattering techniques, along with DFT calculations, allows for assignments to be made without traditional isotopic substitution methods.