Hydration dynamics of oriented DNA films investigated by time-domain terahertz spectroscopy

Abstract

The B to A conformational transition of highly oriented DNA films due to a hydration change is observed with time-domain terahertz spectroscopy. Wet-spun films of calf thymus and salmon DNA are investigated for different film thicknesses and for different polarizations of the terahertz radiation relative to the DNA orientation. A clear polarization dependence is observed. Asynchronous optical sampling allows recording of terahertz absorption and background spectra in a few 10 s, permitting the tracking of the dehydration dynamics on a time scale of minutes. The observation of a phase transition is corroborated by Raman spectroscopy. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2743401͔The conformation of DNA is directly linked to important biological processes such as DNA replication and DNA repair. While DNA in aqueous solution is predominantly in the B form, it is also frequently seen to deviate from this conformation. Dehydration of B-type DNA to below 30% water content by weight converts it into A-type DNA. This conformational transition and its origins in terms of free energy difference between A-and B-type conformations have been intensively studied theoretically 1 and experimentally, e.g., by x-ray diffraction. 2-4Low-frequency vibrational excitations are expected to play an important role in conformational dynamics of DNA, as well as in the hydration shell, and might influence the dynamics of drug binding. 5 In particular, Raman and Brillouin scattering 6,7 as well as thermal neutron scattering 8 have been used to observe low-frequency excitation spectra. Far-infrared absorption spectroscopy in the terahertz frequency range provides another access to the low-frequency vibrational excitations of DNA. Besides Fourier-transform spectroscopy 9-11 and absorption spectroscopy using tunable terahertz light sources, 12,13 time-domain terahertz spectroscopy became available recently to study optical properties of biomolecules and DNA. [14][15][16][17] In this letter, we use a recently developed terahertz spectrometer based on high-speed asynchronous optical sampling ͑ASOPS͒ to measure the terahertz transmission characteristics of highly oriented wet-spun DNA films. We describe the dependences of the terahertz absorption in the range of 100 GHz-2.0 THz on the degree of hydration. Here, we take advantage of the fast sampling possibilities that are inherent to the ASOPS technique. The results from the terahertz experiments are corroborated by x-ray diffraction and Raman scattering experiments.Highly oriented films are produced from DNA sodium salt ͑Sigma-Aldrich Co.͒ using the wet-spinning method developed by Rupprecht. 18 A highly concentrated DNA solution is pumped through a spinneret and precipitated in 77% ethanol solution containing 0.3M NaCl at a temperature of 5°C such that thin DNA fibers are generated and spooled onto a rotating cylinder. Multilayered DNA films are taken off the cylinder and are dried at 5°C and are rehydrated at 75% relative humidity ͑r.h.͒ at room temperature for at least three d...

Terahertz bio-sensing techniques

“…in transmission, resulting in a smaller index of refraction (Brucherseifer et al , 2000 ). Using asynchronous optical sampling THz spectroscopy, the state change of oriented DNA fi lms from hydration to dehydration was measured, and the B to A conformational transition was corroborated by Raman spectroscopy (Kistner et al , 2007 ). Two different types of artifi cially synthesized single-stranded RNA, polyadenylic acid (poly-A) and polycytidylic acid (poly-C), were also distinguished by THz dielectric measurements (Fischer et al , 2005 ).…”

Wavelet Power Spectrum Estimation for High-resolution Terahertz Time-domain Spectroscopy

“…The ASOPS THz-TDS can serve for high-resolution spectroscopy as well as high-speed monitoring of dynamic processes [11]. It should be noted, however, that spectral fluctuation can significantly appear on high-resolution THz spectra measured in ASOPS THz-TDS.…”

High-Speed High-Resolution Terahertz Spectrometers

“…Since the THz region, generally defined as 0.1 -10 THz, spans the range of low-energy excitations in electronic materials [7] and vibrational and rotational transitions in molecules [8], it is a key spectral region for probing fundamental physical interactions, as well as for practical applications. Also, bio-samples such as DNA films [9] and quantum systems such as Rydberg atoms [10] and semiconductor nanostructures [11] can be investigated using THz waves.…”