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The terahertz frequency absorption spectraof DNA molecules reflect low-frequencyinternal helical vibrations involvingrigidly bound subgroups that are connectedby the weakest bonds, including thehydrogen bonds of the DNA base pairs,and/or non-bonded interactions. Althoughnumerous difficulties make the directidentification of terahertz phonon modes inbiological materials very challenging, ourresearch has shown that such measurementsare both possible and fruitful. Spectra ofdifferent DNA samples reveal a large numberof modes and a reasonable level ofsequence-specific uniqueness. In an attemptto show that the long wavelength absorptionfeatures are intrinsic properties ofbiological materials determined by phononmodes, a normal mode analysis has been usedto predict the absorption spectra ofpolynucleotide RNA Poly[G]-Poly[C]. Directcomparison demonstrated a correlationbetween calculated and experimentallyobserved spectra of the RNA polymers, thusconfirming that the fundamental physicalnature of the observed resonance structureis caused by the internal vibration modesin the macromolecules.In this work we demonstrate results fromFourier-Transform Infrared (FTIR)spectroscopy of DNA macromolecules andrelated biological materials in theterahertz frequency range. Carefulattention was paid to the possibility ofinterference or etalon effects in thesamples, and phenomena were clearlydifferentiated from the actual phononmodes. In addition, we studied thedependence of transmission spectra ofaligned DNA and polynucleotide film sampleson molecule orientation relative to theelectromagnetic field, showing the expectedchange in mode strength as a function ofsample orientation. Further, the absorptioncharacteristics were extracted from thetransmission data using the interferencespectroscopy technique, and a stronganisotropy of terahertz characteristics wasdemonstrated.

A detailed investigation of phonon modes in DNA macromolecules is presented. This work presents experimental evidence to confirm the presence of multiple dielectric resonances in the submillimeter-wave spectra (i.e., approximately 0.01-10 THz) obtained from DNA samples. These long-wave (i.e., approximately 1-30 cm(-1)) absorption features are shown to be intrinsic properties of the particular DNA sequence under study. Most importantly, a direct comparison of spectra between different DNA samples reveals a large number of modes and a reasonable level of sequence-specific uniqueness. This work establishes the initial foundation for the future use of submillimeter-wave spectroscopy in the identification and characterization of DNA macromolecules.

Significant progress has been achieved during the last several years relating to experimental and theoretical aspects of terahertz (or submillimetre wave) Fourier transform spectroscopy of biological macromolecules. However, previous research in this spectral range has been focused on bio-materials in solid state since it was common opinion that high water absorption will obscure the spectral signatures of the bio-molecules in solutions. At the same time, the biological functions of DNA and proteins take place in water solutions. In this work, the spectra of DNA samples have been measured in liquid phase (gel) over the spectral range 10–25 cm−1 and compared with spectra obtained from solid films. The results demonstrate that there is very little interference between the spectral features of the material under test and the water background except for the band around 18.6 cm−1. Multiple resonances due to low frequency vibrational modes within biological macromolecules in solutions are unambiguously demonstrated. Higher level of sensitivity and higher sharpness of vibrational modes are observed in the liquid environment in comparison with the solid phase, with the width of spectral lines 0.3–0.5 cm−1. Gel sample spectra are found to be polarization-dependent. The ability of THz spectroscopy to characterize samples in liquid phase could be very important since it permits examination of DNA interactions in real (wet) samples. One demonstrated example of practical importance is the ability to discriminate between spectral patterns for native and denaturated DNA.

We demonstrate submillimetre-wave Fourier transform spectroscopy as a novel technique for biological molecule characterization. Transmission measurements are reported at frequencies 10-25 cm −1 for single-and double-stranded RNA molecules of known base-pair sequences: homopolymers poly[A], poly[U], poly[C] and poly[G], and double-stranded homopolymers poly[A]-poly[U] and poly[C]-poly[G]. Multiple resonances are observed (i.e. in the microwave through terahertz frequency regime).We also present a computational method to predict the low-frequency absorption spectra of short artificial DNA and RNA. Theoretical conformational analysis of molecules was utilized to derive the low-frequency vibrational modes. Oscillator strengths were calculated for all the vibrational modes in order to evaluate their weight in the absorption spectrum of a molecule. Normal modes and absorption spectra of the double-stranded RNA chain poly[C]-poly[G] were calculated. The absorption spectra extracted from the experiment were directly compared with the results of computer modelling thereby, confirming the fact that observed spectral features result from electromagnetic wave interactions with the DNA and RNA macromolecules. Correlation between experimental spectrum and modelling results demonstrates the ability of normal mode analysis to reproduce RNA vibrational spectra.

We present a computational method which couples normal mode analysis in internal coordinates of a molecule with very far IR spectroscopy. The analytical expression for the dependence of IR absorption on frequency incorporates frequencies and optical activities of each normal mode. In order to predict far-IR spectra of a molecule we evaluate the optical activity of each normal mode. This optical activity is determined by the vibration amplitude of the dipole moment produced by a normal mode. We calculated normal modes of DNA double-helical fragments (dA) 12 á (dT) 12 and (dAdT) 6 á (dA-dT) 6 and evaluated their optical activities. These were found to be very sensitive to the DNA basepair sequence. The positions of the resonance peaks in the calculated absorption spectrum of (dA) 12 á (dT) 12 are in a good agreement with those obtained by Fourier transform IR spectroscopy (Powell JW et al. 1987 Phys Rev A 35: 3929±3939).

Hysteresis and plateau-like behavior of the I–V curves of a double-barrier resonant tunneling structure are simulated in the negative differential resistance region. Our simulation results show that the creation of an emitter quantum well after the current passes its maximum value is the key point in understanding the origin of the I–V plateau-like structure. It is demonstrated that the plateau-like behavior of the I–V curves is produced by the coupling between the energy level in the emitter quantum well and that in the main quantum well. The hysteresis is a manifestation of the above-mentioned energy level coupling, the accumulation and distribution of electrons in the emitter, and the coupling between the energy level in the quantum well and the conduction band edge or the three-dimensional continuum states in the emitter. The effects of the structural parameters on the bistability of the I–V curves of resonant tunneling devices are discussed. The creation and disappearance mechanism of the emitter quantum well is presented. The effects of device temperature on the hysteresis and plateau-like behavior of the I–V curves are obtained. These results provide the physical basis for utilizing the plateau-like structure of I–V curves in designing resonant tunneling devices.

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