Investigation of far-infrared vibrational modes in polynucleotides

Vibration spectra of DNA and RNA segments

The dispersion curves and density of states are used to analyze the vibrational characteristics of DNA and RNA segments. This is done using a harmonic Hamiltonian and the Green's function technique. Two configurations of DNA and RNA, finite and cyclic, have been investigated and compared to their infinite counterparts. For the DNA molecule, three models, including a fishbone model, a ldder model, and a fishbone ladder model, have been employed, while the RNA molecule has been represented using a half fishbone model. To enhance the realism of DNA and RNA simulations, the unit cells within each infinite system as well as the length of the finite and cyclic cases are gradually enlarged. The connections between the sub-sites have been modeled using linear springs, where the stiffness of the vertical springs exhibits random variations throughout the length of the DNA and RNA models. Shorter DNA and RNA segments exhibit additional peaks in their density of states, resulting in more bands in dispersion curves. This indicates that as the number of building blocks grows in these segments, their curves resemble those of infinite systems. These findings have practical implications for studying the vibration characteristics of similar macro-systems.

Molecular dynamics study of collective water vibrations in a DNA hydration shell

The structure of DNA double helix is stabilized by water molecules and metal counterions that form the ion-hydration shell around the macromolecule. Understanding the role of the ion-hydration shell in the physical mechanisms of the biological functioning of DNA requires detailed studies of its structure and dynamics at the atomistic level. In the present work, the study of collective vibrations of water molecules around the DNA double helix was performed within the framework of classical all-atom molecular dynamics methods. Calculating the vibrational density of states, the vibrations of water molecules in the low-frequency spectra ranged from [Formula: see text]30 to [Formula: see text]300 [Formula: see text] were analyzed for the case of different regions of the DNA double helix (minor groove, major groove, and phosphate groups). The analysis revealed significant differences in the collective vibrations behavior of water molecules in the DNA hydration shell, compared to the vibrations of bulk water. All low-frequency modes of the DNA ion-hydration shell are shifted by about 15-20 [Formula: see text] towards higher frequencies, which is more significant for water molecules in the minor groove region of the double helix. The interactions of water molecules with the atoms of the macromolecule induce intensity decrease of the mode of hydrogen-bond symmetrical stretching near 150 [Formula: see text], leading to the disappearance of this mode in the DNA spectra. The obtained results can provide an interpretation of the experimental data for DNA low-frequency spectra and may be important for the understanding of the processes of indirect protein-nucleic recognition.

Photo-Switching of Protein Dynamical Collectivity

We examine changes in the picosecond structural dynamics with irreversible photobleaching of red fluorescent proteins (RFP) mCherry, mOrange2 and TagRFP-T. Measurements of the protein dynamical transition using terahertz time-domain spectroscopy show in all cases an increase in the turn-on temperature in the bleached state. The result is surprising given that there is little change in the protein surface, and thus, the solvent dynamics held responsible for the transition should not change. A spectral analysis of the measurements guided by quasiharmonic calculations of the protein absorbance reveals that indeed the solvent dynamical turn-on temperature is independent of the thermal stability/photostate however the protein dynamical turn-on temperature shifts to higher temperatures. This is the first demonstration of switching the protein dynamical turn-on temperature with protein functional state. The observed shift in protein dynamical turn-on temperature relative to the solvent indicates an increase in the required mobile waters necessary for the protein picosecond motions, that is, these motions are more collective. Melting-point measurements reveal that the photobleached state is more thermally stable, and structural analysis of related RFP’s shows that there is an increase in internal water channels as well as a more uniform atomic root mean squared displacement. These observations are consistent with previous suggestions that water channels form with extended light excitation providing O2 access to the chromophore and subsequent fluorescence loss. We report that these same channels increase internal coupling enhancing thermal stability and collectivity of the picosecond protein motions. The terahertz spectroscopic characterization of the protein and solvent dynamical onsets can be applied generally to measure changes in collectivity of protein motions.

Low-frequency vibrations of water molecules in DNA minor groove

Water molecules around the DNA form the hydration shell having different structural and dynamical features in different regions of the double helix. In the DNA minor groove, water molecules are highly ordered and in the case of AT nucleotide sequence, the formation of a hydration spine is observed. In the present research, the vibrations of the hydration spine have been studied to establish the mode of translational vibrations of water molecules in the DNA low-frequency spectra (water-spine vibrations). Using the developed phenomenological model with the parameters determined for different nucleotides of the DNA fragment CGCGAATTCGCG, the frequencies of vibrations of the hydration spine have been obtained within 185 ± 20 cm[Formula: see text] depending on type of nucleotide. The obtained frequencies are in the same region as the translational vibrations of water molecules in the bulk. To select the mode of water-spine vibrations from those modes that are present in the bulk water, the dynamics of DNA with different nucleotide contents has been analyzed, and the possible influence of heavy water has been estimated. The determined features of the mode of water vibrations in the hydration spine of DNA minor groove indicate that this mode may be observed in the experimental spectra.

Far infrared spectroscopy of hydrogen bonding collective motions in complex molecular systems

Far infrared spectroscopy as a tool for the study of inter and intramolecular interactions in complex molecular structures.

First-principles investigation of the lattice vibrational properties of inorganic double helical XY (X = Li, Na, K, Rb, Cs; Y = P, As, Sb)

The vibrational frequencies of the newly discovered inorganic double helical compounds XY (X = Li, Na, K, Rb, Cs; Y = P, As, Sb) are sensitive to either cation or anion or both of them.

Terahertz optical measurements of correlated motions with possible allosteric function

A suggested mechanism for allosteric response is the distortion of the energy landscape with agonist binding changing the protein structure's access to functional configurations. Intramolecular vibrations are indicative of the energy landscape and may have trajectories that enable functional conformational change. Here, we discuss the development of an optical method to measure the intramolecular vibrations in proteins, namely, crystal anisotropy terahertz microscopy, and the various approaches which can be used to identify the spectral data with specific structural motions.

Playing the notes of DNA with light: extremely high frequency nanomechanical oscillations

We use a double nanohole (DNH) optical tweezer with two trapping lasers beating to excite the vibrational modes of single-stranded DNA (ssDNA) fragments in the extremely high frequency range. We find the resonant vibration frequency of a 20 base ssDNA to be 40 GHz. We show that the change in the resonant frequency for different lengths of the DNA strand is in good agreement with one dimensional lattice vibration theory. Thus the DNH tweezer system could distinguish between different lengths of DNA strands with resolution down to a few bases. By varying the base sequence and length, it is possible to adjust the resonance frequency vibration spectrum. The technique shows the potential for use in sequencing applications if we can improve the resolution of the present system to detect changes in resonant frequency for a single base change in a given sequence. The technique is single-molecule and label-free as compared to the existing methods used for DNA characterization like gel electrophoresis.

Terahertz science and technology of carbon nanomaterials

The diverse applications of terahertz (THz) radiation and its importance to fundamental science makes finding ways to generate, manipulate and detect THz radiation one of the key areas of modern applied physics. One approach is to utilize carbon nanomaterials, in particular, single-wall carbon nanotubes and graphene. Their novel optical and electronic properties offer much promise to the field of THz science and technology. This article describes the past, current, and future of THz science and technology of carbon nanotubes and graphene. We will review fundamental studies such as THz dynamic conductivity, THz nonlinearities and ultrafast carrier dynamics as well as THz applications such as THz sources, detectors, modulators, antennas and polarizers.

THz-Spectroscopy of Biological Molecules

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.

Experimental assessment of the contribution of electrodynamic interactions to long-distance recruitment of biomolecular partners: Theoretical basis

Highly specific spatiotemporal interactions between cognate molecular partners essentially sustain all biochemical transactions in living matter. That such an exquisite level of accuracy may result from encountering forces solely driven by thermal diffusive processes is unlikely. Here we propose a yet unexplored strategy to experimentally tackle the long-standing question of a possibly active recruitment at a distance of cognate partners of biomolecular reactions via the action of resonant electrodynamic interactions. We considered two simplified models for a preliminary feasibility investigation of the devised methodology. By taking advantage of advanced experimental techniques nowadays available, we propose to measure the characteristic encounter time scales of dually interacting biopartners and to compare them with theoretical predictions worked out in both the presence and absence of putative long-range electromagnetic forces.

THZ-FREQUENCY SPECTROSCOPIC SENSING OF DNA AND RELATED BIOLOGICAL MATERIALS

The terahertz frequency absorption spectra of DNA molecules reflect low-frequency internal helical vibrations involving rigidly bound subgroups that are connected by the weakest bonds, including the hydrogen bonds of the DNA base pairs, and/or non-bonded interactions. Although numerous difficulties make the direct identification of terahertz phonon modes in biological materials very challenging, recent studies have shown that such measurements are both possible and useful. Spectra of different DNA samples reveal a large number of modes and a reasonable level of sequence-specific uniqueness. This chapter utilizes computational methods for normal mode analysis and theoretical spectroscopy to predict the low-frequency vibrational absorption spectra of short artificial DNA and RNA. Here the experimental technique is described in detail, including the procedure for sample preparation. Careful attention was paid to the possibility of interference or etalon effects in the samples, and phenomena were clearly differentiated from the actual phonon modes. The results from Fourier-transform infrared spectroscopy of DNA macromolecules and related biological materials in the terahertz frequency range are presented. In addition, a strong anisotropy of terahertz characteristics is demonstrated. Detailed tests of the ability of normal mode analysis to reproduce RNA vibrational spectra are also conducted. A direct comparison demonstrates a correlation between calculated and experimentally observed spectra of the RNA polymers, thus confirming that the fundamental physical nature of the observed resonance structure is caused by the internal vibration modes in the macromolecules. Application of artificial neural network analysis for recognition and discrimination between different DNA molecules is discussed.

Acoustic spectroscopy of DNA in the gigahertz range

We find a parametric resonance in the gigahertz range of DNA dynamics, generated by pumping hypersound. The resonance may be accompanied by the formation of localized phonon modes due to the random structure of elastic modulii of DNA.

Dielectric properties of fully hydrated nucleotides in the terahertz frequency range

We use terahertz time domain spectroscopy (THz-TDS) to determine the complex frequency-dependent dielectric response of all four nucleotides at different dilute concentrations. In addition, the suspension model's ability to extract the dielectric response of just the nucleotide with the hydration shell epsilon(b) excluding the dielectric information relating to the bulk will be verified. The suspension model enables us to make the determination that the nucleotides have influences on the water molecules out to the fourth hydration shell. We use a two Debye relaxation fit model for water, all concentrations and all epsilon(b) values. We observed how the nucleotides affect the relaxation parameters in relation to that of pure bulk water. With this information, we notice a transition between purines and pyrimidines, where one is a hydrogen-bond network structure building type material with a low concentration increment and the other is a structure breaking type material with a low concentration decrement. Due to conductivity measurements, we determine that kinetic depolarization is a negligible affect compared to that of dielectric saturation, which we find to dominate where a decrement is found.

Counterion vibrations in the DNA low-frequency spectra

The vibrations of univalent metal cations with respect to phosphate groups of the DNA backbone are described using the four-mass model approach (S.N. Volkov, S.N. Kosevich, J. Biomol. Struct. Dyn. 8, 1069 (1991)) extended in this paper. The force constant of the counterion-phosphate interaction is determined by considering the DNA with counterions as a lattice of ion crystal. For such ion-phosphate lattice the Madelung constant and the dielectric constant are estimated. The obtained value of the Madelung constant is lower than for the NaCl crystal, and its value is about 1.3. The dielectric constant is within 2.3-2.7 depending on the counterion type and form of the double helix. The calculations of the low-frequency spectra show that for the DNA with metal cations Na(+) , K(+) , Rb(+) and Cs(+) the frequency of ion-phosphate vibrations decreases from 174 to 96 cm(-1) as the counterion mass increases. The obtained frequencies agree well with the vibrational spectra of polynucleotides in a dry state which prove our suggestion about the existence of the ion-phosphate lattice around the DNA double helix. The amplitudes of conformational vibrations for DNA in B -form are calculated as well. The results demonstrate that light counterions ( Na(+) do not disturb the internal dynamics of the DNA. However, heavy counterions ( Cs(+) have effect on the internal vibrations of the DNA structural elements.

High-resolution waveguide THz spectroscopy of biological molecules

Low-frequency vibrational modes of biological molecules consist of intramolecular modes, which are dependent on the molecule as a whole, as well as intermolecular modes, which arise from hydrogen-bonding interactions and van der Waals forces. Vibrational modes thus contain important information about conformation dynamics of biological molecules, and can also be used for identification purposes. However, conventional Fourier transform infrared spectroscopy and terahertz time-domain spectroscopy (THz-TDS) often result in broad, overlapping features that are difficult to distinguish. The technique of waveguide THz-TDS has been recently developed, resulting in sharper features. For this technique, an ordered polycrystalline film of the molecule is formed on a metal sample plate. This plate is incorporated into a metal parallel-plate waveguide and probed via waveguide THz-TDS. The planar order of the film reduces the inhomogeneous broadening, and cooling of the samples to 77K reduces the homogenous broadening. This combination results in the line-narrowing of THz vibrational modes, in some cases to an unprecedented degree. Here, this technique has been demonstrated with seven small biological molecules, thymine, deoxycytidine, adenosine, D-glucose, tryptophan, glycine, and L-alanine. The successful demonstration of this technique shows the possibilities and promise for future studies of internal vibrational modes of large biological molecules.

Composite model for DNA torsion dynamics

DNA torsion dynamics is essential in the transcription process; a simple model for it, in reasonable agreement with experimental observations, has been proposed by Yakushevich (Y) and developed by several authors; in this, the nucleotides (the DNA subunits made of a sugar-phosphate group and the attached nitrogen base) are described by a single degree of freedom. In this paper we propose and investigate, both analytically and numerically, a "composite" version of the Y model, in which the sugar-phosphate group and the base are described by separate degrees of freedom. The model proposed here contains as a particular case the Y model and shares with it many features and results, but represents an improvement from both the conceptual and the phenomenological point of view. It provides a more realistic description of DNA and possibly a justification for the use of models which consider the DNA chain as uniform. It shows that the existence of solitons is a generic feature of the underlying nonlinear dynamics and is to a large extent independent of the detailed modeling of DNA. The model we consider supports solitonic solutions, qualitatively and quantitatively very similar to the Y solitons, in a fully realistic range of all the physical parameters characterizing the DNA.

The effect of solvent dynamics on the low frequency collective motions of DNA in solution and unoriented films

Infrared spectroscopy is used to probe the dynamics of in vitro samples of DNA prepared as solutions and as solid unoriented films. The lowest frequency DNA mode identified in the far-infrared spectra of the DNA samples is found to shift in frequency when the solvent influence in the hydration shell is altered. The lowest frequency mode also has characteristics that are similar to beta-relaxations identified in other glass forming polymers.

Where optics meets electronics: recent progress in decreasing the terahertz gap

The terahertz region of the electromagnetic spectrum offers considerable opportunities for exploitation. Recent advances in source and coherent-detection technology have enabled advances to be made in a wide range of applications. This article reviews the historical development of the field, summarizes the current state of system technology and outlines a number of current system applications, including medical imaging. The limitations of present-day technology are discussed and some future possible applications are considered.

Protein flexibility and conformational state: a comparison of collective vibrational modes of wild-type and D96N bacteriorhodopsin

Far infrared (FIR) spectral measurements of wild-type (WT) and D96N mutant bacteriorhodopsin thin films have been carried out using terahertz time domain spectroscopy as a function of hydration, temperature, and conformational state. The results are compared to calculated spectra generated via normal mode analyses using CHARMM. We find that the FIR absorbance is slowly increasing with frequency and without strong narrow features over the range of 2-60 cm(-1) and up to a resolution of 0.17 cm(-1). The broad absorption shifts in frequency with decreasing temperature as expected with a strongly anharmonic potential and in agreement with neutron inelastic scattering results. Decreasing hydration shifts the absorption to higher frequencies, possibly resulting from decreased coupling mediated by the interior water molecules. Ground-state FIR absorbances have nearly identical frequency dependence, with the mutant having less optical density than the WT. In the M state, the FIR absorbance of the WT increases whereas there is no change for D96N. These results represent the first measurement of FIR absorbance change as a function of conformational state.

Correlation between normal modes in the 20-200 cm-1 frequency range and localized torsion motions related to certain collective motions in proteins

In certain biologically relevant collective motions, such as protein domain motions and sub-domain motions, large amplitude movements are localized in one or a few flexible regions consisting of a small number of residues. This paper explores the possible use of normal mode analysis in probing localized vibrational torsion motions in these flexible regions that may be related to certain collective motions. The normal modes of 10 structures of five proteins in different conformation (TRP repressor, calmodulin, calbindin D(9k), HIV-1 protease and troponin C), known to have shear or hinge domain or sub-domain motion, respectively, are analyzed. Our study identifies, for each structure, unique normal modes in the 20-200 cm-1 frequency range, whose corresponding motions are primarily concentrated in the region where large amplitude torsion movements of a known domain or sub-domain motion occur. This suggests possible correlation between normal modes at 20-200 cm-1 frequency range and initial fluctuational motions leading to localized collective motions in proteins, and thus the potential application of normal mode analysis in facilitating the study of biologically important localized motions in biomolecules.

THz time domain spectroscopy of biomolecular conformational modes

We discuss the use of terahertz time domain spectroscopy for studies of conformational flexibility and conformational change in biomolecules. Protein structural dynamics are vital to biological function with protein flexibility affecting enzymatic reaction rates and sensory transduction cycling times. Conformational mode dynamics occur on the picosecond timescale and with the collective vibrational modes associated with these large scale structural motions in the 1-100 cm(-1) range. We have performed THz time domain spectroscopy (TTDS) of several biomolecular systems to explore the sensitivity of TTDS to distinguish different molecular species, different mutations within a single species and different conformations of a given biomolecule. We compare the measured absorbances to normal mode calculations and find that the TTDS absorbance reflects the density of normal modes determined by molecular mechanics calculations, and is sensitive to both conformation and mutation. These early studies demonstrate some of the advantages and limitations of using TTDS for the study of biomolecules.

Label-free probing of genes by time-domain terahertz sensing

A label-free sensing approach for the label-free characterization of genetic material with terahertz (THz) electromagnetic waves is presented. Time-resolved THz analysis of polynucleotides demonstrates a strong dependence of the complex refractive index of DNA molecules in the THz frequency range on their hybridization state. By monitoring THz signals one can thus infer the binding state (hybridized or denatured) of oligo- and polynucleotides, enabling the label-free determination the genetic composition of unknown DNA sequences. A broadband experimental proof-of-principle in a freespace analytic configuration, as well as a higher-sensitivity approach using integrated THz sensors reaching femtomol detection levels and demonstrating the capability to detect single-base mutations, are presented. The potential application for next generation high-throughput label-free genetic analytic systems is discussed.

Nonlinear dynamics of topological solitons in DNA

Dynamics of topological solitons describing open states in the DNA double helix are studied in the framework of a model that takes into account asymmetry of the helix. It is shown that three types of topological solitons can occur in the DNA double chain. Interaction between the solitons, their interactions with the chain inhomogeneities, and stability of the solitons with respect to thermal oscillations are investigated.

Submillimeter-wave phonon modes in DNA macromolecules

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.

Integrated planar terahertz resonators for femtomolar sensitivity label-free detection of DNA hybridization

A promising label-free approach for the analysis of genetic material by means of detecting the hybridization of polynucleotides with electromagnetic waves at terahertz (THz) frequencies is presented. Using an integrated waveguide approach, incorporating resonant THz structures as sample carriers and transducers for the analysis of the DNA molecules, we achieve a sensitivity down to femtomolar levels. The approach is demonstrated with time-domain ultrafast techniques based on femtosecond laser pulses for generating and electro-optically detecting broadband THz signals, although the principle can certainly be transferred to other THz technologies.

The interaction between Terahertz radiation and biological tissue

Terahertz (THz) radiation occupies that region of the electromagnetic (EM) spectrum between approximately 0.3 and 20 THz. Recent advances in methods of producing THz radiation have stimulated interest in studying the interaction between radiation and biological molecules and tissue. Given that the photon energies associated with this region of the spectrum are 2.0 x 10(-22) to 1.3 x 10(-20) J, an analysis of the interactions requires an understanding of the permittivity and conductivity of the medium (which describe the bulk motions of the molecules) and the possible transitions between the molecular energy levels. This paper reviews current understanding of the interactions between THz radiation and biological molecules, cells and tissues. At frequencies below approximately 6 THz. the interaction may be understood as a classical EM wave interaction (using the parameters of permittivity and conductivity), whereas at higher frequencies. transitions between different molecular vibrational and rotational energy levels become increasingly important and are more readily understood using a quantum-mechanical framework. The latter is of particular interest in using THz to probe transitions between different vibrational modes of deoxyribonucleic acid. Much additional experimental work is required in order to fully understand the interactions between THz radiation and biological molecules and tissue.

Mediation of a phase transition in hyaluronate films by the counterions Li, Cs, Mg and Ca as observed by infrared spectroscopy, optical microscopy and optical birefringence

Infrared (IR) spectroscopy and optical microscopy have been performed as a function of relative humidity (rh) on wet-spun oriented films of hyaluronate (HA) prepared with various counterions. Complete swelling measurements have been obtained through optical microscopy for films of Cs-, Mg-, and CaHA. IR spectroscopy of Cs-, Mg-, Ca-, and LiHA films was performed for skeletal vibrations (800-1000 cm(-1)) and for vibrational modes (1150-1300 cm(-1)) attributed to C-C and C-O stretching modes and C-C-H and C-O-H bending modes. These techniques reveal evidence of a counterion-dependent phase transition occuring at high relative humidities. Optical birefringence measurements on the polycrystalline samples showed order before and disorder after the transition from lower to higher humidity.

The effect of structural water molecules on the normal mode spectrum of dTn . dAn x dTn DNA

In this paper we present a theoretical treatment of triplex B type DNA hydration using normal mode calculation techniques. Discrete solvent is added as spines of hydration in the Watson-Crick and Crick-Hoogsteen grooves as well as water bridges between the Phosphate groups. The effect of binding the discrete structural waters on the normal mode of vibration of the system was studied by introducing a parameter, Xw, that is proportional to the degree of water binding and inversely proportional to the relative humidity (RH) of the system. We examined the variation of the dipole moments of characteristic modes with Xw. The results show that there is a direct relationship between the degree of binding of the water molecules to the atoms in the triple helix, the relative humidity of the system and the conformation and stability of the triple helix. At high RH and Xw = 0:0 the triple helix has mostly B type conformation characteristics, with C'2 -endo sugars. The emergence of normal modes of vibration characteristic to the A type conformation (C'3 - endo sugars) at Xw = 0:4 and 60% RH indicates a conformational shift towards A-type for some of the sugars between Xw = 0.2 (80% RH) and Xw = 0.4 (60% RH). These results are in agreement with the "economy of hydration hypothesis" of Saenger (Saenger et al., 1986) which maintains that the main difference in the hydration of A- and B- forms of DNA is the presence of water bridges between adjacent Phosphate groups in the low-hydration A-form but not in the B- form. Free energy calculations for the triplex DNA with structural waters show that there is a minimum of the free energy at Xw = 0.2 and the free energy increases with Xw and becomes larger than the free energy of the B conformation without structural waters for Xw equal to and larger than 0.4. This result indicates that the B conformation is more stable with bound structural water molecules (for degrees of water binding that are not over 20% higher than the degree of binding between bulk water molecules). The structural water molecules are bound much tighter in the A conformation than in the B conformation. The model predicts that the B to A transition occurs at higher relative humidities in D2O than in H2O. Part of these results (Dadarlat, 1997) have been subsequently confirmed by the experimental work and MD simulations of Ouali (Ouali et al., 1997). The experimental results showed that the N-type sugars corresponding to the A conformation are clearly detected below 75% RH.

Stability of triple-helical poly(dT)-poly(dA)-poly(dT) DNA with counterions

Structural conformation of triple-helical poly(dT)-poly(dA)-poly(dT) has been a very controversial issue recently. Earlier investigations, based on fiber diffraction data and molecular modeling, indicated an A-form conformation with C'3-endo sugar pucker. On the other hand, Raman, solution infrared spectral, and NMR studies show a B-form structure with C'2-endo sugars. In accordance with these experimental results, a theoretical model with B-form, C'2-endo sugars was proposed in 1993. In the present work we investigate the dynamics and stability of the two conformations within the effective local field approach applied to the normal mode calculations for the system. The presence of counterions was explicitly taken into account. Stable equilibrium positions for the counterions were calculated by analyzing the normal mode dynamics and free energy of the system. The breathing modes of the triple helix are shifted to higher frequencies over those of the double helix by 4-16 cm-1. The characteristic marker band for the B conformation at 835 cm-1 is split up into two marker bands at 830 and 835 cm-1. A detailed comparison of the normal modes and the free energies indicates that the B-form structure, with C'2-endo sugar pucker, is more stable than the A-form structure. The normal modes and the corresponding dipole moments are found to be in close agreement with recent spectroscopic findings.

Stabilization of the B conformation in unoriented films of calf thymus DNA by NaCl: a Raman and IR study

Unoriented films of calf thymus NaDNA with either 3.0 or 5.0 NaCl per base pair were prepared by dehydrating unstressed gels. These films were studied by Raman and ir spectroscopy. The 5.0 samples showed very strong vibrational modes characteristic of the B conformation at relative humidities (RH) as low as 30%, indicating that those samples were entirely in the B conformation. The 3.0 samples showed weaker features: some of the DNA in these samples were in the B conformation at 80% RH while the DNA is essentially in a disordered phase at 30% RH.

On a simple model of low-frequency vibrations in DNA macromolecules

A simple quasi-continuity model of low-frequency dynamics of DNA macromolecules is presented. The model is based on the phenomenological theory by Volkov and Kosevich (J. Biomol. Struct. Dyn. 8, 1069 (1991)). We propose improvements to their theory by correcting the model energy and corresponding equations of motion, recalculating the model DNA parameters, and estimating the effects of hydration and counterion binding on the masses of DNA subunits. Comparing the calculated low-frequency vibration spectrum and the results of classical Raman scattering experiments from the literature, the values for the model force constants are determined. These values differ significantly from the estimations made by Volkov and Kosevich. A good quantitative agreement with experiment can be obtained both for the case when the "25 cm-1 mode" is an external one, and when it is considered as an intrahelical mode. Problems connected with the qualitative assignments of the calculated and experimentally observed modes are discussed.

A study of Na-DNA films containing NaCl via scanning electron and tunneling microscopies

Unoriented films of Na-DNA were prepared by dessicating a gel with different amounts of NaCl. For low salt concentrations, the resulting films were smooth and patternless. For high salt concentrations, the resulting films had visible patterns that were very similar to those recently reported by Sclavi et al. [(1994) Biopolymers, Vol. 34, pp. 1105-1113]. Scanning electron and tunneling microscopies and electron dispersive spectroscopy studies were used to study the spatial distribution of NaCl throughout these films. Two main conclusions were reached about the NaCl: (1) some NaCl is distributed throughout every film, and (2) the visible patterns observed in films with a large amount of NaCl were caused by the growth of NaCl crystals during the dehydration process.

Geometry and energetics of DNA basepairs and triplets from first principles quantum molecular relaxations

A first principles model for calculating hydrogen bonding interactions, previously applied to water, is here applied to the more difficult problem of interactions between DNA bases. We first consider the energetics and geometry for the A-T and the G-C basepairs, comparing our results to other calculated results as well as to experiment. Next, we study the interactions of isomorphic DNA base triplet structures, which are important because of their suggested role in the recombination process. We find that energetically the third base in the triplet tends to favor a position along the dyadic axis, where it is hydrogen bonded to both bases in the duplex.

Sequence and temperature dependence of the interbase hydrogen-bond breathing modes in B-DNA polymers: comparison with low-frequency Raman peaks and their role in helix melting

We carry out temperature-dependent lattice dynamics calculations to determine the vibrational normal modes associated with the interbase H-bond breathing motion in several B-DNA copolymers at temperatures from room temperature to the melting temperatures. We take into consideration Raman selection rules and incorporate a simple empirical model of Raman susceptibility in the interbase H bonds in our calculation and compare them to Raman measurements. Our calculations are carried out using empirical force constants that are not further refined to low-frequency spectra. Our calculations show the existence of strong interbase H-bond breathing modes at frequencies and with relative oscillator strengths close to the observed Raman peaks in the range of 60-140 cm-1 for the DNA sequences considered except for one helix. The correlation between the calculated and observed frequencies and oscillator strengths indicates that the observed Raman peaks in the frequency range are likely interbase H-bond breathing modes. We find that these modes exhibit sizable temperature as well as sequence dependence. We show the softening of these modes on approaching thermal denaturation that is also in agreement with the observed behavior in Raman and melting measurements. The sensitivity of the calculation on the empirical model of Raman susceptibility and the possible reasons for the discrepancy between a few calculated values and observations are discussed.

Fractal-like patterns in DNA films, B form at 0% relative humidity, and antiheteronomous DNA: an IR study

This report details the observation of (a) the intact double helix of DNA at 0% relative humidity in poly(dA).poly(dT) and poly(dA-dT).poly(dA-dT) films, (b) the fractal-like growth of DNA crystals in films of poly(dA).poly(dT), and (c) poly(dA).poly(dT) with adenines in the B form and thymines in the A form. Observation (a) is based on the behavior of the middle ir signature of double-helical base stacking, the 1714 cm-1 peak, at low water activity. This observation is modeled as a trapping of water in the polycrystalline part of these films. We interpret the glycosidic region of the middle ir spectra of the polycrystalline films of poly(dA).poly(dT) at 0% relative humidity to indicate that at least part of the adenine strand in is the B conformation and the thymine strand is in the A form, whereas previous assignments of poly(dA).poly(dT) films at 75% relative humidity indicated the opposite strand conformation assignment [E. Taillandier et al. (1987) Biochemistry, Vol. 26 p. 3361].

Optical third harmonic generation study of the hydration of DNA films

Optical third harmonic generation (THG) has been observed for the first time from DNA films. The THG signal is observed from NaDNA films exposed to relative humidities (RHs) between 0% and 98%. A strong enhancement (approximately 5x) of the THG signal from NaDNA is observed at 84% RH; no enhancement is observed for RbDNA. The most likely mechanism for such an enhancement is an increased coherence length. A model calculation using estimates of the refractive indices at both the fundamental and third harmonic frequencies supports this interpretation. The observed THG signal has the same polarization as the incident (fundamental) light. For the A conformation, the THG signal polarized perpendicular to the helical axis is approximately twice as strong as the signal polarized parallel to the helical axis. No such anisotropy is observed for either the disordered conformation (below about 50% RH) or the B conformation (above 92% RH).

Nonlinear dynamics of biopolymers: theoretical models, experimental data

Nonlinear dynamics of biopolymers is a new and rapidly developing field of biophysical science. It can be considered as a part of the general dynamics which deals with the internal mobility of biopolymers. Theoreticians define it also as the next (anharmonic or nonlinear) approximation after the first harmonic or linear one.

Far-infrared study of the vibrational modes of 5'-GMP gels and crystals of Na+ and K+

Five Far-Infrared (50-600 cm-1) spectra are presented: the sodium and potassium salts of 5' Guanosine Monophosphate (GMP), each salt in both the gel and crystal conformations, and poly(rG). Measurements were performed at a sample temperature of 10 K under vacuum with a liquid He-cooled bolometer. The spectra were fit with Lorentzians and assignments are suggested. There are noteworthy differences in oscillator strengths and frequencies of the bands between all spectra. We report the tentative observation of a 100 cm-1 mode which is in the neighborhood of a mode observed by Raman spectroscopy in solution (1) and dried gels (2).

Plasmon interpretation of 25 cm-1 mode in DNA

An extension of the effective field approach for the normal mode dynamics of dissolved DNA polymers has been applied to study the vibrational modes of DNA-hydration sheath-counterion system, to include the effect of site bound counterions on the system dynamics. An alternative interpretation has been suggested for a 25 cm-1 mode recently observed in DNA samples and interpreted earlier as an interhelical mode. Analysing the eigenvectors this mode is found to possess a large electric dipole moment with longitudinal collective oscillations of the system. These characteristics identify this mode as a collective plasmon mode. Possible physical reasons for the existence of this character have been presented.