Interpretation of DNA vibrational spectra by normal coordinate analysis

Non-covalent binding interaction of bioactive coumarin esculetin with calf thymus DNA and yeast transfer RNA: A detailed investigation to decipher the binding affinities, binding location, interacting forces and structural alterations at a molecular level

Esculetin is a well-known coumarin derivative found abundantly in nature possessing an extensive array of pharmacological and therapeutic properties. Consequently, to comprehend its molecular recognition mechanism, our objective is to conduct a complete investigation of its interactions with the nucleic acid, specifically ct-DNA, and t-RNA, using spectroscopic and computational techniques. The intrinsic fluorescence of esculetin is quenched when it interacts with ct-DNA and t-RNA, and this occurs through a static quenching mechanism. The thermodynamic parameters demonstrated that the interaction is influenced by hydrogen bonding and weak van der Waals forces. CD and FT-IR results revealed no conformational changes in ct-DNA and t-RNA structure on binding with esculetin. Furthermore, competitive displacement assay with ethidium bromide, melting temperature, viscosity measurement, and potassium iodide quenching experiments, reflected that esculetin probably binds to the minor groove of ct-DNA. The molecular docking results provided further confirmation for the spectroscopic findings, including the binding location of esculetin and binding energies of esculetin complexes with ct-DNA and t-RNA. Molecular dynamics simulation studies demonstrated the conformational stability and flexibility of nucleic acids.

Experimental detection of conformational transitions between forms of DNA: problems and prospects

Under different conditions, the DNA double helix can take different geometric forms. Of the large number of its conformations, in addition to the "canonical" B form, the A, C, and Z forms are widely known, and the D, Hoogsteen, and X forms are less known. DNA locally takes the A, C, and Z forms in the cell, in complexes with proteins. We compare different methods for detecting non-canonical DNA conformations: X-ray, IR, and Raman spectroscopy, linear and circular dichroism in both the infrared and ultraviolet regions, as well as NMR (measurement of chemical shifts and their anisotropy, scalar and residual dipolar couplings and inter-proton distances from NOESY (nuclear Overhauser effect spectroscopy) data). We discuss the difficulties in applying these methods, the problems of theoretical interpretation of the experimental results, and the prospects for reliable identification of non-canonical DNA conformations.

Rapid Discrimination of Neuromyelitis Optica Spectrum Disorder and Multiple Sclerosis Using Machine Learning on Infrared Spectra of Sera

Neuromyelitis optica spectrum disorder (NMOSD) and multiple sclerosis (MS) are both autoimmune inflammatory and demyelinating diseases of the central nervous system. NMOSD is a highly disabling disease and rapid introduction of the appropriate treatment at the acute phase is crucial to prevent sequelae. Specific criteria were established in 2015 and provide keys to distinguish NMOSD and MS. One of the most reliable criteria for NMOSD diagnosis is detection in patient’s serum of an antibody that attacks the water channel aquaporin-4 (AQP-4). Another target in NMOSD is myelin oligodendrocyte glycoprotein (MOG), delineating a new spectrum of diseases called MOG-associated diseases. Lastly, patients with NMOSD can be negative for both AQP-4 and MOG antibodies. At disease onset, NMOSD symptoms are very similar to MS symptoms from a clinical and radiological perspective. Thus, at first episode, given the urgency of starting the anti-inflammatory treatment, there is an unmet need to differentiate NMOSD subtypes from MS. Here, we used Fourier transform infrared spectroscopy in combination with a machine learning algorithm with the aim of distinguishing the infrared signatures of sera of a first episode of NMOSD from those of a first episode of relapsing-remitting MS, as well as from those of healthy subjects and patients with chronic inflammatory demyelinating polyneuropathy. Our results showed that NMOSD patients were distinguished from MS patients and healthy subjects with a sensitivity of 100% and a specificity of 100%. We also discuss the distinction between the different NMOSD serostatuses. The coupling of infrared spectroscopy of sera to machine learning is a promising cost-effective, rapid and reliable differential diagnosis tool capable of helping to gain valuable time in patients’ treatment.

Micronomicin/tobramycin binding with DNA: fluorescence studies using of ethidium bromide as a probe and molecular docking analysis

Two aminoglycosides, micronomicin (MN), and tobramycin (TB), binding with DNA were studied using various spectroscopic techniques including fluorescence, UV-Vis, FT-IR, and CD spectroscopy coupled with relative viscosity and molecular docking. Studies of fluorescence quenching and time-resolved fluorescence spectroscopy all revealed that MN/TB quenching the fluorescence of DNA-EB belonged to static quenching. The binding constants and binding sites were obtained. The values of ΔH, ΔS, and ΔG suggested that van der Waals force or hydrogen bond might be the main binding force. FT-IR and CD spectroscopy revealed that the binding of MN/TB with DNA had an effect on the secondary structure of DNA. Binding mode of MN/TB with DNA was groove binding which was ascertained by viscosity measurements, CD spectroscopy, ionic strength, melting temperature (T_m), contrast experiments with single stranded (ssDNA), and double stranded DNA (dsDNA). Molecular docking analysis further confirmed that the groove binding was more acceptable result.

Vibrational spectra of DNA in the confined interglobular volume of photonic crystal

The impact of confinement of DNA molecules in a limited volume of the cavity of photonic crystals (PC) on the vibrational properties of the DNA molecule and its conformation is studied. According to our preliminary study, the aqueous shell is removed when the DNA molecules are infiltrated into the PC cavities. Raman scattering (RS) DNA marker lines showed a dramatic conformational change of DNA in the PC cavities and the appearance of new unknown conformational states. We observed the enhancement of vibrational modes of DNA in the PC in comparison with free DNA of about tenfold and the absence of vibrational modes in DNA bases in a region of 1450-1700 cm-1. The observed features in the RS spectra of DNA are explained by the impact of confined interglobular volume and strong localization of the electromagnetic field. Namely, FDTD simulations in linear regime demonstrate the localization of light in cavities of PC with an approximately ninefold enhancement of the electric field within the photonic stop-band, which is the main reason for RS amplification.

Robust IR-based detection of stable and fractionally populated G-C+ and A-T Hoogsteen base pairs in duplex DNA

Noncanonical G-C+ and A-T Hoogsteen base pairs can form in duplex DNA and play roles in recognition, damage repair, and replication. Identifying Hoogsteen base pairs in DNA duplexes remains challenging due to difficulties in resolving syn versus antipurine bases with X-ray crystallography; and size limitations and line broadening can make them difficult to characterize by NMR spectroscopy. Here, we show how infrared (IR) spectroscopy can identify G-C+ and A-T Hoogsteen base pairs in duplex DNA across a range of different structural contexts. The utility of IR-based detection of Hoogsteen base pairs is demonstrated by characterizing the first example of adjacent A-T and G-C+ Hoogsteen base pairs in a DNA duplex where severe broadening complicates detection with NMR.

Structural effects of simvastatin on liver rat [corrected] tissue: Fourier transform infrared and Raman microspectroscopic studies

Simvastatin is one of the most frequently prescribed statins because of its efficacy in the treatment of hypercholesterolemia, reducing cardiovascular risk and related mortality. Determination of its side effects on different tissues is mandatory to improve safe use of this drug. In the present study, the effects of simvastatin on molecular composition and structure of healthy rat livers were investigated by Fourier transform infrared and Raman imaging. Simvastatin-treated groups received 50 mg/kg/day simvastatin for 30 days. The ratio of the area and/or intensity of the bands assigned to lipids, proteins, and nucleic acids were calculated to get information about the drug-induced changes in tissues. Loss of unsaturation, accumulation of end products of lipid peroxidation, and alterations in lipid-to-protein ratio were observed in the treated group. Protein secondary structure studies revealed significant decrease in α-helix and increase in random coil, while native β-sheet decreases and aggregated β-sheet increases in treated group implying simvastatin-induced protein denaturation. Moreover, groups were successfully discriminated using principal component analysis. Consequently, high-dose simvastatin treatment induces hepatic lipid peroxidation and changes in molecular content and protein secondary structure, implying the risk of liver disorders in drug therapy.

Fourier Transform Infrared spectroscopy discloses different types of cell death in flow cytometrically sorted cells

Fourier Transform Infrared (FTIR) spectroscopy is a label free methodology showing promise in characterizing different types of cell death. Cervical adenocarcinoma (HeLa) and African monkey kidney (Vero) cells were treated with a necrosis inducer (methanol), novel apoptotic inducers (diphenylphosphino gold (I) complexes) and positive control, auranofin. Following treatment, cells stained with annexin-V and propidium iodide were sorted using a Fluorescence Activated Cell Sorter (FACS Aria) to obtain populations consisting of either viable, necrotic or apoptotic cells. Transmission Electron Microscopy confirmed successful sorting of all three populations. Four bands were identified which could discriminate between viable and necrotic cells namely 989 cm(-1), 2852 cm(-1), 2875 cm(-1) and 2923 cm(-1). In HeLa cells viable and induced apoptosis could be distinguished by 1294 cm(-1), while four bands were different in Vero cells namely; 1626 cm(-1), 1741 cm(-1), 2852 cm(-1) 2923 cm(-1). Principal Component Analysis showed separation between the different types of cell death and the loadings plots indicated an increase in an additional band at 1623 cm(-1) in dead cells. FTIR spectroscopy can be developed into an invaluable tool for the assessment of specific types of chemically induced cell death with notably different molecular signatures depending on whether the cells are cancerous and mechanism of cell death.

Cellular injury evidenced by impedance technology and infrared microspectroscopy

Fourier Transform Infrared (FTIR) spectroscopy is finding increasing biological application, for example in the analysis of diseased tissues and cells, cell cycle studies and investigating the mechanisms of action of anticancer drugs. Cancer treatment studies routinely define the types of cell-drug responses as either total cell destruction by the drug (all cells die), moderate damage (cell deterioration where some cells survive) or reversible cell cycle arrest (cytostasis). In this study the loss of viability and related chemical stress experienced by cells treated with the medicinal plant, Plectranthus ciliatus, was investigated using real time cell electronic sensing (RT-CES) technology and FTIR microspectroscopy. The use of plants as medicines is well established and ethnobotany has proven that crude extracts can serve as treatments against various ailments. The aim of this study was to determine whether FTIR microspectroscopy would successfully distinguish between different types of cellular injury induced by a potentially anticancerous plant extract. Cervical adenocarcinoma (HeLa) cells were treated with a crude extract of Pciliatus and cells monitored using RT-CES to characterize the type of cellular responses induced. Cell populations were then investigated using FTIR microspectroscopy and statistically analysed using One-way Analysis of Variance (ANOVA) and Principal Component Analysis (PCA). The plant extract and a cancer drug control (actinomycin D) induced concentration dependent cellular responses ranging from nontoxic, cytostatic or cytotoxic. Thirteen spectral peaks (915cm(-)(1), 933cm(-)(1), 989cm(-)(1), 1192cm(-)(1), 1369cm(-)(1), 1437cm(-)(1), 1450cm(-)(1), 1546cm(-)(1), 1634cm(-)(1), 1679cm(-)(1) 1772cm(-)(1), 2874cm(-)(1) and 2962cm(-)(1)) associated with cytotoxicity were significantly (p value<0.05, one way ANOVA, Tukey test, Bonferroni) altered, while two of the bands were also indicative of early stress related responses. In PCA, poor separation between nontoxic and cytostatic responses was evident while clear separation was linked to cytotoxicity. RT-CES detected morphological changes as indicators of cell injury and could distinguish between viable, cytostatic and cytotoxic responses. FTIR microspectroscopy confirmed that cytostatic cells were viable and could still recover while also describing early cellular stress related responses on a molecular level.

In vitro protection of biological macromolecules against oxidative stress and in vivo toxicity evaluation of Acacia nilotica (L.) and ethyl gallate in rats

BACKGROUND

Recently, enormous research has been focused on natural bioactive compounds possessing potential antioxidant and anticancer properties using cell lines and animal models. Acacia nilotica (L.) is widely distributed in Asia, Africa, Australia and Kenya. The plant is traditionally used to treat mouth, ear and bone cancer. However, reports on Acacia nilotica (L.) Wild. Ex. Delile subsp. indica (Benth.) Brenan regarding its toxicity profile is limited. Hence in this study, we investigated the antioxidant capacity and acute toxicity of ethyl gallate, a phenolic antioxidant present in the A. nilotica (L.) leaf extract.

METHODS

The antioxidant activity of ethyl gallate against Fenton's system (Fe3+/H2O2/ascorbic acid) generated oxidative damage to pBR322 DNA and BSA was investigated. We also studied the interaction of ethyl gallate to CT-DNA by wave scan and FTIR analysis. The amount of ethyl gallate present in the A. nilotica (L.) leaf extract was calculated using HPLC and represented in gram equivalence of ethyl gallate. The acute toxicity profile of ethyl gallate in the A. nilotica (L.) leaf extract was analyzed in albino Wistar rats. Measurement of liver and kidney function markers, total proteins and glucose were determined in the serum. Statistical analysis was done using statistical package for social sciences (SPSS) tool version 16.0.

RESULTS

Ethyl gallate was found to be effective at 100 μg/mL concentration by inhibiting the free radical mediated damage to BSA and pBR322 DNA. We also found that the interaction of ethyl gallate and A. nilotica (L.) leaf extract to CT-DNA occurs through intercalation. One gram of A. nilotica (L.) leaf extract was found to be equivalent to 20 mg of ethyl gallate through HPLC analysis. Based on the acute toxicity results, A. nilotica (L.) leaf extract and ethyl gallate as well was found to be non-toxic and safe.

CONCLUSIONS

Results revealed no mortality or abnormal biochemical changes in vivo and the protective effect of A. nilotica (L.) leaf extract and ethyl gallate on DNA and protein against oxidative stress in vitro. Hence, A. nilotica (L.) leaf extract or ethyl gallate could be used as potential antioxidants with safe therapeutic application in cancer chemotherapy.

DNA interaction studies of an anticancer plant alkaloid, vincristine, using Fourier transform infrared spectroscopy

The binding of vincristine with DNA has been investigated using Fourier transform infrared spectroscopy. Various changes in the double helical structure of DNA after addition of vincristine have been examined. It is evident from Fourier transform infrared results that vincristine-DNA interaction occurs through guanine and cytosine base pairs. External binding of vincristine with phosphate backbone of the DNA is also observed. Vincristine perturbs guanine band at 1714 cm(-1), cytosine band at 1488 cm(-1), and the phosphate vibrations at 1225 and 1086 cm(-1). The UV-visible spectra of vincristine-DNA complex show hypochromic and bathochromic shifts, indicating the intercalation of vincristine into the double helical structure of DNA. Both intercalative and external binding modes are observed for vincristine binding with DNA, with an estimated binding constant K = 1.0 × 10(3) M(-1).

Vibrational spectroscopic analysis of cytosine monohydrate and its copper(II) complex

Single crystals of cytosine monohydrate and its copper(II) complex were grown by slow evaporation in an aqueous solution at room temperature. The solubility studies were carried out at different temperatures in deionized water. Cytosine monohydrate and the copper(II) complex of cytosine were characterized by recording IR, Raman and UV spectra. The various vibrational modes of the crystals have been classified using factor group and site group analysis. Vibrational assignments were proposed for both the systems based on the spectral investigations.

Differentiation of mesophilic and thermophilic bacteria with fourier transform infrared spectroscopy

In the present study the characterization and differentiation of mesophilic and thermophilic bacteria were investigated by using Fourier transform infrared (FT-IR) spectroscopy. Our results showed significant differences between the FT-IR spectra of mesophilic and thermophilic bacteria. The protein-to-lipid ratio was significantly higher for thermophiles compared to mesophiles. The absorption intensity of the CH(3) asymmetric stretching vibration was higher in thermophilic bacteria, indicating a change in the composition of the acyl chains. The higher intensity/area observed in the CH(2) symmetric stretching mode at 2857 cm(-1), and the CH(2) bending vibration band at 1452 cm(-1), indicated a higher amount of saturated lipids in thermophilic bacteria. The lipid C=O stretching vibration at 1739 cm(-1), which was observed in the mesophilic group, was not observed clearly in the thermophilic group, indicating a difference in packing that is presumably due to the decreased proportion of unsaturated acyl chains in thermophilic bacteria. In addition, the carbonyl groups become hydrogen bonded and the cellular DNA content was lower in thermophilic bacteria. Moreover, in the 1000-400 cm(-1) frequency region, the spectra of each bacterial species belonging to both the mesophilic and thermophilic bacterial groups, showed characteristic differences that were discriminated via dendrogram using cluster analysis. The current study implies that FT-IR spectroscopy could be successfully applied for the rapid comparison of bacterial groups and species to establish either similarities or discrepancies, as well as to confirm biochemical or physiological characteristics.

On the interpretation of Raman spectra of 1-aminooxy-spermine/DNA complexes

By FT-Raman spectroscopy, we have investigated the effect of 1-aminooxy-spermine (AOSPM) on aggregation and stability of calf-thymus DNA and selected oligonucleotide chains. AOSPM is able to mimic spermine in some macromolecular interactions, but is unable to substitute polyamines to maintain cell proliferation, suggesting pharmacological applications. Raman spectra of solutions containing AOSPM and either genomic DNA or two 15mer oligodeoxyribonucleotides, with GC or AT sequences, were recorded. Precipitation was observed for calf-thymus DNA, aggregated structures and appearance of several Z marker bands were observed for the 15mer GC sequence, and no macromolecular changes were detected for the 15mer AT sequence. Specific binding sites between the aminooxy group and the base residues were also evidenced. Assignment of the AOSPM Raman bands was supported on a normal mode calculation for the molecule NH(2)-O-CH(3), as a model. The theoretical results, in combination with the analysis of the Raman bands, demonstrated that the aminooxy group played a relevant role in the AOSPM-DNA interaction. Preferential binding by the major groove was evidenced in the absence of macromolecular changes. When either precipitation or aggregation occurred, the interaction involved both the major and minor grooves. The specific interaction between AT/GC base pairs and the aminooxy group has also been theoretically investigated. The biological relevance of this work is discussed.

Cisplatin adducts of d(CCTCTG*G*TCTCC).d(GGAGACCAGAGG) in aqueous solution by vibrational circular dichroism spectroscopy

The vibrational circular dichroism (VCD) and IR absorption spectra of a dodecamer d(CCTCTGGTCTCC).d(GGAGACCAGAGG) coordinated with cisplatin are distinct compared to those of the dodecamer without cisplatin. Although the intensity of PO(2)/deoxyribose absorptions (1150-850 cm(-1)) increases noticeably relative to those of the carbonyl and ring deformations of the bases (1750-1500 cm(-1)), the VCD spectra differ to a much greater extent. Overlapping positive and negative bands can be assigned relatively easily to individual vibrational modes. The effect of platination on the dodecamer duplex is expressed most prominently in VCD arising solely from the vibrations of the guanines bound to the platinum atom. The effect on the VCD features of other bases leads to minute wavenumber shifts at most. These observations are in agreement with previous NMR and X-ray experiments on the same oligonucleotide. The assignment of the absorption and VCD bands strongly resembles those of the octamer duplex d(CCTGGTCC).d(GGACCAGG) when coordinated with platinum. The spectra of the dodecamer did not indicate any isomerization of the complex with time, as is clearly the case for the octamer.

Characterization and property of DNA incorporated bilayer lipid membranes

Calf-thymus DNA-incorporated bilayer lipid membranes supported on a glassy carbon (GC) electrode was prepared by making layers of phosphatidylcholine dimyristoyl (DMPC) on GC electrode. DNA in the BLM was characterized by cyclic voltammetry, IR and AFM, and lipid layers formed on the GC electrode were demonstrated to be a bilayer lipid membrane by electrochemical impedance experiment. In IR and AFM experiments the findings indicated that DNA was incorporated into BLM. The ion channel of bilayer lipid membranes incorporated was studied. The result showed that the ion channel was opened in the presence of the stimulus quinacrine. In the absence of quinacrine the channel was switched. The process can repeat itself many times. The impedance spectroscopy measurements demonstrate that the stimulus quinacrine opens the channel for permeation of marker ion. The mechanism of forming an ion channel was investigated.

Fourier transform Raman study of the structural specificities on the interaction between DNA and biogenic polyamines

Biogenic polyamines putrescine, spermidine, and spermine are essential molecules for proliferation in all living organisms. Direct interaction of polyamines with nucleic acids has been proposed in the past based on a series of experimental evidences, such as precipitation, thermal denaturation, or protection. However, binding between polyamines and nucleic acids is not clearly explained. Several interaction models have also been proposed, although they do not always agree with one another. In the present work, we make use of the Raman spectroscopy to extend our knowledge about polyamine-DNA interaction. Raman spectra of highly polymerized calf-thymus DNA at different polyamine concentrations, ranging from 1 to 50 mM, have been studied for putrescine, spermidine, and spermine. Both natural and heavy water were used as solvents. Difference Raman spectra have been computed by subtracting the sum of the separated component spectra from the experimental spectra of the complexes. The analysis of the Raman data has supported the existence of structural specificities in the interactions, at least under our experimental conditions. These specificities lead to preferential bindings through the DNA minor groove for putrescine and spermidine, whereas spermine binds by the major groove. On the other hand, spermine and spermidine present interstrand interactions, whereas putrescine presents intrastrand interactions in addition to exo-groove interactions by phosphate moieties.

Multilayer films containing immobilized nucleic acids. Their structure and possibilities in biosensor applications

Langmuir-Blodgett (LB) and film technologies based on electrostatic attraction self-assembly (SA) are shown to be useful for immobilization of nucleic acids (DNA, polynucleotides) onto solid supports in sensor devices. The nucleic acids were immobilized in complexes with cationic surfactants (for LB) and polycations (for SA). Infrared spectral studies showed that DNA unfolds in multilayer LB films with octadecylamine and conserves its double helical structure in the LB films with dioctadecyldimethylammonium and in the SA films with polyallylamine, polyethylenimine and poly-L-lysine. Atomic groups and the types of interactions determining the complex formation of these films have been identified. The hydration of LB and SA films was studied to find out binding sites of water molecules and to evaluate the flexibility of nucleic acid compounds in the multilayer films. The possibilities of biosensor applications of these LB and SA films were monitored on binding of specific reagents for DNA by DNA-containing films and mononucleotides by a complementary single-stranded polynucleotide immobilized on a positively charged solid support.

Structure and drug interactions of parallel-stranded DNA studied by infrared spectroscopy and fluorescence

The infrared spectra of three different 25-mer parallel-stranded DNAs (ps-DNA) have been studied. We have used ps-DNAs containing either exclusively dA x dT base pairs or substitution with four dG x dC base pairs and have them compared with their antiparallel-stranded (aps) reference duplexes in a conventional B-DNA conformation. Significant differences have been found in the region of the thymine C = O stretching vibrations. The parallel-stranded duplexes showed characteristic marker bands for the C2 = O2 and C4 = O4 carbonyl stretching vibrations of thymine at 1685 cm-1 and 1668 cm-1, respectively, as compared to values of 1696 cm-1 and 1663 cm-1 for the antiparallel-stranded reference duplexes. The results confirm previous studies indicating that the secondary structure in parallel-stranded DNA is established by reversed Watson--Crick base pairing of dA x dT with hydrogen bonds between N6H...O2 and N1...HN3. The duplex structure of the ps-DNA is much more sensitive to dehydration than that of the aps-DNA. Interaction with three drugs known to bind in the minor groove of aps-DNA--netropsin, distamycin A and Hoechst 33258--induces shifts of the C = O stretching vibrations of ps-DNA even at low ratio of drug per DNA base pair. These results suggest a conformational change of the ps-DNA to optimize the DNA-drug interaction. As demonstrated by excimer fluorescence of strands labeled with pyrene at the 5'-end, the drugs induce dissociation of the ps-DNA duplex with subsequent formation of imperfectly matched aps-DNA to allow the more favorable drug binding to aps-DNA. Similarly, attempts to form a triple helix of the type d(T)n.d(A)n.d(T)n with ps-DNA failed and resulted in the dissociation of the ps-DNA duplex and reformation of a triple helix based upon an aps-DNA duplex core d(T)10.d(A)10.

FT-IR spectroscopic study of the poly(amino2dA-dT) duplex in Mg(2+)-containing solution and in films

The alternative structures of the synthetic poly(amino2dA-dT) duplex have been studied using infrared spectroscopy in films and in solution (D2O and H2O) in the presence and in the absence of magnesium salt. In solution without magnesium salt, the polynucleotide exists in a B genus conformation with some of the sugar puckers possibly in the C3'-endo/anti geometry. In magnesium-containing solution (66 mM MgCl2), however, we report infrared spectra of Mg(2+)-poly(amino2dA-dT) which have characteristic marker bands of the A form. Film samples in 70% relative humidity (RH) give similar infrared spectra to those of the polynucleotide obtained using Mg2+. Thus, when analyzed in comparison with previously reported infrared spectra of other oligo and polynucleotides, our data show that double helical poly(amino2dA-dT) goes into the same (or very closely related) conformation in dehydrated films as in solutions containing Mg2+.