Mercury-induced DNA polymorphism: probing the conformation of Hg(II)-DNA via staphylococcal nuclease digestion and circular dichroism measurements

Differential Charging in Photoemission from Mercurated DNA Monolayers on Ferromagnetic Films

Spin-dependent and enantioselective electron-molecule scattering occurs in photoelectron transmission through chiral molecular films. This spin selectivity leads to electron spin filtering by molecular helices, with increasing magnitude concomitant with increasing numbers of helical turns. Using ultraviolet photoelectron spectroscopy, we measured spin-selective surface charging accompanying photoemission from ferromagnetic substrates functionalized with monolayers of mercurated DNA hairpins that constitute only one helical turn. Mercury ions bind specifically at thymine-thymine mismatches within self-hybridized single-stranded DNA, enabling precise control over the number and position of Hg²⁺ along the helical axis. Differential charging of the organic layers, manifested as substrate-magnetization-dependent photoionization energies, was observed for DNA hairpins containing Hg²⁺; no differences were measured for hairpin monolayers in the absence of Hg²⁺. Inversion of the DNA helical secondary structure at increased metal loading led to complementary inversion in spin selectivity. We attribute these results to increased scattering probabilities from relativistic enhancement of spin-orbit interactions in mercurated DNA.

A review of heavy metal cation binding to deoxyribonucleic acids for the creation of chemical sensors

Various human activities lead to the pollution of ground, drinking, and wastewater with toxic metals. It is well known that metal ions preferentially bind to DNA phosphate backbones or DNA nucleobases, or both. Foreman et al. (Environ Toxicol Chem 30(8):1810-1818, 2011) reported the use of a DNA-dye based assay suitable for use as a toxicity test for potable environmental water. They compared the results of this test with the responses of live-organism bioassays. The DNA-based demonstrated that the loss of SYBR Green I fluorescence dye bound to calf thymus DNA was proportional to the toxicity of the water sample. However, this report raised questions about the mechanism that formed the basis of this quasi-quantitatively test. In this review, we identify the unique and preferred DNA-binding sites of individual metals. We show how highly sensitive and selective DNA-based sensors can be designed that contain multiple binding sites for 21 heavy metal cations that bind to DNA and change its structure, consistent with the release of the DNA-bound dye.

Hg2+-trapping beads: Hg2+-specific recognition through thymine-Hg(II)-thymine base pairing

Mercury pollution poses a severe threat to human health. To remove Hg(2+) from contaminated water, we synthesized Hg(2+)-trapping beads that include oligo-thymidine functionalities that can form thymine-Hg(II)-thymine base pairs on the solid support. The beads can selectively trap Hg(2+) even in the presence of other metal cations. More interestingly, Hg(2+)-trapping efficiency was higher in the presence of the co-existing cations. Thus, the developed Hg(2+)-trapping beads can capture Hg(2+) without affecting the mineral balance of water so much. The Hg(2+)-trapping beads presented here show promise for removing Hg(2+) from environmental water.

Studies on the thymine–mercury–thymine base pairing in parallel and anti-parallel DNA duplexes

Parallel and anti-parallel T–Hg–T base pairs have different thermal stabilities and conformational influences on DNA duplex structures.

A reusable and sensitive biosensor for total mercury in canned fish based on fluorescence polarization

In this work, we developed a sensitive and selective sensor technique for total mercury (Hg) detection in canned fish samples based on the fluorescence polarization (FP) method. The detection principle was that ssDNA containing thymine (T) bases was modified on magnetic nanoparticles (MNPs), which were used as enhancement probe. In the presence of Hg(2+), the ssDNA on MNPs can hybridize with the fluorophore labeled aptamer owing to the specific interaction between T bases and Hg(2+). The formation of thymine-Hg(2+)-thymine (T-Hg(2+)-T) complexes leads to the molar mass increase of fluorophore molecules, resulting in the enhancement of FP signal. The increase of FP was in a good linearity with the concentration of Hg(2+) in range of 2.0 nM-1.0 mM and the limit of detection was 0.49 nM (3.29 SB/m, according to the recent recommendation of IUPAC). Moreover, the proposed biosensor can be reused for 6 cycling times and was successfully applied in monitoring Hg(2+) in real samples.

Detection of mercury-TpT dinucleotide binding by Raman spectra: a computational study

The Hg(2+) ion stabilizes the thymine-thymine mismatched base pair and provides new ways of creating various DNA structures. Recently, such T-Hg-T binding was detected by the Raman spectroscopy. In this work, detailed differences in vibrational frequencies and Raman intensity patterns in the free TpT dinucleotide and its metal-mediated complex (TpT·Hg)(2) are interpreted on the basis of quantum chemical modeling. The computations verified specific marker Raman bands indicating the effect of mercury binding to DNA. Although the B3LYP functional well-describes the Raman frequencies, a dispersion correction had to be added for all atoms including mercury to obtain realistic geometry of the (TpT·Hg)(2) dimer. Only then, the DFT complex structure agreed with those obtained with the wave function-based MP2 method. The aqueous solvent modeled as a polarizable continuum had a minor effect on the dispersion interaction, but it stabilized conformations of the sugar and phosphate parts. A generalized definition of internal coordinate force field was introduced to monitor covalent bond mechanical strengthening and weakening upon the Hg(2+) binding. Induced vibrational frequency shifts were rationalized in terms of changes in electronic structure. The simulations thus also provided reliable insight into the complex structure and stability.

Interactions of tetracationic mercury(II), cadmium(II) and lead(II) porphyrins with DNA and their effects on DNA cleavage

The effects of tetrakis(1-methylpyridinium-4-yl)porphyrinatomercury(II) [ Hg ( TMPyP )]4+(6) along with [ Pb ( TMPyP )]4+(7), [ Cd ( TMPyP )]4+(8), and [ H2( TMPyP )]4+(1) (free base porphyrin) on DNA cleavage by Haemophilus aegyptius (HaeIII) have been studied using gel electrophoresis, UV-vis absorption spectroscopy, and circular dichroism (CD) spectroscopy. The gel electrophoresis results indicate that in the absence of 6, HaeIII restriction enzyme could not cleave plasmid DNA at a low concentration of 0.2 units μl-1after 1 h incubation at 37 °C. However, DNA cleavage occurred in the presence of 6 at the concentrations of 1.0 × 10-6and 1.0 × 10-7M and 0.2 units μl-1of HaeIII under the same conditions. In addition, acceleration of DNA cleavage occurred at a higher concentration of HaeIII (0.4 units μl-1) in the presence of a wider concentration range of 6 from 1.0 × 10-5to 1.0 × 10-7M . 8, 7, and 1 could not enhance DNA cleavage in the presence of HaeIII (0.2 units μl-1). However, when the concentration of HaeIII was increased to 0.4 units μl-1, only 8 inhibited DNA cleavage by HaeIII at higher concentrations (1.0 × 10-5-10-6M ) whereas 6, 7, and 1 did not. The UV-vis and CD spectroscopic results indicate that these porphyrins interact differently with DNA based on their binding modes. 6 enhanced DNA cleavage due to the synergistic effect of the Hg2+ions and the free base porphyrin 1 that dissociated from the mercury(II) porphyrin complex upon binding to DNA which resulted in the enhanced transformation of the DNA conformation from the original B-form to a Z-like conformation, while 8 inhibited HaeIII activity at higher concentrations and 7 and 1 neither enhanced nor inhibited DNA cleavage. A mechanism for this phenomenon is suggested.

A DNA-based assay for toxic chemicals in wastewater

Chemical toxicants, particularly metal ions, are a major contaminant in global waterways. Live-organism bioassays used to monitor chemical toxicants commonly involve measurements of activity or survival of a freshwater cladoceran (Ceriodaphnia dubia) or light emitted by the marine bacterium Vibrio fischeri, used in the commercial Microtox® bioassay. Here we describe a novel molecule-based assay system employing DNA as the chemical biosensor. Metals bind to DNA, causing structural changes that expel a bound (intercalated) fluorescent reporter dye. Analyses of test data using 48 wastewater samples potentially contaminated by metal ions show that the DNA-dye assay results correlate with those from C. dubia and Microtox bioassays. All three assays exhibit additive, antagonistic, and synergistic responses that cannot be predicted by knowing individual metal concentrations. Analyses of metals in these samples imply the presence of chemical toxicants other than metal ions. The DNA-dye assay is robust, has a 12-month shelf life, and is only slightly affected by sample pH in the range 4 to 9. The assay is completed in a matter of minutes, and its portability makes it well suited as a screening assay for use in the field. We conclude that the DNA-dye test is a surrogate bioassay suitable for screening chemical toxicity.

NMR spectroscopic study of a DNA duplex with mercury-mediated T-T base pairs

Recently, we reported that T-T mismatches can specifically recognize Hg(II) (T-Hg(II)-T pair formation). In order to understand the properties of the T-Hg(II)-T pair, we recorded NMR spectra for a DNA duplex, d(CGCGTTGTCC).d(GGACTTCGCG), with two successive T-T mismatches (Hg (II)-binding sites). We assigned 1H resonances for mercury-free and di-mercurated duplexes, and performed titration experiments with Hg(II) by using 1D 1H NMR spectra. Because of the above mentioned assignments, we could confirm the existence of mono-mercurated species, because individual components gave independent NMR signals in the titration spectra.

Probing Mercury Species−DNA Interactions by Capillary Electrophoresis with On-Line Electrothermal Atomic Absorption Spectrometric Detection

The interactions of inorganic mercury Hg(II), methylmercury (MeHg(I)), ethylmercury (EtHg(I)), and phenylmercury (PhHg(I)) with DNA have been probed by capillary electrophoresis with on-line electrothermal atomic absorption spectrometric detection (CE-ETAAS) in combination with circular dichroism and Fourier transform infrared spectroscopy. The CE-ETAAS assay allows sensitive probing of the level of DNA damage by mercury species, extraction of thermodynamic and kinetic information on the interactions of mercury species with DNA, and provides direct evidence for the formation of mercury species-DNA adducts. The binding affinity of mercury species to DNA increases in order of Hg(II) < EtHg(I) approximately PhHg(I) approximately MeHg(I). The interactions of mercury species with DNA follow a first-order kinetics for mercury species and zero-order kinetics for DNA. Mercury highly covalently coordinates to endocyclic and exocyclic N sites of DNA bases. However, the interactions of DNA with mercuric species cause no transition of the DNA original conformation. The results reveal that organomercuric species exhibit stronger affinity and faster binding to DNA and show more potential damage to DNA than Hg(II) in view of the kinetic and thermodynamic evaluations. Moreover, MeHg(I) exhibits the fastest binding to DNA, suggesting that MeHg(I) enjoys superiority over the other mercuric species for rapid formation of a stable complex with DNA, whereas Hg(II) shows the slowest binding to DNA. The present study provides new evidence and understanding of the binding modality of mercuric species to DNA.

MercuryII-Mediated Formation of Thymine−HgII−Thymine Base Pairs in DNA Duplexes

The very specific binding of the HgII ion unexpectedly and significantly stabilizes naturally occurring thymine-thymine base mispairing in DNA duplexes. Following this finding, we prepared DNA duplexes containing metal-mediated base pairs at the desired sites, as well as novel double helical architectures consisting only of thymine-HgII-thymine pairs.

Enhanced conformational changes in DNA in the presence of mercury(II), cadmium(II) and lead(II) porphyrins

The interactions of the metalloporphyrins of tetrakis (1-methylpyridinium-4yl)porphyrin ([M(TMPyP)](4+)) where M=Hg(II), Cd(II) and Pb(II)) with pBluescript II plasmid DNA have been studied by the measurement of circular dichroism (CD), UV-visible and fluorescence spectra at 0.1 M NaNO(3), pH 7.5 and 25 degrees C. The CD spectra of the DNA changed quite significantly, with the conformational changes in the presence of the metalloporphyrins being much more enhanced compared to that of their free metal ion counterparts. The conformational changes in DNA upon binding to the Hg(II) porphyrin and Hg(II) were, however, different from those of the Cd(II) porphyrin, Pb(II) porphyrin, Pb(II), Cd(II) and H(2)(TMPyP)(4+). In the concentration range of 0-2.30 x 10(-5) M of DNA, the absorption spectra of H(2)(TMPyP)(4+) showed substantial hypochromicity at 423 nm and a red shift of Deltalambda=16 nm in the presence of DNA whereas the Hg(II)-, Pb(II)- and Cd(II) porphyrins showed blue shifts of absorption maximum wavelengths of Deltalambda=-17 nm, Deltalambda=-35 nm and Deltalambda=-4.5 nm, respectively. Furthermore, the shifted absorption maximum wavelengths/nm of the porphyrins in excess amount of DNA were comparable; 438, 439, 440 and 440 for H(2)(TMPyP)(4+), Hg(II)-, Pb(II)- and Cd(II) porphyrins, respectively. The changes in absorption spectra for Hg(II)-, Pb(II)- and Cd(II) porphyrins revealed that these metalloporphyrins dissociated upon binding to DNA which was confirmed by CD as well as fluorescence spectra. The CD results, UV-Vis and fluorescence data indicate that the metalloporphyrins interact differently with DNA based on their binding modes. And the enhanced changes in conformation of DNA in the presence of the metalloporphyrins are due to the synergistic effects of the simultaneous binding of the metal ions and the free base porphyrin to DNA compared to their free metal ion counterparts: [M(TMPyP)](4+)+DNA+2H(+) right harpoon over left harpoon [M(II)(DNA)H(2)(TMPyP)(4+)]. The detailed equilibrium reactions have been described along with suggestions of possible applications in the medical and biological fields.

Unusually wide co-factor tolerance in a metalloenzyme; divalent metal ions modulate endo-exonuclease activity in T5 exonuclease

T5 5'-3' exonuclease is a member of a homologous group of 5' nucleases which require divalent metal co-factors. Structural and biochemical studies suggest that single-stranded DNA substrates thread through a helical arch or hole in the protein, thus bringing the phosphodiester backbone into close proximity with the active site metal co-factors. In addition to the expected use of Mg(2+), Mn(2+) and Co(2+) as co-factors, we found that divalent zinc, iron, nickel and copper ions also supported catalysis. Such a range of co-factor utilisation is unusual in a single enzyme. Some co-factors such as Mn(2+) stimulated the cleavage of double-stranded closed-circular plasmid DNA. Such endonucleolytic cleavage of circular double-stranded DNA cannot be readily explained by the threading model proposed for the cleavage of substrates with free 5'-ends as the hole observed in the crystal structure of T5 exonuclease is too small to permit the passage of double-stranded DNA. We suggest that such a substrate may gain access to the active site of the enzyme by a process which does not involve threading.

Long-range hole transport in DNA

Abstract A guanine radical cation (G(+•)) was site-selectively generated in double-stranded DNA and the hole transport from G(+•) to a GGG unit in G(+•)(TTG)(N)GG sites (N=1-4) was analyzed. The overall rate of the charge transfer exhibits a weak (algebraic) distance dependence, i. e. k ∝ N (η) with η = 1.7±0.2. This result supports that long-range hole migration in mixed DNA strands is a multistep hopping process between G bases.

M-DNA: A complex between divalent metal ions and DNA which behaves as a molecular wire

M-DNA is a complex of DNA with divalent metal ions (Zn(2+), Co(2+), or Ni(2+)) which forms at pH conditions above 8. Upon addition of these metal ions to B-DNA at pH 8.5, the pH decreases such that one proton is released per base-pair per metal ion. Together with previous NMR data, this result demonstrated that the imino proton in each base-pair of the duplex was substituted by a metal ion and that M-DNA might possess unusual conductive properties. Duplexes of 20 base-pairs were constructed with fluorescein (donor) at one end and rhodamine (acceptor) at the other. Upon formation of M-DNA (with Zn(2+)) the fluorescence of the donor was 95 % quenched. Fluorescence lifetime measurements showed the presence of a very fast component in the decay kinetics with tau</=10 ps. The fast component was absent in B-DNA and in M-DNA lacking an acceptor chromophore; a result which is only consistent with electron transfer. Efficient signal transduction was also observed between the two fluorophores separated by 54 base-pairs (over 150 A) in an M-DNA duplex. The addition of a sequence-specific DNA-binding protein prevented the flow of electrons and this was reversed by protease digestion. Therefore, M-DNA behaves as a molecular wire and could be manipulated to prepare self-assembling electronic circuits.

Site-specific targeting of aflatoxin adduction directed by triple helix formation in the major groove of oligodeoxyribonucleotides

The targeted adduction of aflatoxin B1- exo -8,9-epoxide (AFB1- exo -8,9-epoxide) to a specific guanine within an oligodeoxyribonucleotide containing multiple guanines was achieved using a DNA triplex to control sequence selectivity. The oligodeoxyribonucleotide d(AGAGAAGATTTTCTTCTCTTTTTTTTCTCTT), designated '3G', spontaneously formed a triplex in which nucleotides C27*G2*C18 and C29*G4*C16 formed base triplets, and nucleotides G7*C13formed a Watson-Crick base pair. The oligodeoxyribonucleotide d(AAGAAATTTTTTCTTTTTTTTTTCTT), designated '1G', also formed a triplex in which nucleotides C24*G3*C24 formed a triplet. Reaction of the two oligodeoxyribonucleotides with AFB1-exo-8,9-epoxide revealed that only the 3G sequence formed an adduct, as determined by UV absorbance and piperidine cleavage of the 5'-labeled adduct, followed by denaturing polyacrylamide gel electrophoresis. This site was identified as G7by comparison to the guanine-specific cleavage pattern. The chemistry was extended to a series of nicked bimolecular triple helices, constructed from d(AAAGGGGGAA) and d(CnTTCTTTTTCCCCCTTTATTTTTTC5-n) (n = 1-5). Each oligomer in the series differed only in the placement of the nick. Reaction of the nicked triplexes with AFB1- exo -8,9-epoxide, piperidine cleavage of the 5'-labeled adduct, followed by denaturing polyacrylamide gel electrophoresis, revealed cleavage corresponding to the guanine closest to the pyrimidine strand nick. By using the appropriate pyrimidine sequence the lesion was positioned within the purine strand.

Mercury(II) Site-Selective Binding to a DNA Hairpin. Relationship of Sequence-Dependent Intra- and Interstrand Cross-Linking to the Hairpin-Duplex Conformational Transition

Hg(II) interacted site selectively with only one of three deoxyribooligonucleotides examined; these "oligos" each had a different number of unmatched T residues. Thus, Hg(II) formed an intrastrand T-Hg-T cross-link between the first and fourth T residues of the hairpin, d(GCGCTTTTGCGC) (T4). The DNA strand formed a loop around the Hg, as if the Hg atom had been lassoed. The interactions of Hg(II) with two other oligos, d(ATGGGTTCCCAT) (T2) and d(GCGCTTTGCGC) (T3), were less specific. Previously, we found that at high DNA and salt concentrations, T2 was a mixture of hairpin and duplex forms while T3 and T4 had the hairpin form; modeling studies showed that in the free T4 hairpin the two T's at the ends of the (T)(4) loop form a T.T wobble base pair. Only in T4 are the T residues positioned to form an intrastrand cross-link readily. The Hg(II)-oligo adducts formed as a function of added Hg(II) were investigated by titrations monitored by UV, CD, and (1)H NMR spectroscopy. The appearance of a new set of (1)H signals with the concomitant decay of the free oligo (1)H signals indicated that 1:1 Hg(II):T2, 1.5:1 Hg(II):T3, and 1:1 Hg(II):T4 adducts were formed with Hg(NO(3))(2). In H(2)O, these adducts all had spectra with very downfield signals for the exchangeable TN(3)H and GN(1)H groups, a characteristic of base-paired regions. All upfield N(3)H signals from the (T)(2) and (T)(3) sequences of the free oligo disappeared in the spectra of the 1:1 Hg(II):T2 and 1.5:1 Hg(II):T3 adducts. The disappearance of the NH signals, the UV spectral changes, and the stoichiometries (1:1 Hg(II):T2 and 1.5:1 Hg(II):T3) indicate that these adducts are duplexes containing two and three T-Hg-T interstrand cross-links for T2 and T3, respectively. The (1)H and (13)C signals of the 1:1 Hg(II):T4 adduct in D(2)O were nearly completely assigned by 2D NMR spectroscopy. The spectrum of the adduct in H(2)O had only two of the four original TN(3)H signals from the (T)(4) sequence present in the spectrum of T4; this result is consistent with the presence of a TN3-Hg-TN3 cross-link. The (13)C chemical shift changes upon Hg(II) binding indicated that the TN3-Hg-TN3 cross-link was between the T's at each end of the (T)(4) loop. The NOESY, CD, and UV spectra were all consistent with a hairpin conformation for the 1:1 Hg(II):T4 adduct. A hairpin conformation also appeared reasonable from molecular modeling calculations. In conclusion, the length of the central (T)(n)() sequence influenced the type of T-Hg-T cross-link formed and, in turn, the conformation of the adducts. For (T)(2) and (T)(3), interstrand T-Hg-T cross-linking favored the duplex form. In contrast, for (T)(4), intrastrand T-Hg-T cross-linking stabilized the hairpin form.

An A-form of poly[d(A-C)].poly[d(G-T)] induced by mercury (II) as studied by UV and FTIR spectroscopies

The conformational changes of poly[d(A-C)].poly[d(G-T)] induced by Hg(ClO4)2 in aqueous solution have been studied using UV absorption and fourth derivative spectrophotometries, and FTIR spectroscopy. The UV absorption and fourth derivative spectra reflect changes in the polynucleotide stacking interactions as a result of the metal-polynucleotide interaction. The fourth derivative spectra do not indicate a Z-form either at low or at high metal-to-polynucleotide ratios. Furthermore, the infrared spectrum at high metal-to-polynucleotide ratio (r = 1.2; r = [Hg(ClO4])2/[nucleotide] molar ratio) has the main features of an A-form, in contrast with previous CD studies which proposed that the polynucleotide adopts a Z-form under these conditions. The nature of a different conformation of the polynucleotide induced at low r-ratios (r < or = 0.2) is discussed.

Effect of Hg(II) on the spectroscopic properties of poly[d(A-T).d(A-T] and poly[d(A).d(T)] and their constituent subunits (deoxyadenosine and thymidine monomers and dimers)

Adding, respectively, increasing amounts of Hg(ClO4)2 (identical to Hg(II)) to poly[d(A-T).d(A-T)] (I), poly[d(A).d(T)] (II) as well as to their constituent subunits 2'-deoxyadenosine (identical to dA), thymidine (identical to dT), 2'-deoxyadenosine-5'- monophosphate (identical to dAp), thymidine-5'-monophosphate (identical to dTp), 2'-deoxyadenylyl-(3'-->5')-2'-deoxyadenosine (identical to d(ApA)), 2'-deoxyadenylyl-(3'-->5')-thymidine (identical to d(ApT)), thymidylyl- (3'-->5')-2'-deoxyadenosine (identical to d(TpA)), and thymidylyl- (3'-->5')-thymidine (identical to dTpT))--all dissolved in 0.1 M NaClO4, 5 mM cacodylic acid, pH 7--generates changes in their UV spectra that (a) progress in (I) in a pattern that is distinctly different from the one occurring in (II) and that (b) reveal a strong sequence dependence in the case of the dinucleoside phosphates. The spectroscopic parameters D (dipole strength), f (oscillator strength), and h (hypochromicity) were determined as a function of Hg(II) concentration for both polymers as well as for all dimers. Also determined were D and f of the monomers. D and f of dA, dAp, and d(ApA) display a different dependence on Hg(II) concentration than do D and f of dT, dTp, and d(TpT). The corresponding parameters of the mixed-sequence dimers d(ApT) and d(TpA) vary with Hg(II) in a 'mirror'-like fashion. Increase in base stacking subsequent to mercury binding is noted with d(TpT) and d(TpA). The opposite occurs in d(ApT). Hg(II) exerts only marginal effects on the base stacking in d(ApA). Both D and f of polymers (I) and (II) increase with increasing levels of Hg(II), i.e. Hg(II) binding decreases base stacking (loss of hypochromicity).(ABSTRACT TRUNCATED AT 250 WORDS)

The mercury(II) and high-salt-induced conformational B<==>Z transitions of poly[d(G-m5C).d(G-m5C)] as studied by non-polarized (ultraviolet) and circularly polarized (CD) ultraviolet spectroscopy

The B<==>Z transition of poly[d(G-m5C).d(G-m5C)] in buffered solution (0.002 M sodium cacodylate, pH 7) was studied by CD and ultraviolet spectroscopy as a function of the supporting electrolyte concentration (0.002-1.1 M NaClO4) in the absence of Hg(ClO4)2 [Hg(II)], and as a function of the Hg(II) concentration at a given NaClO4 level. NaClO4 alone changes the conformation of the polymer from B<==>Z at approximately 0.7 M NaClO4. The spectral changes caused by Hg(II) in the B-form polymer (e.g. at 0.002 M < or = [Na] < or = 0.7) resemble those generated by salt alone during the B<==>Z transition; the changes generated by Hg(II) in the Z-form polymer (e.g. in 1.1 M [Na]) leave principally intact the Z-form spectrum obtained at the higher levels of NaClO4 (e.g. at [Na] > 0.7 M) in the absence of Hg(II). It is concluded that particularly the long-wavelength positive-CD band, located at 274 nm, is a correct indicator of duplex DNA right<==>left-screwness inversion. According to generally accepted criteria, the NaClO4-induced left-handed form is Z DNA; Hg(II) generates a left-handed form termed here ZHg(II). This form is close to (but not identical with) the salt-induced Z-form. All Hg(II)-induced spectral changes are reversible upon removal of Hg(II) with a suitable complexing reagent (e.g. NaCN).

Interaction of inorganic mercury salts with model and red cell membranes: importance of lipid binding sites

The effect of two mercury salts, HgCl2 and Hg(NO3)2, on the thermotropic properties of phosphatidylserine (PS) model membranes and sonicated rat red cell membranes was investigated by fluorescence polarization. Both Hg(II) salts abolished the phase transition and decreased the membrane fluidity by interacting with PS. Maximal effect was observed at a Hg/PS ratio of 2.5-5 for mercuric chloride and at 1.5 for the nitrate salt. For both mercury compounds, 10 mM NaCl protected model membranes from the effects of Hg(II). HgCl2 and Hg(NO3)2 also decreased the fluidity of rat red cell membranes. Maximal effect was observed for 0.4 mM HgCl2 and 0.6 mM Hg(NO3)2, with 0.0125 mg protein/ml. Addition of NaCl to the Hg(II)-red cell system decreased the Hg(II)-induced perturbation of the thermotropic properties. For both membrane systems, the effects observed with Hg(NO3)2 were greater than those with HgCl2, which can be accounted for by the absence of competition with chloride ions in samples containing Hg(NO3)2.[Cl-] governs the availability of Hg(II) by determining its chemical speciation: increasing [Cl-] generates HgCl3- and HgCl4(2-), which do not interact with lipid binding sites. These results indicate that besides protein thiol groups, Hg(II)-lipid binding sites play an important role in the interaction of Hg(II) with red cell membranes that is qualitatively different from Hg(II) binding to protein thiol groups.

The effect of mercuric ions on the excited states of DNA-intercalated ethidium bromide

The first excited triplet state of DNA-intercalated ethidium bromide is produced with a quantum yield of 0.01 +/- 0.002 on irradiation at 532 nm. A difference extinction coefficient of 1.5 +/- 0.2 x 10(3) m2 mol-1 is measured for the triplet state at 380 nm. Mercuric ions quench the first excited singlet state of DNA-intercalated ethidium bromide via induced spin orbit coupling to give an increased yield of ethidium triplet states. The same mercuric ion that quenches the singlet state then quenches the triplet state, via the same mechanism, with a rate constant of ca 3.5 x 10(3) s-1. An upper limit for the rate of detachment of Hg2+ from its binding site in DNA may be fixed at ca 10(3) s-1.

Changes in poly[d(T-G).d(C-A)] chirality due to Hg(II)-binding: circular dichroism (CD) studies

The conformation of poly[d(T-G).d(C-A)] in aqueous solution (0.1 M NaClO4, 5 mM cacodylic acid buffer, pH 6.9) was studied by circular dichroism (CD) spectroscopy in the ultraviolet. The conformation of the polynucleotide, as reflected by its chiroptical signature, changes in a highly cooperative fashion in the presence of Hg(ClO4)2. The CD changes signal transitions first from the B to a modified B-state (B*), or to a non-B structure termed X, and finally to a form that is presumably Z. The alterations are totally reversible subsequent to the removal of mercury with the help of a suitable complexing agent such as sodium cyanide, indicating that mercuration does not disrupt Watson-Crick hydrogen bonding to any extent.