Mercury-induced transitions between right-handed and putative left-handed forms of poly[d(A-T).d(A-T)] and poly[d(G-C).d(G-C)]

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.

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.

Mercury-thymine interaction with a chair type G-quadruplex architecture

The T-Hg-T bond was utilized to cross-link two lateral loops of chair-type G-quadruplex DNA (G4-DNA) structures. Two out of five cross-linking geometries are able to increase the melting temperature and simultaneously reduce the polymorphism of the G4-DNA conformations.

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.

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).

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.