Concerted dynamics of metallo-base pairs in an A/B-form helical transition

Metal-mediated base pairs expand the repertoire of nucleic acid structures and dynamics. Here we report solution structures and dynamics of duplex DNA containing two all-natural C-Hg^II-T metallo base pairs separated by six canonical base pairs. NMR experiments reveal a 3:1 ratio of well-resolved structures in dynamic equilibrium. The major species contains two (N3)T-Hg^II-(N3)C base pairs in a predominantly B-form helix. The minor species contains (N3)T-Hg^II-(N4)C base pairs and greater A-form characteristics. Ten-fold different ¹J coupling constants (¹⁵N,¹⁹⁹Hg) are observed for (N3)C-Hg^II (114 Hz) versus (N4)C-Hg^II (1052 Hz) connectivities, reflecting differences in cytosine ionization and metal-bonding strengths. Dynamic interconversion between the two types of C-Hg^II-T base pairs are coupled to a global conformational exchange between the helices. These observations inspired the design of a repetitive DNA sequence capable of undergoing a global B-to-A-form helical transition upon adding Hg^II, demonstrating that C-Hg^II-T has unique switching potential in DNA-based materials and devices.

HgII binds to C-T mismatches with high affinity

Binding reactions of HgII and AgI to pyrimidine-pyrimidine mismatches in duplex DNA were characterized using fluorescent nucleobase analogs, thermal denaturation and 1H NMR. Unlike AgI, HgII exhibited stoichiometric, site-specific binding of C-T mismatches. The on- and off-rates of HgII binding were approximately 10-fold faster to C-T mismatches (kon ≈ 105 M-1 s-1, koff ≈ 10-3 s-1) as compared to T-T mismatches (kon ≈ 104 M-1 s-1, koff ≈ 10-4 s-1), resulting in very similar equilibrium binding affinities for both types of 'all natural' metallo base pairs (Kd ≈ 10-150 nM). These results are in contrast to thermal denaturation analyses, where duplexes containing T-T mismatches exhibited much larger increases in thermal stability upon addition of HgII (ΔTm = 6-19°C), as compared to those containing C-T mismatches (ΔTm = 1-4°C). In addition to revealing the high thermodynamic and kinetic stabilities of C-HgII-T base pairs, our results demonstrate that fluorescent nucleobase analogs enable highly sensitive detection and characterization of metal-mediated base pairs - even in situations where metal binding has little or no impact on the thermal stability of the duplex.

DNA-Targeted Inhibition of MGMT

The cationic porphyrin 5,10,15,20-tetrakis (diisopropyl-guanidine)-21H,23H-porphine (DIGPor) selectively binds to DNA containing O⁶ -methylguanine (O⁶ -MeG) and inhibits the DNA repair enzyme O⁶ -methylguanine-DNA methyltransferase (MGMT). The O⁶ -MeG selectivity and MGMT inhibitory activity of DIGPor were improved by incorporating Zn^II into the porphyrin. The resulting metal complex (Zn-DIGPor) potentiated the activity of the DNA-alkylating drug temozolomide in an MGMT-expressing cell line. To the best of our knowledge, this is the first example of DNA-targeted MGMT inhibition.

DNA Polymerase Inhibition by High Kinetic Stability of T-HgII-T Base Pairs

A fluorescent surrogate of thymidine called DMAT was used for the first fluorescence-based study of HgII binding to discrete T-T sites in duplex DNA. The fluorescent properties of DMAT-A base pairs were highly sensitive to wild-type T-HgII-T base pair formation at an adjacent site, allowing for a determination of the precise thermodynamic and kinetic parameters of these metal binding reactions. T-HgII-T complexes exhibited equilibrium dissociation constants of Kd ? 8–50 nM. These high-affinity binding interactions are characterized by very slow association and dissociation kinetics (kon ? 104– 105 M–1s–1, koff ? 10–4 – 10–3s–1), revealing exceptionally high kinetic stabilities of T-HgII-T base pairs (half-lives = 0.3–1.3 h). Duplex DNA containing DMAT and no T-T mismatch exhibited nonspecific HgII binding affinities of Kd ? 2.0 ?M. The high kinetic stabilities of T-HgII-T resulted in the inhibition of dynamic processes such as DNA strand invasion and strand displacement during enzymatic DNA synthesis, which led to premature chain termination. These results demonstrated that T-HgII-T base pairs are kinetically distinct from T-A base pairs and therefore are likely to disrupt DNA metabolism in vivo.

A fluorescent surrogate of thymidine in duplex DNA

DMAT is a new fluorescent thymidine mimic composed of 2′-deoxyuridine fused to dimethylaniline.

[The hyper-reactive bronchial system. Pathophysiologic aspects and therapeutic consequences]

We talk about a hypersensitive bronchial system, when under certain endogen prepositions or exogen influences, upon which a healthy persons reacts hardly or not at all, it comes to a significant raise of the airway resistances. Probably this is the prestadium of a chronic obstructive airway disease, which often occurs with patients suffering from an existant bronchial disease. Therefore it is important for actual clinical practice to discuss those problems, which interfere with the raise of sensitivity of the bronchial airways. The discussion should be based on the actual state of science, because several therapeutic problems and consequences ensue.

The bronchodilator effect of NAB 365

1. Dose response relationships for salbutamol and a new bronchodilator drug NAB 365 have been obtained in patients with reversible airways obstruction. These show that the latter is about one hundred times more potent than the former. 2. NAB 365 has a very long half-life. The implications of this are discussed.

[Airways obstruction and disodium-cromoglycate--a new therapeutic principle (Clinical aspects) (author's transl)]

A short outline of the causes, pathology and genetic background of airways obstruction is presented in which the special importance of allergic and immune reactions is emphasised. The place of disodium-cromoglycate (DSCG) in treatment is shown to lie in interrupting the chain of events which leads to an asthmatic attack. The main field of application of this drug is in allergic asthma, but it also has a place in the treatment of other forms of labile airways obstruction where an allergic cause in not so obvious. Examples of this are given and the protective effect of DSCG on the irritable bronchial tree is also demonstrated. Dosage and technique of administration are described and it is shown that conventional drug therapy can be reduced when this drug is properly used.