Jlilirobioloyie und experimentelle Therapie der Deutschen Akademie der Synopsis Spectrophotometric, sedimentation, infrared, optical rotatory dispersion (Oltl)), and circxilar dichroisni (CI)) methods have beeti used to demonstrate the structural changes in DNA induced by the iiiteractioii of copper(I1) with bases and to elucidate the complex binding sites. As showii by the electrolyt,e-induced reversion (addit ion of salts) of temperat~ire-deriat~ired cwpper I ) S A the effectiveness of re-formation of the doublestranded structure depends o i l the temperature, copper(I1) ioii cwiiceiitration, and on the base compositioii of the I ) S A . ICxposure of heat-deliat tired c:.)pper I)NA to higher temperatures decreases the reversioii effert on addition of e1ec.t rolyte. The reilllts indicate that a greater frartioii with a c*ooperative transitioii appwrs o i i heating I>NA to 80 or 100°C at a Cii2+/l>?;A-P ratio of 2: 1 than at a Cii2+~I)XA-1' ratio of 1 : 1 . With AT-rich copper IIXA, reveixioii t o the iiaiive I ) S A striictiire wa-; i i o t observed. Selective methylatiori of griariiiie residiies i n I)XA also afl'ects the electrol?te-itiduced reversion, itidicatiiig the importance of GC pairs for copper(I1) binding aiid the reversion to the native structure. Temperature-deriat Ilred copper 1)XA shows an increased sedimentation coefficient which decreases again after electrolyte-iiidl1c.e 1 reversion. This change in s is reduced by selective methylatioii ( J f IINA. Chrnplex formation bet ween c:opper(II) and the bases is accompaiiied by a coiiformatioiial change of the I)NA double-helical striicliire as demonstrated by O I t I ) atid CI> experiments. The O l t l ) profile of GC-rich I)NA is murh more affected by copper(I1) than that of AT-rich ones. I*;ven at very low copper(I1) coricentr:ttio~rs, e.g., at O.W aiid 0.2 Ca2+/1)NA-P, the 0111) and CU measuremeiits exhibit cotiformat ioiial changes of the DNA secondary st rrictiire at room temperattire. By comparing the itifrared spectra of deoxytiiideosides with that. of I)NA of dift'ererit (;C content it has been shown t,hat both guanine and cyt.osirie are involved i n the formation of the complex of copper(I1) with IINA. N-7 and 0 at C-6 in guanine aiid N-3 as well as 0 of C-2 in cytosine are discussed as the most probable binding sites in I)NA. A binding model for the c:oorditiation of the copper(I1) ion between giiariine arid cytosine of the opposite >traiids is sriggested. The results are in good agreement with the assumptioris :uid predictions made by Kichhorn and Clark ahout the complexirig of copper(I1) with IINA. The recent proposal made by Schreiber aiid I):iriiie :tholit :tii ititerwtioii of the type g~i : i i i i t i e -~~i 2 + -g~~: i i i i i i e canriot. he exc*ltided :is an :idditioii:il kiiid of c~ooi~diii:itioii o f (*oppw(Il) i t 1 1)X.k.Various divaleiit metal ions are kno\vii to interact nit11 riucleic acids.L-LO Iri some investigations the results of metal determination in carefully isolated D S A preparations iiidicat e that certain metal io...

The binding of the antibiotics netropsin and distamycin A to DNA has been studied by thermal melting, CD and sedimentation analysis. Netropsin binds strongly at antibiotic/nucleotide ratios up to at least 0.05. CD spectra obtained using DNA model polymers reveal that netropsin binds tightly to poly (dA) . poly (dT), poly (dA-dT) . poly(dA-dT) and poly (dI-dC) . poly (dI-dC) but poorly, if at all, to poly (dG) . poly (dC). Binding curves obtained with calf thymus DNA reveal one netropsin-binding site per 6.0 nucleotides (K(a)=2.9 . 10(5) M(-1)); corresponding values for distamycin A are one site per 6.1 nucleotides with K(a)= 11.6 . 10(5) M(-1). Binding sites apparently involve predominantly A.T-rich sequences whose specific conformation determines their high affinity for the two antibiotics. It is suggested that the binding is stabilized primarily by hydrogen bonding and electrostatic interactions probably in the narrow groove of the DNA helix, but without intercalation. Any local structural deformation of the helix does not involve unwinding greater than approximately 3 degrees per bound antibiotic molecule.

No abstract

The specific interaction of distamycin A and analogs with DNA's and synthetic deoxypolynucleotide duplexes were studied in detail by means of circular dichroism and the data were analyzed together with viscosity results of several natural DNA's. At low ligand to nucleotide ratio the previously reported specific binding to (A-T) pairs of DNA is verified by a highly favoured interaction with (A-T)-enriched segments of distamycins containing four and five methylpyrrole carboxamide units. At higher distamycin concentration a second specific binding to (G-C) pairs most probably through hydrogen bonding is established. Viscometric results suggest a distamycin-induced local bending of the helix and could support the idea of a preferential alignment of the ligand molecule along only one strand in the groove which differs from the netropsin interaction mechanism. The possibility of an overlapping binding of the oligopeptides in the small groove is discussed.

Experiments were made to demonstrate the predominant protonation effects and structural changes of the ordered double helical DNA structure and denatured state of DNA. Spectrophotometric titrations performed at different wavelengths indicate that cytosine can be protonated in the DNA double helical molecule to a high extent without breakdown of the secondary structure. With DNA heat-denatured under severe conditions the protonation of cytosine can be measured at 280, 295, and 300 mμ: the apparent pK value obtained was ∼4.6. The protonated double helical conformation of the DNA molecule differs from the unprotonated state, which follows from the decrease of the thermal stability and from changes in the ORD curves. The ORD of a GC-rich DNA indicates a novel Cotton effect with positive rotations at ∼260 mμ in 0.02M KCl below pH 4.0 to pH 3.3. The occurrence of the new peak parallels the extent of protonated cytosine measured by the spectrophotometric titrations. It is concluded that the protonated cytosine in the double helical structure is responsible for the difference between the protonated DNA conformation and the native state at neutral pH.