Hexamminecobalt(III)-induced condensation of calf thymus DNA: circular dichroism and hydration measurements

Abstract

The interaction of hexamminecobalt(III), Co(NH(3))(6)(3+), with 160 and 3000-8000 bp length calf thymus DNA has been investigated by circular dichroism, acoustic and densimetric techniques. The acoustic titration curves of 160 bp DNA revealed three stages of interaction: (i) Co(NH(3))(6)(3+) binding up to the molar ratio [Co(NH(3))(6)(3+)]/[P] = 0.25, prior to DNA condensation; (ii) a condensation process between [Co(NH(3))(6)(3+)]/[P] = 0.25 and 0.30; and (iii) precipitation after [Co(NH(3))(6)(3+)]/[P] = 0.3. In the case of 3000-8000 bp DNA only two processes were observed: (i) binding up to [Co(NH(3))(6)(3+)]/[P] = 0.3; and (ii) precipitation after this point. In agreement with earlier observations, long DNA aggregates without changes in its B-form circular dichroism spectrum, while short DNA demonstrates a positive B-->Psi transition after [Co(NH(3))(6)(3+)]/[P] = 0.25. From ultrasonic and densimetric measurements the effects of Co(NH(3))(6)(3+) binding on volume and compressibility have been obtained. The binding of Co(NH(3))(6)(3+) to both short and long DNA is characterized by similar changes in volume and compressibility calculated per mole Co(NH(3))(6)(3+): DeltaV = 9 cm(3) mol(-1) and Deltakappa = 33 x 10(-4) cm(3) mol(-1) bar(-1). The positive sign of the parameters indicates dehydration, i.e. water release from Co(NH(3))(6)(3+) and the atomic groups of DNA. This extent of water displacement would be consistent with the formation of two direct, hydrogen bonded contacts between the cation and the phosphates of DNA.

Synthesis, Characterization, Cytotoxic Activities, and DNA-Binding Studies of Ternary Copper(II) Complexes with New Coumarin Derivatives

“…43,44) It is similar to that observed for [Ru(NH 3 ) 4 (qdppz)] 2ϩ and [Co(NH 3 ) 6 ] 3ϩ bound to DNA of short lengths with 160 base pairs. 45,46) The results obtained here validate those obtained from the UV-Vis spectral studies.…”

“…43,44) It is similar to that observed for [Ru(NH 3 ) 4 (qdppz)] 2ϩ and [Co(NH 3 ) 6 ] 3ϩ bound to DNA of short lengths with 160 base pairs. 45,46) The results obtained here validate those obtained from the UV-Vis spectral studies.Viscosity Studies Hydrodynamic measurements that are sensitive to the length change (i.e., viscosity and sedimentation) are regarded as the least ambiguous and the most critical tests of a binding model in solution in the absence of crystallographic structural data. …”

Structural Analysis of DNA Interactions with Biogenic Polyamines and Cobalt(III)hexamine Studied by Fourier Transform Infrared and Capillary Electrophoresis

“…Indeed, pure DNA when hydrated at 75% relative humidity exhibits the same behavior with a major band at 860 cm Ϫ1 and a weak band at 836 cm Ϫ1 (74). From ultrasonic and densimetric measurements, it has been found that cobalt(III)hexamine reduce the hydration of the atomic group of DNA (79).…”

Ultrasonic and Densimetric Titration Applied for Acid-base Reactions

“…Their deprotonation constants are: K1 = 10 -2.35 and K2 = 10 -9.87 (glycine hydrochloride), K1 = 10 -4. 16 and K2 = 10 -5.61 (succinic acid). 19 Note that a relatively low variability occurred even at high initial concentrations of the reagents.…”

“…Acoustic titration-like experiments were also made in order to characterize the acid-induced denaturation of staphylococcal nuclease at different urea concentrations, 13 to study the binding of the cationic surfactant dodecyltrimethylammonium bromide to short fragments of DNA in dilute solutions at 1 mM NaCl, 14 to analyze the binding of alamethicin to unilamellar DMPC vesicles, 15 and also to study interactions of Co(NH3)6 3+ with calf thymus DNA 16 and Cs + and Mg 2+ with polynucleotides. 17 All of the above listed reports encouraged us to test if sound speed or density measurements can be applied for monitoring simple reactions, like acid-base titration, and how to describe the observed changes.…”