A "Double-Diamond Superlattice" Built Up of Cd17S4(SCH2CH2OH)26 Clusters.

A simple preparation of Cd(17)S(4)(SCH(2)CH(2)OH)(26) clusters in aqueous solution leads to the formation of colorless blocky crystals. X-ray structure determinations revealed a superlattice framework built up of covalently linked clusters. This superlattice is best described as two enlarged and interlaced diamond or zinc blende lattices. Because both the superlattice and the clusters display the same structural features, the crystal structure resembles the self-similarities known from fractal geometry. The optical spectrum of the cluster solution displays a sharp transition around 290 nanometers with a large absorption coefficient ( approximately 84,000 per molar per centimeter).

High energy radiation effects in single histones. I. Preparation of histones and irradiation of histone H2B.

Histone H2B from calf thymus was irradiated with 50 or 100 ns pulses of 16 MeV electrons in N2O-saturated aqueous solution at pH 9 in the presence of NaN3. All tyrosine moieties in the histone were found to be freely accessible to the attack of .N3 radicals (formed by the reaction .OH + N3(-)----OH- + .N3). At sufficiently high concentrations of H2B, tyrosyl radicals were formed with G(TyrO.) = 5.4/100 eV and dityrosine groups with G(dityr) = 1.6/100 eV, indicating that about 60 per cent of tyrosyl radicals formed bisphenolic products. There is no polymer effect with respect to G(dityr) as inferred from comparison with other authors' data obtained with low molecular weight compounds. Kinetic measurements revealed that tyrosyl radicals reacted in two modes, a fast one with a value of tau 1/2 of about several milliseconds and a slow second order process also in the millisecond range. The fast process is assigned to intramolecular reactions of tyrosyl radicals generated in close proximity to each other and the slow process to intermolecular self reactions of isolated tyrosyl radicals distributed statistically in the solution. There is a polymer effect with respect to the rate constant of the slow process: 2k8 = 4.8 X 10(7) dm3 mol-1 s-1 (H2B) and 2k8 = 4 X 10(8) dm3 mol-1 s-1 (Lys-Tyr-Lys, Prütz et al. (1983)). The five histones contained in calf thymus were isolated chromatographically with the aid of two gels, Bio-Gel P-60 (BioRad) and Sephadex G100 (Pharmacia).

Radiation-induced strand breakage in poly(riboadenylic acid). A pulse radiolysis study on the protective action of cysteamine under anoxic conditions.

Poly(riboadenylic acid) was irradiated in N2O-saturated dilute aqueous solution at pH 7.8 with single pulses (50-200 ns) of 16 MeV electrons. With the aid of Rayleigh light scattering measurements both the extent and rate of main-chain scission were measured in the absence and the presence of cysteamine (RSH). In both cases two modes of light scattering intensity (LSI) decrease were detected. From this finding it was concluded that two chemically different radical sites, giving rise to main-chain breakage, were produced by OH attack on the macromolecules. Cysteamine reduced the extent of LSI decrease and accelerated the rate of the slow mode of LSI decrease significantly. The fast mode of the LSI decrease is due to fragment diffusion and therefore the influence of cysteamine on the rate of decay of the short-lived radical could not be studied. The assessment of the relative importance of .OH scavenging (RSH + .OH----RS. + H2O) and of repair (PA. + RSH----PAH + RS.) with respect to protection yielded the following: in the case of the long-lived radical, repair contributes significantly at low RSH concentration (less than 10(-4) M); in the case of the short-lived radical, OH scavenging dominates and repair becomes noticeable only at [RSH] greater than 10(-4)M. The following data were evaluated in this work: 100 eV-yield of main-chain scission, G(sb) = 1.1; lifetimes of radicals: less than 50 microseconds (short-lived), 0.6 s (long-lived), kPA. + RSH = 3.4 X 10(6)M-1 S-1 (for long-lived radicals).

High energy radiation-induced crosslinking of histone octamer complexes.

Calf thymus histone octamer complexes were irradiated in the native state in N2O-saturated dilute aqueous solution (0.5 g/l, pH 9, [NaClO4] = 1 -4 mol/l) with 50 or 100 ns pulses of 16 MeV electrons or 60Co-gamma-rays. Time resolved light scattering measurements and optical absorption measurements yielded the following: the octamers underwent a volume contraction due to intra-complex-crosslinking induced by the attack of OH-radicals. Crosslinking proceeded to a certain extent via 2,2'-biphenol coupling as inferred from product analyses.