Search

We have investigated in detail radiation and thermal stabilities and transformations of adenosine mono- and triphosphates in liquid and frozen solid aqueous solutions within a wide range of absorbed radiation dose (up to 75 kGy) and temperature (up to 160 degrees C). Dephosphorylation is the main pathway of high temperature hydrolysis of adenine nucleotides. Basic thermodynamic and kinetic parameters of this process have been determined. Radiolysis of investigated compounds at room temperature results in scission of N-glycosidic bond with a radiation yield about of 1 mol/100 eV. Solution freezing significantly enhances radiation stability of nucleotides as well as other biomolecules. This circumstance is essential in the discussion of panspermia concepts.

Crude extracts of E. coli endo I(-) and E. coli endo I(-)uvrA6(-) possess the ability to remove thymine products of the 5,6-dihydroxy-dihydrothymine type from gamma-irradiated or osmium tetroxide-oxidized poly(dA-dT). It is shown that the uvrA-gene product, which is responsible for incision close to photodimers in prereplication ultraviolet repair in E. coli, is not required for, but may aid in, the excision of gamma-ray products of the 5,6-dihydroxy-dihydrothymine type. Ring-damaged thymine products are also removed by E. coli extracts from osmium tetroxide-oxidized poly(dA-dT), which contains only 5,6-dihydroxy-dihydrothymine but no strand breakage, indicating that product excision occurs in the absence of radiation-induced breaks. On the average, 8 to 16 nucleotides are removed from the polymer per ring-damaged thymine residue excised by extracts from both strains and for gamma-irradiated and osmium tetroxide-oxidized polymer.

Pancreatic ribonuclease and chicken lysozyme possess gross similarities that are responsible for a common ability to form enormous light-scattering centers in cooperation with homopolyribonucleotides. The light-scattering power of the mixtures is highest when [homopolymer]/[protein] assumes some critical value that is unique for each homopolymer-protein pair. In some respects the scatterers resemble very large antigen-antibody networks. A criterion is established to ascertain the relative abilities of the homopolymers to form the centers with the two proteins. Both see polyinosinic acid (poly-I) as most and polyadenylic acid (poly-A) as least efficient in this respect.

Ultraviolet photosensitivity of the single-stranded forms of poly(A) and poly(dA) is compared. In contrast to poly(A), poly(dA) is rather sensitive to ultraviolet irradiation. Relatively low doses of ultraviolet light lead to characteristic changes of the ultraviolet and circular dichroism spectra of poly(dA), suggesting formation of a specific photoproduct in poly(dA) that cannot be formed in poly(A). The quantum yields in poly(dA) (varphi = 2.5 x 10(-3)) are about a factor of 10 higher than in poly(A), indicating a high structure specificity. The base-pairing capacity of irradiated probes of poly(dA) to poly(dT) is inhibited. These results demonstrate a relatively high probability of DNA photolesions, that cannot be repaired by simple repair enzyme systems.