Interferon induction by poly(inosinic acid).poly(cytidylic acid) segmented by spin-labels

Poly(inosinic acid).poly(cytidylic acid) [(I)n.(C)n] duplexes of which the (C)n strand was modified to various degrees chemically or enzymatically with nitroxide radicals (spin-labels) were evaluated for interferon-inducing activity. Upon annealing of the chemically modified (C)n, (1C,Cx)n (X = 1000 or 16), with (I)n, the interferon-inducing activity was similar to that of (I)n.(C)n in PRK cell cultures. However, to overcome hydrolysis of the spin-label linkage in (1C,Cx)n, and enzymatic approach was taken to synthesize (1S(4)U,Cx)n copolymers with x = 100, 38, 16, and 8. The (1s(4)U,Cx)n copolymers were chemically stable, and upon annealing with (I)n the correlation time of the nitroxide moiety in (I)n. (1s(4)U,Cx)n was determined. A comparison of this correlation time with that measured for (RUGT,U100)n.(A)n, which contains the nitroxide moiety in position 5 of the U moiety, suggests that the 1s(4)U residue is in a nonintrahelical conformation and partitions the duplex into double-helical segments of varying size. The interferon-inducing activity of (I)n. (1s(4)U,Cx)n was evaluated in primary rabbit kidney, human skin fibroblast (strain VGS), and mouse L-929 cell cultures as well as in rabbits. The 1s(4)U residue did not cause a significant change in the interferon induction as compared to (I)n.(C)n in most systems tested unless x less than 16. These findings indicate that double-helical segments of approximately 16 base pairs partitioned by nonintrahelical 1s(4)U residues suffice to trigger the interferon response in all systems studied.     

Poly(8-aminoguanylic acid): formation of ordered self-structures and interaction with poly(cytidylic acid).

Poly(8-aminoguanylic acid) has in neutral solution a novel ordered structure of high stability. The 8-amino group permits formation of three hydrogen bonds between two residues along the "top", or long axis, of the purines. The usual hydrogen bonding protons and Watson-Crick pairing sites are not involved in the association. The bonding scheme has a twofold rotation axis and is hemiprotonated at N(7). Poly(8NH2G) is converted by alkaline titration (pK = 9.7) to a quite different ordered structure, which is the favored form over the range approximately pH 10-11. The bonding scheme appears to be composed of a planar, tetrameric array of guanine residues, in which the 8-amino group does not participate in interbase hydrogen bonding. Poly (8NH2G) does not interact with poly(C) in neutral solution because of the high stability of the hemiprotonated G-G self-structure. Titration to the alkaline plateau, however, permits ready formation of a two-stranded Watson-Crick helix. In contrast to the monomer 8NH2GMP, poly(8NH2G) does not form a triple helix with poly(C) under any conditions. The properties of the ordered structures are interpreted in terms of a strong tendency of the 8-amino group to form a third interbase hydrogen bond, when this possibility is not prevented by high pH.     

The photochemistry of the paleoatmosphere

Summary The ideas of Harold Urey on the origin and evolution of the atmosphere have dominated thinking in this area for 3 decades. Recent progress in this area is reviewed, with particular emphasis on photochemical modeling studies of atmospheric evolution. Research into the paleoatmosphere can be divided into 3 distinct areas: (1) The photochemistry/chemistry of the prebiological paleoatmosphere, (2) the evolution of oxygen and the transition to an oxidizing atmosphere, and (3) the origin and evolution of ozone. Photochemical calculations indicate that the stability of a heavily reducing paleoatmosphere of CH₄—NH₃ was extremely shortlived, if such a prebiological atmosphere ever existed at all. A more mildly reducing early atmosphere of CO₂—N₂ is favored by photochemical considerations. Recent calculations of O₂ in the prebiological paleoatmosphere vary from less than 10^–14 of present atmospheric level (PAL) to 10^–1 PAL. Clearly, additional work is indicated. The evolution of O₃ as a function of O₂ level has been investigated with increasingly detailed photochemical models that have included the photochemistry/chemistry of the oxygen, hydrogen, nitrogen, carbon, and chlorine species, as well as the effects of eddy transport, the rainout of water-soluble species, dry deposition and lightning as a source of trace atmospheric gases.     

X-ray refinement study on the binding of cytidylic acid (2'-CMP) to ribonuclease A

The X-ray structure of the inhibitor complex of bovine ribonuclease A with cytidylic acid (2'-CMP) has been determined at 2.3 A (1 A = 0.1 nm) resolution and refined by restrained least-squares refinement to R = 0.132 for 5650 reflections. Incorporation of the inhibitor molecule has occurred with little disturbance of the protein main-chain atoms, although significant displacement of some side-chain atoms has occurred, particularly in the region of the active site. The binding of 2'-CMP to ribonuclease A is different from that of the related cytidine-N(3)-oxide 2'-phosphate, which has an extra oxygen on N(3) of the cytidine base. The PO4(2-) group is held by hydrogen bond interactions to the side-groups of His 12, Glu 11 and His119. Thr45 is involved in stabilizing the enzyme-ligand complex by forming hydrogen bond interactions between O(gamma) and the pyrimidine base N(3) atom and between the main-chain N(45) and O(2) of the base. Phe120 is much closer to the inhibitor than in the cytidine N(3)-oxide 2'-phosphate structure.     

The photochemistry of thiophene S-oxides.

Thiophene S-oxides have been prepared and their photoreactivity has been studied. In many cases, the sulfur moiety of the thiophene S-oxides is deoxygenated. Here, the corresponding thiophenes or hydroxylated thiophenes are isolated. In the case of the photoirradiation of tert-butyl substituted thiophene S-oxides, the oxygen of the sulfoxy unit is incorporated into the heterocyclic ring system and the corresponding furans have been isolated. Also, structures with two thiophene cores have been oxidized to the respective thiophene S-oxides. These molecules undergo photodeoxygenation, a reaction, which is catalysed by (non-oxidized) thiophenes.     

The purification and properties of chicken liver RNase: An enzyme which is useful in distinguishing between cytidylic and uridylic acid residues

A heat-stable endoribonuclease isolated from chicken liver has been purified to homogeneity as evidenced by the presence of a single protein band upon polyacrylamide gel electrophoresis. The enzyme can, in limit digests of 5 S rRNA and 5.8 S rRNA, dinstinguish between cytidylic and uridylic acids bonds at a ratio of 61:1 and, therefore, may be useful in RNA sequence analysis. The means by which the enzyme hydrolyzes substrate is unusual in that kinetic data do not support a simple formation and breakdown of an enzyme . substrate complex. Rather, the existence of a second complex, consisting of 2 mol of substrate and one of enzyme, derived from the initial enzyme . substrate complex, is postulated. In common with the other endonucleases, enzyme activity is inhibited by free poly(A) or tracts of the polypurine present at the 3'-terminus of RNA. Reversal of inhibition and restoration of activity may be achieved by the addition of low concentrations of spermidine to reaction mixtures.