Fluorescence quenching and spin-label electron spin resonance studies of stacking self-association in aqueous solutions of 2-aminopurine riboside and its 5'-mono- and -diphosphate

The autoassociation of 2-aminopurine riboside (rn²Pur) and its 5'-mono-(P-rn²Pur) and 5'-diphosphate (PP-rn²Pur) in neutral aqueous solutions was investigated using fluorescence quenching and ESR spin-label methods within the range 276–358 K. Respective equilibrium constants and thermodynamic functions were derived therefrom assuming two models of infinite autoassociation: (i) an isodesmic one (K₂ =K₃ = · K_p). and (ii) one in which K₂≠ K₃ = K₄ = · K_p. Comparative analysis of these data and that of the parent 2-aminopurine, obtained previously, allowed us to formulate the following conclusions: (1) the mechanism of autoassociation of rn²Pur varies with temperature in such a way that at T = 318 K the isodesmic model is fulfilled (K₂ = K_p); at higher temperatures $\text{K}{}_{\mathrm{p}}\text{K}{}_2$> 1. i.e., the process is cooperative, while at lower temperatures it becomes anticooperative (K_p/K₂ < I); (2) at 298 K the tendency to autoassociation decreases in the order; rn²Pur>P-rn²Pur>PP-rn²Pur; (3) rn²Pur forms highly packed complexes with the bases stacked and the ribofuranose residues interacting via hydrogen bonds or water bridges: (4) autoassociation of P-rn²Pur and PP-rn²Pur is mainly governed by stacking of the bases, while the ribose phosphate residues attain a trans configuration corresponding to the lowest electrostatic repulsion between charged phosphate groups: even at high ionic strength (I = 0.8). a positive electrostatic contribution to the free enthalpy of autoassociation is observed: (5) the two methods employed gave similar results for P-rn²Pur, but somewhat different ones for rn²Pur because the presence of the spin label (nitroxide stable radical) at the 2'(3')-OH group of the ribose residue prevents its interaction via hydrogen bonding with an unlabelcd one of an adjacent nucleoside.     

NAD+ boosting reduces age-associated amyloidosis and restores mitochondrial homeostasis in muscle

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Solution conformation and crystal structure of methyl 3,6-dideoxy-β-d-ribohexopyranoside,an immunodominant sugar of O-antigens

The crystal structure of methyl 3,6-dideoxy-β-d-ribohexopyranoside monohydrate was determined by direct methods. Crystals are monoclinic, space group P2₁, with cell dimensions $\text{a = 9.089(1), b = 7.668(1), c = 6.956(1)}\text{A}\text{?}\text{, β = 101.12°}$. The molecule adopts the ⁴C₁ chair conformation. The same conformation was also found in both aqueous and chloroform solutions. The pyranose ring is only slightly distorted, and the consequences of this observation on antigen structure are discussed.     

Isolation and Preliminary Characterization of ATPase from Olive Calli Grown at Different Auxin/Cytokinin Ratio

ATPase activity was studied in calli from olive (Olea europaea L.) petioles cultured in media modified in their auxin/cytokinin ratio in order to induce different morphogenetic responses. Addition of 0.54 μM α-naphthaleneacetic acid (NAA) or 14 μM zeatin (ZEA) did not induce any morphogenesis in calli and proton pump activity in vivo was very low, while calli produced roots at 27 or 11 μM NAA + 0.28 μM ZEA and possessed clearly detectable proton pump activity. ATPase activity associated with microsomes isolated by differential centrifugation from different callus cultures had the same pH optimum and similar sensitivity toward nitrate and azide. However, microsomes isolated from non-morphogenetic calli had higher specific ATPase activity which was very poorly (6 %) inhibited by vanadate. Also, the fractionation of these microsomes on a continuous sucrose gradient showed two peaks of ATPase activity, the more pronounced one co-purifying with the Golg i marker enzyme, Triton-stimulated UDPase activity, suggesting thus the presence of very high ATPase activity in Golgi secretory vesicles. On the contrary, ATPase activity of microsomes from calli producing roots was more sensitive to vanadate (30 - 40 % inhibition). Furthermore, the component of ATPase activity attributable to Golgi secretory vesicles was less abundant. This revised version was published online in July 2006 with corrections to the Cover Date.