Alkali-catalyzed isomerization of some nitrogen-bridged furanosyl uracils to the corresponding pyranosyl cyclonucleosides

With a view to examine the possibility of introducing an "up" amino residue into the sugar moiety of uracil nucleosides through 2,3'-N-cyclonucleosides, 2,3'-imino and 2,3'-substituted imino-1-(5'-O-benzoyl-beta-D-lyxofuranosyl)uracils, 2a4, 2b5 and 2c-f, were synthesized and debenzoylated correspondingly to 3a4, 3b5 and 3c-f. Hydrolysis of 3a,c,d,e with one to one mixture of 6N-NaOH and EtOH allowed the isolation of the corresponding 2,3'-N-bridged lyxopyranosyl nucleosides 4 in optically active, crystalline form. This is the first example of furanosyl to pyranosyl conversion in the field of pyrimidine cyclonucleosides.     

Cyclization of polyenes: XII. Direct brominative ring closure of polyenes

In connection with a program directed toward the biogenetic-type synthesis of bromine-containing terpenoids, we have developed a new reagent system which provides for selective brominative cyclization of polyenes. The mechanism of Br⁺-formation in nature is discussed on the basis of the new reagent system.     

Oligomerization and ring closure of immunoglobulin E class antibodies by divalent haptens

Cross-linking of antibodies constitutes a widespread initiation signal for their respective effector functions. Cross-linking IgE-class antibodies provide the triggering signal to mast cells for their degranulation process. To obtain a quantitative insight into these cross-linking processes, the interactions between a DNP-specific monoclonal antibody of the IgE class and a series of divalent DNP haptens with spacers of different length and flexibility have been studied by fluorescence titration experiments. These were analyzed by employing the theoretical model developed by Dembo and Goldstein [Dembo, M., & Goldstein, B. (1978) J. Immunol. 121, 345-353] in a fitting procedure. Equilibrium constants that describe the aggregation and ring-closure processes caused by divalent hapten binding have been used as free parameters. The intrinsic binding constants were determined by fluorescence titrations with corresponding monovalent haptens. The main results are the following: (1) The divalent haptens with a short and flexible spacer [i.e., N alpha, N epsilon-di-(DNP)-L-lysine,meso-bis[(DNP-beta-Ala)amino]succinate, and bis[(DNP-tri-D-Ala)amino]heptane, having a maximal DNP-DNP distance of gamma = 14, 21, and 45 A, respectively] effect aggregation of the antibodies mainly into closed dimers. (2) The divalent hapten family with long and rigid oligoproline spacers di(DNP)-Ahx-Asp-(Pro)n-Lys with n = 24, 27, and 33 (i.e., gamma = 100, 110, and 130 A) causes aggregation of the antibodies predominantly into closed dimers and trimers. The corresponding equilibrium constants of the respective ring-closure processes decrease significantly with longer spacer length. (3) Evidence was found that intramolecularly monomeric ring closure of the IgE antibodies is caused by haptens containing oligoproline spacers with n = 37 or 42 (gamma = 130-150 A). The equilibrium constant of the ring-closure process increases with spacer length. This increase in stability indicates a difference in the imposed strain. Furthermore, the latter results imply that the distance between the two binding sites of the IgE molecule lies in the range dictated by the rigid oligoproline part of the respective hapten's spacer, i.e., 115-130 A. (4) Nearly all oligomeric ring-closure processes proceed relatively slowly with an approximate lower limit of a half-life of 5-10 s. This slowing down of the aggregation and ring-closure processes most probably reflects steric factors.     

Structural basis for antibody catalysis of a disfavored ring closure reaction.

The catalysis of disfavored chemical reactions, especially those with no known natural enzyme counterparts, is one of the most promising achievements of catalytic antibody research. Antibodies 5C8, 14B9, 17F6, and 26D9, elicited by two different transition-state analogues, catalyze disfavored endo-tet cyclization reactions of trans-epoxy alcohols, in formal violation of Baldwin's rules for ring closure. Thus far, neither chemical nor enzyme catalysis has been capable of emulating the extraordinary activity and specificity of these antibodies. X-ray structures of two complexes of Fab 5C8 with the original hapten and with an inhibitor have been determined to 2.0 A resolution. The Fab structure has an active site that contains a putative catalytic diad, consisting of AspH95 and HisL89, capable of general acid/base catalysis. The stabilization of a positive charge that develops along the reaction coordinate appears to be an important factor for rate enhancement and for directing the reaction along the otherwise disfavored pathway. Sequence analysis of the four catalytic antibodies, as well as four inactive antibodies that strongly bind the transition-state analogues, suggests a conserved catalytic mechanism. The occurrence of the putative base HisL89 in all active antibodies, its absence in three out of the four analyzed inactive antibodies, and the rarity of a histidine at this position in immunoglobulins support an important catalytic role for this residue.     

Dependence of polynucleotide helix–coil transition theory on the ring closure exponent

Formulas are derived which relate the maximum slope of the helix-coil transition in synthetic polynucleotides to the stacking free energy and to the exponent k in the expression for the probability of ring closure for polymer chains. The exact value of k has not yet been determined. By use of the derived formulas, it is then shown that estimates of the stacking free energy are sensitive enough to the value chosen for k, so that exact values of these energies must await a precise determination of k.     

Bond-optimized ring closure for proline: comparison of conformations and semiempirical energies with small molecule X-ray structures

A method is described for generating proline ring structures by successive addition of atoms, wherein ring closure is achieved by optimizing the fit to known ring bond-angles and one closing bond-length ("bond-optimized ring closure"). Two ring torsion angles are fixed independently within broad, allowed ranges, and the remaining torsion angles are determined uniquely in most cases. The independent torsion angles are chosen as phi and chi 2, and ring closure is achieved without prohibitive strain through most of the ranges -130 degrees less than phi less than -20 degrees and -60 degrees less than chi 2 less than 60 degrees. Comparisons of predicted ring structures to 191 X-ray diffraction structures from the literature, starting with the known values of phi and chi 2, yielded root-mean-square deviations of 4.8 degrees in chi 1, 4.7 degrees in chi 3, 8.3 degrees in chi 4, and 0.3-2% in the ring bond angles and the N-C delta distance. Semiempirical energies were calculated for the optimized structures using three sets of energy parameters from the literature. The energy surfaces show broad minima coinciding with the torsion angle regions in which the highest concentrations of observed structures are found. Two of the sets of energy parameters produce double minima corresponding to the "up" and "down" puckered conformations.     

Aim44p regulates phosphorylation of Hof1p to promote contractile ring closure during cytokinesis in budding yeast

Whereas actomyosin and septin ring organization and function in cytokinesis are thoroughly described, little is known regarding the mechanisms by which the actomyosin ring interacts with septins and associated proteins to coordinate cell division. Here we show that the protein product of YPL158C, Aim44p, undergoes septin-dependent recruitment to the site of cell division. Aim44p colocalizes with Myo1p, the type II myosin of the contractile ring, throughout most of the cell cycle. The Aim44p ring does not contract when the actomyosin ring closes. Instead, it forms a double ring that associates with septin rings on mother and daughter cells after cell separation. Deletion of AIM44 results in defects in contractile ring closure. Aim44p coimmunoprecipitates with Hof1p, a conserved F-BAR protein that binds both septins and type II myosins and promotes contractile ring closure. Deletion of AIM44 results in a delay in Hof1p phosphorylation and altered Hof1p localization. Finally, overexpression of Dbf2p, a kinase that phosphorylates Hof1p and is required for relocalization of Hof1p from septin rings to the contractile ring and for Hof1p-triggered contractile ring closure, rescues the cytokinesis defect observed in aim44∆ cells. Our studies reveal a novel role for Aim44p in regulating contractile ring closure through effects on Hof1p.     

Replicative helicase loaders: ring breakers and ring makers

Helicase loaders transfer the ring-shaped replicative helicases onto DNA. They assort into two classes: ring breakers, which place stabile hexameric rings on DNA via transient gaps at subunit interfaces; and helicase makers, which assemble hexameric rings around DNA from monomeric helicase units.     

Guidelines for pharyngostome closure

The reconstructive procedure for pharyngostome closure includes single-stage restoration in three steps: lining, intermediate layer, and covering. The lining repair is the key factor to the successful outcome of the operation. Three clinical situations may be distinguished. First, the mucosa is sufficient to restore a new gullet. In this case, it is widely undermined and sutured along the midline without any tension. Second, the mucosa is only partially sufficient. The same procedure as above is adopted to close the lower two-thirds of the pharyngostome, while an advancement flap is outlined from the base of the tongue to restore the upper third. Third, the mucosa is not sufficient. A musculocutaneous flap solves the problem. Reconstruction of the intermediate layer involves rotation of one (or both) sternomastoid muscle(s), if present. The possibilities for coverage include a submandibular flap, a thoracoacromial flap, and/or musculocutaneous flaps. By following these guidelines, the authors have successfully closed 37 pharyngostomes.