Synthesis, and carbon-13 n.m.r. study, of O-α- -rhamnopyranosyl-(1→3)-O-α- -rhamnopyranosyl-(1→2)- -rhamnopyranose and O-α- -rhamnopyranosyl-(1→3)-O-α- -rhamnopyranosyl (1→3)- -rhamnopyranose, constituents of bacterial, cell-wall polysaccharides

O-α- -Rhamnopyranosyl-(1→3)- -rhamnopyranose (19) and O-α- -rhamnopyranosyl-(1→2)- -rhamnopyranose were obtained by reaction of benzyl 2,4- (7) and 3,4-di-O-benzyl-α- -rhamnopyranoside (8) with 2,3,4-tri-O-acetyl-α- -rhamnopyranosyl bromide, followed by deprotection. The per-O-acetyl α-bromide (18) of 19 yielded, by reaction with 8 and 7, the protected derivatives of the title trisaccharides (25 and 23, respectively), from which 25 and 23 were obtained by Zemplén deacetylation and catalytic hydrogenolysis, With benzyl 2,3,4-tri-O-benzyl-β- -galactopyranoside, compound 18 gave an ≈3:2 mixture of benzyl 2,3,4-tri-O-benzyl-6-O-[2,4-di-O-acetyl-3-O-(2,3,4-tri-O-acetyl-α- -rhamnopyranosyl)-α- -rhamnopyranosyl]-β- -galactopyranoside and 4-O-acetyl-3-O-(2,3,4-tri-O-acetyl-α- -rhamnopyranosyl)-β- -rhamnopyranose 1,2-(1,2,3,4-tetra-O-benzyl-β- -galactopyranose-6-yl (orthoacetate). The downfield shift at the α-carbon atom induced by α- -rhamnopyranosylation at HO-2 or -3 of a free α- -rhamnopyranose is 7.4-8.2 p.p.m., ≈1 p.p.m. higher than when the (reducing-end) rhamnose residue is benzyl-protected (6.6-6.9 p.p.m.). α- -Rhamnopyranosylation of HO-6 of gb- -galactopyranose deshields the C-6 atom by 5.7 p.p.m. The 1 2-orthoester ring structure [O₂,C(me)OR] gives characteristic resonances at 24.5 ±0.2 p.p.m. for the methyl, and at 124.0 ±0.5 p.p.m. for the quaternary, carbon atom.     

α-, β-, and γ-Tubulin Polymerization in Response to DNA Damage

Microtubules provide structural support for a cell and play key roles in cell motility, mitosis, and meiosis. They are also the targets of several anticancer agents, indicating their importance in maintaining cell viability. We have investigated the possibility that alterations in microtubule structure and tubulin polymerization may be part of the cellular response to DNA damage. In this report, we find that γ-radiation stimulates the production and polymerization of α-, β-, and γ- tubulin in hematopoeitic cell lines (Ramos, DP16), leading to visible changes in microtubule structures. We have found that this microtubule reorganization can be prevented by caffeine, a drug that concomitantly inhibits DNA damage-induced cell cycle arrest and apoptosis. Our results support the idea that microtubule polymerization is an important facet of the mammalian response to DNA damage.