Biomimetic Construction of a syn-2,7-Dimethyloxepane Ring via 7-Endo Cyclization.

syn-2,7-Dimethyloxepane is a unique structure observed in natural ladder-shaped polycyclic ethers (LSPs), such as Caribbean ciguatoxins, gymnocin-B, and brevisulcenal-F that exhibit potent biological activities. Thus, the successful construction of this seven-membered ring is desirable, but its ring strain and the 1,3-repulsion between its two methyl groups makes this process difficult. Herein, we prepared syn-2,7-dimethyloxepanes via 7-endo cyclizations of vinyl epoxides that break Baldwin's rules. Such a biomimetic approach to syn-2,7-dimethyloxepanes has not yet been reported; however, we achieved this challenging cyclization with the aid of a cis-olefin tether and an unsubstituted vinyl group. The NO-ring fragment of gymnocin-B was also prepared from one of these 7-endo cyclized products, demonstrating the potential application of this strategy in constructing bioactive LSPs.

The hyperthermophilic cystathionine γ-synthase from the aerobic crenarchaeon Sulfolobus tokodaii: expression, purification, crystallization and structural insights.

Cystathionine γ-synthase (CGS; EC 2.5.1.48), a pyridoxal 5'-phosphate (PLP)-dependent enzyme, catalyzes the formation of cystathionine from an L-homoserine derivative and L-cysteine in the first step of the transsulfuration pathway. Recombinant CGS from the thermoacidophilic archaeon Sulfolobus tokodaii (StCGS) was overexpressed in Escherichia coli and purified to homogeneity by heat treatment followed by hydroxyapatite and gel-filtration column chromatography. The purified enzyme shows higher enzymatic activity at 353 K under basic pH conditions compared with that at 293 K. Crystallization trials yielded three crystal forms from different temperature and pH conditions. Form I crystals (space group P21; unit-cell parameters a = 58.4, b = 149.3, c = 90.2 Å, ß = 108.9°) were obtained at 293 K under acidic pH conditions using 2-methyl-2,4-pentanediol as a precipitant, whereas under basic pH conditions the enzyme crystallized in form II at 293 K (space group C2221; unit-cell parameters a = 117.7, b = 117.8, c = 251.3 Å) and in form II' at 313 K (space group C2221; unit-cell parameters a = 107.5, b = 127.7, c = 251.1 Å) using polyethylene glycol 3350 as a precipitant. X-ray diffraction data were collected to 2.2, 2.9 and 2.7 Šresolution for forms I, II and II', respectively. Structural analysis of these crystal forms shows that the orientation of the bound PLP in form II is significantly different from that in form II', suggesting that the change in orientation of PLP with temperature plays a role in the thermophilic enzymatic activity of StCGS.