1-methylidenesqualene and 25-methylidenesqualene as active-site probes for bacterial squalene:hopene cyclase.

1-methylidenesqualene and 25-methylidenesqualene were converted to 30-methylidenehop-22(29)-ene by squalene:hopene cyclase from Alicyclobacillus acidocaldarius. It was remarkable that both analogues generated the same product. The hopanyl intermediate cation, stabilized by the methylidene residue, enabled a rotation of the isobutenyl group at C-21 prior to the final proton elimination. In contrast, in the formation of hop-22(29)-ene, the final proton abstraction takes place regiospecifically from the Z-methyl group, which was verified by cyclization of (1,1,1,24,24,24-(2)H(6))squalene into (23,23,23,30,30,30-(2)H(6))hop-22(29)-ene. [reaction: see text]