ON THE STRUCTURE, BIOSYNTHESIS, FUNCTION AND PHYLOGENY OF ISOARBORINOL AND MOTIOL

The solid-state conformations of the C-3 acetates of two isomeric hopanoids - 1, isoarborinol (D:C-friedo-3-beta,5-gamma,8-alpha,10-beta,14-alpha, 17-beta,18-?? alpha, 21-beta) and 2, motiol (D:C-friedo-B1:A1-neogammacer-7(8)-en-3-beta-ol?? [3-beta,5-alpha,9-alpha,10-beta,13-alpha,14-beta,17-alpha,?? 18-beta,21-alpha])-have been determined by X-ray crystallography. The data show that whereas both molecules are planar, 1 orients into a chair-halfchair-chair-chair-halfchair conformation while 2 orients into a chair-sofa-twist-half-chair-halfchair conformation. To explain the biogenesis of 1 and 2 from squalene oxide, a step-wise mechanism is proposed which proceeds through the protosteroid cation (for 1) and dammarenyl cation (for 2). After ring enlargement from the corresponding 13(17)bond followed by concerted 1,2-migrations and loss of the 11-beta-H and 7-beta-H as protons, respectively, a 9,11-double bond (in 1) and a 7,8-double bond (in 2) is introduced into the nucleus. The mechanism is discussed in relation to the classical view of a non-stop cyclization process where, for example, squalene oxide folds in a chair-chair-chair-chair-boat conformation to give a cyclized product (motiol) presumably with the same conformational disposition as the cyclizing material. The three-dimensional geometry of 1 and 2 was found to be structurally dissimilar from sterols. For instance, 1 and 2 are shorter and volumetrically smaller molecules than cholesterol, and this may explain their diminished importance as membrane inserts compared with sterols in eukaryote evolution.