1a,25(OH)2D3 1a,25(OH)2D3

(6-s-fransor (6-s-c/sor extended conformation) steroid-like conformation)

FIGURE 9-7 Conformational flexibility of vitamin D molecules using la,25(OH)2-vitamin D3 [la,25(OH)2D3] as an example. (A) The dynamic single bond rotation of the cholesterol-like side chain of la,25(OH)2D3 (i.e., 360° rotations about five single carbon bonds and the oxygen, as indicated by the curved arrows). The dots indicate the position in three-dimensional space of the 25-hydroxyl group for some 394 readily identifiable side chain conformations which have been determined from energy minimization calculations. Two orientations of the C/D/side chain are presented: a "top" view and an "in-plane" view. (B) The rapid (thousands of times per second) chair-chair interconversion of the A-ring of the secosteroid which effectively equilibrates the la-hydroxyl between the axial and equatorial orientations. (C) The 360° rotational freedom about the 6,7 carbon-carbon bond of the seco /3-ring which generates conformations ranging from the more steroid-like (6-s-cis) conformation, to the open and extended (6-s-frans) conformation of la,25(OH)2D3.

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