Sterols are a class of pristine natural products which have intrigued scientists
for 150 years. M.E. Chevreul, a 19th century French chemist, is often associated
with the title “Father of Steroidology” (the study of sterols and related
steroids). In 1815, Chevreul discovered that a gallstone substance possessed
physical properties different from other lipoidal materials, notably waxes. He
named the substance “cholesterine” from the Greek “chole” for bile and “stereos”
for solid; from cholesterine evolved cholesterol. The term sterol, however, is
used with different shades of meaning by different writers. We define R-sterols,
which are produced by living systems and the subject of this monograph, as any
chiral (“handed”) tetracyclic isopentenoid which is formed by the epoxidation
and cyclization of squalene through a transition state possessing stereochemistry
similar to the trans-syn-trans-anti-trans-anti configuration, i.e., the protosteroid
cation, and retains in the cyclized product an hydroxyl group at C-3, an
all-trans anti stereochemistry in the ring system and a side chain 20-R configuration.
A conformational outcome of these stereochemical determinants is to
produce a sterol which is flat and for the side chain to orient to the right. The
resulting amphiphilic properties and length of the molecule, ca. 20 R, allow
sterols to function as architectural components of membranes. Sterols may also
play additional functions, such as bioregulators