Page 63 - Color Atlas of Biochemistry
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54 Biomolecules
Steroid structure onto which the much more mobile side chain
is attached.
Steroids are relatively apolar (hydropho-
A. Steroid building blocks
bic). Some polar groups—e. g., hydroxyl and
Common to all of the steroids is a molecular oxo groups—give them amphipathic proper-
core structure consisting of four saturated ties. This characteristic is especially pro-
rings, known as gonane. At the end of the nounced with the bile acids (see p. 314).
steroid core, many steroids also carry a side
chain, as seen in cholestane,the basic compo-
nent of the sterols (steroid alcohols). C. Thin-layer chromatography
Thin-layer chromatography (TLC) is a power-
ful, mainly analytic, technique for rapidly sep-
B. Spatial structure
arating lipids and other small molecules such
The four rings of the steroids are distin- as amino acids, nucleotides, vitamins, and
guished using the letters A, B, C, and D. Due drugs. The sample being analyzed is applied
to the tetrahedral arrangement of the single to a plate made of glass, aluminum, or plastic,
carbon bonds, therings arenot flat, but puck- which is covered with a thin layer of silica gel
ered. Various ring conformations are known or other material (1). The plate is then placed
by the terms “chair,” “boat,” and “twisted” in a chromatography chamber that contains
(not shown). The chair and boat conforma- some solvent. Drawn by capillary forces, the
tions are common. Fivemembered rings fre- solvent moves up the plate (2). The substan-
quently adopt a conformation referred to as ces in the sample move with the solvent. The
an “envelope”. Some rings can be converted speed at which they move is determined by
from one conformation to another at room their distribution between the stationary
temperature, but with steroids this is dif cult. phase (the hydrophilic silica), and the mobile
Substituents of the steroid core lie either phase (the hydrophobic solvent). When the
approximately in the same plane as the ring solvent reaches the top edge of the plate,
(e = equatorial) or nearly perpendicular to it the chromatography is stopped. After evapo-
(a = axial). In threedimensional representa- ration of the solvent, the separated substan-
tions, substituents pointing toward the ob- ces can be made visible using appropriate
server are indicated by an unbroken line (β staining methods or with physical processes
position), while bonds pointing into the plane (e. g., ultraviolet light) (3). The movement of a
of thepageare indicatedby a dashed line (α substance in a given TLC system is expressed
position). The so-called angular methyl as its R f value. In this way, compounds that are
groups at C-10 and C-13 of the steroids always not known can be identified by comparison
adopt the β position. with reference substances.
Neighboring rings can lie in the same plane A process in which the polarity of the sta-
(trans; 2) or at an angle to one another (cis; 1). tionary and mobile phases is reversed—i. e.,
This depends on the positions of the substitu- the stationary phase is apolar and the solvent
ents of the shared ring carbons, which can be is polar—is known as “reversed-phase thin-
arranged either cis or trans to the angular layer chromatography” (RP-TLC).
methyl group at C-10. The substituents of ste-
roid that lie at the points of intersection of the
individual rings are usually in trans position.
As a whole, the core of most steroids is more
or less planar, and looks like a flat disk. The
only exceptions to this are the ecdysteroids,
bile acids (in which A:B is cis), cardiac glyco-
sides, and toad toxins.
A more realistic impression of the three-
dimensional structure of steroids is provided
by the space-filling model of cholesterol (3).
The four rings form a fairly rigid scaffolding,
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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