/ | \ / \
/ | \ / N[trb](CH_{3})_{3}
HO CH_{2}OOC.R O /
HO

fatty acid
/
Or, glycerol-fatty acid + Choline = Lecithin + H_{2}O
\
phosphoric acid

There are many different possible linkages of the constituent groups which
make up the lecithin molecule. In the first place, if the (OH) groups of
the glycerol molecule be numbered (1) and (2), thus,

CH_{2}OH (1)
|
CHOH (2)
|
CH_{2}OH (1)

the fatty acid radicals may be attached either in one (1) position and one
(2) position, or in the two (1) positions; hence, two forms of
glycero-phosphoric acid are possible, thus

fatty acid fatty acid
/ /
(A) glycerol--fatty acid (B) glycerol--phosphoric acid
\ \
phosphoric acid fatty acid

Again, the choline may be attached to the phosphoric acid either through
its alcoholic (OH) group or through its basic (N) group, thus

OH OH
/ /
-P=O C_{2}H_{4} -P=O
\ / \ \
O N[trb](CH_{3})_{3} or, O-N[trb](CH_{3})_{3}
/ \
HO C_{2}H_{4}OH

The facts that in the arrangement (B) the central carbon atom of the
glycerol would be asymmetric, and that both lecithin and the
glycero-phosphoric acid derived from it by hydrolysis are optically active,
prove that formula (B) correctly represents the arrangement of that part of
the lecithin molecule; and there is ample theoretical and experimental
evidence to prove that the choline linkage is through the alcoholic (OH)
group. Hence the formula for lecithin indicating the linkage as shown above
is the correct one.

The fatty acids in the lecithin molecule may be different i