Once the configuration of the glyceric acids was known (in relation to the glyceraldehydes),
it was then possible to relate other compounds to either of these, and
each time a new compound was related, others could be related to it. In this way,
many thousands of compounds were related, indirectly, to D- or L-glyceraldehyde,
and it was determined that 37, which has the D configuration, is the isomer that
rotates the plane of polarized light to the left. Even compounds without asymmetric
atoms, such as biphenyls and allenes, have been placed in the D or L series.95 When
a compound has been placed in the D or L series, its absolute configuration is said to
be known.96
In 1951, it became possible to determine whether Rosanoff’s guess was right.
Ordinary X-ray crystallography cannot distinguish between a D and a L isomer,
but by use of a special technique, Bijvoet was able to examine sodium rubidium
tartrate and found that Rosanoff had made the correct choice.97 It was perhaps historically
fitting that the first true absolute configuration should have been determined
on a salt of tartaric acid, since Pasteur made his great discoveries on
another salt of this acid.
In spite of the former widespread use of D and L to denote absolute configuration,
the method is not without faults. The designation of a particular enantiomer as D or
L can depend on the compounds to which it is related. Examples are known where
an enantiomer can, by five or six steps, be related to a known D compound, and by
five or six other steps, be related to the L enantiomer of the same compound. In a
case of this sort, an arbitrary choice of D or L must be used. Because of this and
other flaws, the DL system is no longer used, except for certain groups of compounds,
such as carbohydrates and amino acids.