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Bibliography and Further Reading
T. Petrova, H. Steuber etal., Factorizing Selectivity Determinants
of Inhibitor Binding toward Aldose and Aldehyde Reductases:
Structural and Thermodynamic Properties of the Aldose Reduc-
tase Mutant Leu300Pro-Fidarestat Complex, J. Med. Chem., 48,
5659–5665 (2005)
B. Baum etal., More than a simple lipophilic contact: A detailed
thermodynamic analysis of nonbasic residues in the S1-pocket of
Thrombin, J. Mol. Biol., 390, 56–69 (2009)
M. Neeb etal., Occupying a Flat Subpocket in a tRNA-modifying
Enzyme with Ordered or Disordered Sidechains: Favorable or Un-
favorable for Binding? Bioorg. Med. Chem., 24, 4900–4910 (2016)
B. P. Morgan, J. M. Scholtz, M. D. Ballinger, I. D. Zipkin and P. A.
Bartlett, Differential Binding Energy: A Detailed Evaluation of the
Inuence of Hydrogen-Bonding and Hydrophobic Groups on the
Inhibition of Thermolysin by Phosphorous-Containing Inhibitors,
J. Am. Chem. Soc., 113, 297–307 (1991)
E. Rühmann, etal., Boosting afnity by correct ligand preorganization
for the S2 pocket of thrombin: A study by ITC, MD and high
resolution crystal structures, ChemMedChem, 11, 309–319 (2016)
A. Sandner, etal., Strategies for Late-stage Optimization: Proling
Thermodynamics by Preorganization and Salt Bridge Shielding,
J. Med. Chem., 62, 9753–9771 (2019)
C. Gerlach, M. Smolinski, etal., Thermodynamic Inhibition Prole of
a Cyclopentyl- and a Cyclohexyl Derivative Towards Thrombin:
The Same, but for Deviating Reasons, Angew. Chem. Int. Ed., 46,
8511–8514 (2007).
B. Baum, etal., Non-additivity of functional group contributions in
protein-ligand binding: a comprehensive study by crystallography
and isothermal titration, J. Mol. Biol., 397, 1042–1054 (2010)
A. T. Fenley, H. S. Muddana and M. K. Gilson, Entropy–enthalpy
transduction caused by conformational shifts can obscure the
forces driving protein–ligand binding, Proc. Natl. Acad. Sci. USA,
109, 20006–20011 (2012)
Optical Activity and Biological
Effect
Contents
5.1 Louis Pasteur Sorts Crystals – 68
5.2 Structural Basis of Optical Activity – 69
5.3 The Isolation, Synthesis,
and Biosynthesis of Enantiomers – 71
5.4 Lipases Separate Racemates – 72
5.5 Dierences in the Activity of Enantiomers – 74
5.6 Image and Mirror Image:
Why Is It Dierent for the Receptor? – 78
5.7 An Excursion into the World of Stereoisomers – 80
5.8 Synopsis – 81
Bibliography and Further Reading – 81
© The Author(s), under exclusive license to Springer-Verlag GmbH, DE, part of Springer Nature 2024
G. Klebe, Drug Design, https://doi.org/10.1007/978-3-662-68998-1_5
Chapter • Optical Activity and Biological Eect
5
The three-dimensional shape of amolecule has adecisive
inuence on its biological activity. The conguration of
amolecule is made up of the bonds between the atoms.
Substances with an asymmetric center at the tetracoordinated carbon atom, which are considered here, are optically active and exist in two different forms. They are
asymmetric and have arelationship to each other like
an image and its mirror image. They are called chiral. It
is impossible to change one form into the other without
breaking and reforming bonds. Chirality is often unimportant to chemists because the image and mirror image
behave exactly the same in asymmetrical environment.
When they are placed in an asymmetric environment,
such as the binding site of aprotein, this is no longer
true. The consequences of this for drug design and therapy are the subject of this chapter.
In the early 19th century, Jean Baptiste Biot observed
that some quartz crystals rotated the plane of linearly
polarized light to the right, while others rotated it to the
left. Macroscopically, this optical activity is imprinted
in the asymmetric, handed (enantiomorphic) shape of
the crystals; they exist as left- and right-handed mirror
images. Alittle later, Biot found that not only crystals,
but also organic compounds such as turpentine or sugar
solutions rotate polarized light in aparticular direction.
5.1 Louis Pasteur Sorts Crystals
The decisive experiment was performed by the then
26-year-old Louis Pasteur in Paris in 1848. Several reports in the literature were inconsistent with his theory
that there must be an obvious relationship between crystal forms and their optical properties. While carefully
studying the sodium ammonium salt of optically inactive
tartaric acid, he discovered that the crystals had different shapes. They had either right-handed or left-handed
symmetry and could be sorted by hand. The crystals of
enantiomers5.1 and5.2 (. Fig.5.1) gave solutions that
rotated the plane of linearly polarized light in opposite
directions. This conrmed his suspicions. Before Pasteur
could present his results to the Academy of Sciences, he
had to repeat the experiment in public (!) in the presence of Biot at the Collège de France. He was lucky. His
experiment was successful only because his solutions
were allowed to evaporate slowly at room temperature.
Above the critical temperature of 28 °C, astoichiometric
1:1 mixture of the two enantiomeric forms, aracemate,
would have crystallized in ahomogeneous crystal form
(Sect.5.4).
Afew years later, Pasteur made another important
observation: mold contamination of aracemic solution
. Fig. 5.1 Optical isomerism in tartaric acid. The enantiomers
(−)-tartaric acid5.1 (m.p. 168–170 °C, [α]d20 = −12°) and (+)-tartaric acid5.2 (m.p. 168–170 °C, [α]d20 = +12°) cannot be superimposed
upon each other either in the plane of the paper or in 3D space. The
sole symmetry element that they have is atwofold rotational axis (or-
ange axes) that dissect the central C–C bond. Each mirror image rotates the plane of polarized light in opposite directions to the other.
In contrast, meso-tartaric acid 5.3 (m.p. = 140 °C) has an inversion
center of symmetry (the purple center on the central C–C bond).
Solutions of meso-tartaric acid have no optical activity because the
contribution from each stereogenic center compensates for the other.
Racemic tartaric acid (m.p. = 206 °C, no rotation) is a1:1 mixture of
both enantiomers of tartaric acid5.1 and5.2. Such mixtures are optically inactive and are called racemates (Lat. racemus, the grape–tartaric acid is found in grapes and wine). (7 https://sn.pub/GCWqem)
. • Structural Basis of Optical Activity
of tartaric acid caused optical activity to develop. One
enantiomer of tartaric acid is metabolized much faster
than the other. He, thus, discovered two important methods for separating racemates into enantiomers. While
mechanical sorting is limited to afew examples, enzy-
matic kinetic resolution of enantiomers has found wide
application (Sect.5.4).
5.2 Structural Basis of Optical Activity
An explanation for the optical isomerism was possible
with the help of the theory of the tetrahedral carbon atom
developed independently by Jacobus Henricus van’t Hoff
and Joseph-Achille Le Bel in 1874. When acarbon atom
carries four different substituents, it creates an asymme-
tric or, as it is also called, stereogenic center. This prop-
erty is not limited to carbon; nitrogen (in ammonium
salts) or silicon atoms with four different substituents,
phosphorus, for example, in phosphonic or phosphoric
acid esters, or even sulfur atoms in sulfoxides (with two
different substituents, oxygen, and the lone pair of electrons) can also be asymmetric. The spatial orientation
of these compounds gives rise to two mirror-image isomers, each of which rotates polarized light in the opposite direction to the same degree. These forms are called
enantiomers (formerly antipodes). Except for their optical activity, enantiomers are identical in all chemical and
physicochemical properties, but only as long as they are
in an achiral environment.
Compounds with two chiral centers congured as image and mirror image within the same molecule do not
macroscopically exhibit optical activity; meso-tartaric
acid 5.3 (. Fig. 5.1), an inversion-symmetrical molecule, exists as aracemic mixture of chiral conformers.
Each conformer exists as an “internal” racemic mixture
because the molecule has inversion symmetry in an energetically favored conformation. Its left part can be inverted to its right part by point reection through the
center of the central C–C bond. Therefore, the R,S- or
S,R-meso-forms are identical and cannot be resolved into
enantiomers (R,S assignment, see below).
Optical activity is also present in other forms of molecular asymmetry. One example is any regular or irregular tetrahedral orientation of different substituents on
any skeleton other than asingle carbon atom. Another
case can be found in compounds where two groups are
strongly rotationally hindered around acommon bond.
This results in an asymmetric center, which gives rise to
optically active rotational isomers, called atropisomers
(. Fig.5.2).
The experimentally determined rotational value (+)
or (−) (previously calledd orl) is used to characterize
enantiomeric compounds. The spatial conguration of
astereogenic center in amolecule is described asd orl
(Lat. dextro, levo). This notation is based on the Fischer
. Fig. 5.2 Even molecules without stereogenic centers can form an
image/mirror-image pair because of their spatial construction; an example is twistane5.4. If rotation around the bonds is limited, as in the
case of the sedative methaqualone5.5, enantiomers will be separable (so-called atropisomers). In nonplanar fused ring systems like the
dibenzocycloheptadiene derivative 5.6, the enantiomeric separation
depends on the barrier of inversion for the central ring system
convention and is related to the absolute conguration of
d- and l-glyceraldehyde,5.7 and5.8 (. Fig.5.3). Most
sugars, for instance glucose5.9, can be traced back to
d-glyceraldehyde 5.7, and the natural amino acids of
proteins, for instance alanine 5.10, can be traced back to
l-glyceraldehyde5.8. For this reason, today the d/l no-
menclature is still frequently applied to sugars and amino
acids. The enantiomers of tartaric acid correspond to the
d-(−) or l-(+) form.
. Fig. 5.3 The rotation (+ or−) and the Fischer assignment (d orl)
is reported as part of the characterization of optically active compounds. To determine the Fischer assignment, the longest carbon
chain is drawn vertically with the highest-oxidized carbon atom on top
(e.g.,5.9). The standard is set by the asymmetric carbon (red) of the d-
and l-glyceraldehyde pair (5.7 and5.8). With sugars (e.g., glucose5.9)
or amino acids (e.g., alanine 5.10), the carbon that is marked with the
arrow decides whether the molecule isd orl
5
Chapter • Optical Activity and Biological Eect
. Fig. 5.4 The R/S nomenclature that was proposed by R.S. Cahn,
C.K. Ingold, and V.Prelog is unambiguous. Priority rules for each
of the four different substituents on the tetrahedral stereogenic center
The Cahn–Ingold–Prelog rules allow an unambiguous
stereochemical assignment (. Fig.5.4). By convention,
the optical center is oriented so that the substituent with
the lowest atomic number is at the back (e.g., ahydrogen atom or alone pair of electrons). To use an intuitive
explanatory model, we want to assign this substituent to
the column of asteering wheel. Then the other substituents lie in the plane of the steering wheel. If these substituents are considered in descending order of atomic
number and this order follows arotation to the right,
the stereogenic center will have an R-conguration; the
opposite direction is the S-conguration (from the Latin:
rectus and sinister). The only disadvantage of this nomenclature system is that the assignment of the stereocenter can change simply because of the atomic number,
valency, or oxidation state. The homologous L-amino
acids serine and cysteine, which are structurally stereo-
were established. The substituent with the lowest priority is placed in
the back, and the direction of remaining substituents determine the
direction of rotation by decreasing priority
chemical analogues differing only by the exchange of an
oxygen for asulfur atom, are classied as (S)-serine and
(R)-cysteine.
If there is one stereogenic center in amolecule, there
will be two enantiomers. Each additional symmetry-independent stereogenic center increases the number of
enantiomers by afactor of2. For nasymmetric centers,
there are 2n optical isomers. They occur as 2
n−1
racemic
mixtures because each has two isomers that are mirror
images of each other. Diastereomers cannot be superimposed by translation and rotation in space or by creating
amirror image because the chirality of the stereocenters
is different relative to one another. As aresult, they have
different physicochemical and chemical properties. All
pairwise racemates of adiastereomeric mixture exist as
a1:1 mixture of enantiomers, but their relative proportions in the total composition can vary widely. Labeta-
. • The Isolation, Synthesis, and Biosynthesis of Enantiomers
. Fig. 5.5 Because it has two different asymmetric centers, labeta-
lol 5.11 is adiastereomeric mixture of four different compounds with
different activities on the same receptor. The antagonistic potency on
the α1 receptor of the (R,R)-, (R,S)-, (S,R)-, and (S,S)-isomers is: S,R
>>S,S≈R,R>R,S; and on the β1 receptor is: R,R >>R,S>S,S≈S,R;
and on the β2 receptor is: R,R >>R,S >>S,S≈S,R
lol 5.11 (. Fig.5.5) is just such apair of diastereomers,
consisting of two racemates, that is, two pairs of enantiomers. As amixed antagonist, it acts on α-,
β
-adrenergic receptors (see Sect.29.3). Due to the asym-
2
β
-, and
1
metric architecture of biological macromolecules, the individual components of this mixture vary signicantly in
their qualitative and quantitative biological properties
(Sects.5.5, 5.6 and5.7).
5.3 The Isolation, Synthesis,
and Biosynthesis of Enantiomers
Racemic acids and bases can often be separated by using other enantiomerically pure, optically active bases
and acids because the formed diastereomeric salts have
different solubilities. The chemical reaction of racemic
acids, amines, and alcohols with optically active alcohols
or acids results in diastereomeric reaction products. Because of their different properties, it is possible to separate them and nally isolate the desired optically active
product by chemical cleavage.
Syntheses that do not start with optically active starting materials and do not use optically active auxiliaries
always result in racemic mixtures, that is, an exact 50:50
mixture of the two enantiomers. Access to optically active compounds can be obtained by taking synthetic reaction components from the “chiral pool.” All optically
active natural products, their derivatives and degradation
products that are available in an optically pure form can
be used as easily accessible synthetic building blocks.
Syntheses using chiral catalysts are particularly elegant.
In most cases, the optimization of yield and enantiomeric
purity, expressed as the ee value (ee =enantiomeric ex-
. Fig. 5.6 The biotechnological production of ephedrine is accom-
plished by the fermentation of sugar with baker’s yeast Saccharomy-
ces cerevisiae to pyruvic acid. Pyruvic acid is coupled to benzaldehyde
with decarboxylation to form (R)-(–)-1-hydroxy-1-phenylacetone
5.12. Upon further chemical transformation (1R,2S)-(−)-ephedrine
5.13 is obtained in optically pure form. (1S,2S)-(+)-pseudoephedrine
5.14 is adiastereomer of ephedrine. The conguration of one of the
two chiral centers is different
cess), requires considerable process development. The
chromatographic separation of racemates on optically
active solid supports is more suitable for analytical or
semipreparative purposes.
Enzymatic and biotechnological techniques have
become increasingly popular in recent years. Proteases,
esterases, lipases, or hydantoinases react more or less selectively, preferentially with adistinctly different reaction
rate; only one enantiomer of aracemic mixture is transformed to the product. The selectivity and yield of such
areaction can be optimized by the careful selection of
the medium and other reaction conditions.
The production of optically pure ephedrine is an
example of an industrial application of biotechnological synthesis that has been used for decades. This phytopharmacon is used in combination preparations for
the adjuvant therapy of rhinitis, bronchitis, and asthma.
The synthetic intermediate 5.12 (. Fig.5.6) is obtained
from amixture of benzaldehyde, sugar, and yeast. It is
then transformed to (1R,2S)-(−)-ephedrine 5.13, which
is identical to the natural product in both of its optical
centers. The C1 isomer (1S,2S)-(+)-pseudoephedrine
5.14 is adiastereomer of ephedrine. Its optical rotation,
melting point, and biological properties differ from those
of ephedrine.
Chapter • Optical Activity and Biological Eect
. Fig. 5.7 Starting with enantiomerically pure valine, valinol is formed by reduction, which condenses with phosgene to an oxazolidinone.
After abstraction of the amidic proton, the compound reacts with propionyl chloride to form the starting material for the Evans synthesis
5
Numerous other microbial syntheses provide optically pure products with or without the use of achiral,
racemic, or enantiomerically pure starting materials. Of
particular economic importance are the biotechnological
syntheses of various antibiotics, especially penicillins and
cephalosporins (Sects.2.4 and23.7). The biotechnologi-
cal production of synthetic intermediates for chiral drugs
is also becoming increasingly important.
In the last 35years, organic synthesis has developed
avariety of methods for the stereoselective preparation
of compounds. Metal catalysts, which create alocal chiral environment due to their spatial geometry, are mainly
used for this purpose. In this environment, the conversion
of one of the stereoisomeric reaction products is preferentially catalyzed. In addition, the conformational properties (Chap.16) of the molecules to be reacted can also
be used to steer areaction preferentially in the direction
of aparticular product.
David Evans at Harvard University, Cambridge,
USA, has proposed aprocedure for the stereoselective
introduction of alkyl groups in the α-position on carboxylic acids. It starts, for example, with an amino acid
such as valine, which is reduced to the amino alcohol
valinol and then condensed with phosgene to an oxazolidinone (. Fig.5.7). In the next step, acarboxylic acid,
such as propionic acid, to which the alkyl group is to be
added in the α-position, is coupled to the oxazolidinone
via an amide bond. The conformational properties of the
oxazolidinone ring are crucial for the further course of
the reaction. The ve-membered ring adopts ahalf-chair
geometry. The conformer in which the large iso-propyl
substituent is equatorial is energetically more favorable.
One of the terminal methyl groups of the iso-propyl substituent is oriented towards the bottom of the ring plane
of the heterocycle (. Fig.5.8). With astrong base such
as lithium iso-propylamide, a proton is extracted from
the reactant to form an enolate. The lithium ion plays an
important role by forming achelate ring with the enolate
and the carbonyl oxygen, stabilizing aplanar geometry in
this part of the molecule. If, for example, benzyl bromide
is used as areagent, abenzyl group will be introduced
stereoselectively. The newly formed C–C bond forms via
2 reaction step. It proceeds through atrigonal–bi-
an S
N
pyramidal transition state. The new C–C bond to the
enolic carbon is formed simultaneously with the displacement of the bromine atom as abromide ion. This
attack can only take place “from above,” since the lower
surface of the oxazolidinone ring is spatially shielded by
the iso-propyl group (. Fig.5.8, red semicircle). In this
way, only one stereoisomer is predominantly formed.
How can the benzyl group be introduced from the
other side? The key is that now the upper face of the oxazolidinone ring must be sterically shielded. If amethyl and
aphenyl group are attached to the two tetrahedral carbon
atoms of the heterocycle with an upwards orientation, the
larger phenyl substituent on the half-chair will assume the
equatorial orientation. The methyl group will then be axially positioned towards the upper side of the ring. Now
the attack can only occur from the bottom. The benzyl
group introduced at the α-position is on the bottom side
of the product. The other stereoisomer is formed. After
hydrolysis of the amide bond, the enantioselective carboxylic acid is obtained. For the second reaction with one
methyl and one phenyl group, another stereochemically
uniform amino alcohol is required. (1S,2R)-Ephedrine
can be used for this purpose. The example of the Evans
synthesis with achiral auxiliary is exemplary for many
stereoselective syntheses that follow such aconcept.
5.4 Lipases Separate Racemates
Enzymes are well suited to resolve racemates because of
their asymmetric structure. This can occur either because
one of the two enantiomeric substrates is more strongly
bound and more rapidly converted. Alternatively, achemical reaction may take place in the binding pocket of the
protein with different efciencies. Lipases are often used
for kinetic resolution of racemates because they are stable
in organic solvents due to their molecular composition
and lipophilic surface. They belong to the large group of
hydrolyzing enzymes (Chap.23). A nucleophilic serine is
present in the catalytic center that forms an acyl–enzyme
complex upon hydrolysis of an amide or ester substrate.
The protein is then itself converted to an ester through the
OH group of the serine, the so-called acyl form is produced
(Sect.23.2). Such acomplex can then react with another
nucleophile, for instance an amine. The amine attacks the
internal enzyme ester, the bond to the serine oxygen atom
is broken, and anew amide bond is formed. If one employs
the right- or left-handed form of an amine, one form will
react preferentially. In this way, the racemate is resolved.
. • Lipases Separate Racemates
. Fig. 5.8 Example of astereoselective Evans synthesis for the for-
mation of α-alkylated carboxylic acids using achiral auxiliary. Stereochemically uniform oxazolidinones with either an iso-propyl (upper
row) or methyl/phenyl substituent (bottom row) are converted into the
corresponding enolates with LiN(iPr)2. The oxazolidinone ring assumes ahalf-chair conformation. Here, the largest substituent (either
iso-propyl or phenyl) is preferentially equatorially oriented. In an SN2
reaction, the formed enolates are reacted with benzyl bromide. Upper
row In the rst case, the attack takes place from the sterically more accessible upper face. Bottom row In the second case, attack occurs from
How does the enzyme distinguish between the two
enantiomers of an amine? The reaction of (R)- and
(S)-phenylethylamine 5.15 and 5.16 with the lipase Can-
dida antarctica was carefully investigated (. Fig.5.9). The
energy barrier for the faster-reacting R-form is lower than
for the slower S-form. Adetailed evaluation of the kinetic
parameters showed that this is above all due to an enthalpic
advantage of the R-amine. The S-amine has an entropic
advantage. Altogether the enthalpic component is in excess
so that the free energy difference (∆G) favors the R-form
(. Fig.5.9). How is this discrimination to be understood?
Structural transition state analogues have been synthesized.
In the place of the unstable tetrahedral carbon atom intermediate, aphosphorus atom was introduced (5.17 and
5.18, . Fig.5.10). This trick gives astable compound that
is very similar to the transition state formed at the carbon
atom. These analogues were synthesized with both enan-
the lower face. In the product, therefore, the introduced side chain is
either up (R-) or down (S-). Depending on the reaction conditions
chosen, the Evans synthesis can achieve adiastereomeric excess of
93–98 (7 https://sn.pub/kicyt3)
tiomeric amines, and complexes with the lipase were prepared. Marco Bocola managed to obtain acrystal structure of both. Interestingly the transition state analogue of
the faster-reacting R-form ts well into the binding pocket
(. Fig.5.10). On the other hand, the S-form demonstrated
great residual mobility in the catalytic center.
Computer simulations of the molecular dynamics of
both forms conrmed the picture: The R-analogue is well
dened and temporally stable in ageometry that is ideal
for the chemical reaction. In contrast, the S-analogue appears to be quite mobile. It is much less likely to remain
in an arrangement that allows the catalytic reaction to
occur in the lipase. Successful catalytic transformation
of this substrate occurs much less frequently. The R-an-
alogue, xed as in avise and waiting to be transformed,
forms good enthalpic contacts with the enzyme. It practically takes ashape complementary to the enzyme pocket.
5
Chapter • Optical Activity and Biological Eect
. Fig. 5.9 The reaction of (R)- and (S)-phenylethylamine, 5.15 and
5.16, with Candida antarctica lipase begins with the formation of an
acyl–enzyme complex, E–A. The faster-reacting R-amine 5.15 (red)
forms alower-energy transition state that leads to the free enzyme and
the R-amide (E+R). Analogously the S-amide (E+S) forms from the
This results in its great enthalpic advantage. Entropically,
however, this xation comes at aprice. With its methyl
group at the stereogenic center, the R-analogue becomes
entangled in asmall niche of the binding pocket. The
S-analogue misses this opportunity because this crucial methyl group is oriented in the opposite direction.
Therefore, it lacks this anchor for attachment to the
binding pocket. It has ahigh degree of mobility in the
catalytic center, so it does not lose as many degrees of
freedom compared to the situation before enzyme binding. Entropically, this is favorable. But enthalpically, this
substrate substrate cannot form good interactions. The
complementary t to the protein is rarely achieved. In
the end, the enthalpic component predominates and the
resulting Gibbs free energy difference leads to amuch
faster conversion of the R-amine. This is sufcient to
produce practically only the R-amide in high yield. The
lipase can also be immobilized onto asolid support and
loaded into aglass column. After the acyl form is prepared on the column, aracemic mixture of the amine
only needs to be poured onto the column. The S-amine
and R-amide are then simply collected in aask. If the
solvent is well chosen, the amide will crystallize directly
from the solution and can be mechanically separated
whereas the amine will remain in solution.
Interestingly, the enantiopreference of the kinetic resolution is lost with increasing temperature or enlarging the
enzyme pocket. Enlargement can be achieved by exchanging atryptophan along the rim of the catalytic pocket for
ahistidine. The higher temperature or increased space in
higher-energy E–S transition state (blue) from the S-amine 5.16. Dif-
ference in ∆G‡ is −19.4 kJ/mol and favors the Rform. The ∆G‡ difference is based on acombined enthalpic and entropic contribution in
which the R-form is enthalpically favored, and entropically disfavored.
The S-form is enthalpically disfavored but has an entropic advantage
the binding pocket enhances the mobility of both substrates in the lipase pocket. The enthalpic advantage of
the faster-reacting R-amine is lost. The entropic difference of both substrates levels out under these conditions.
This example shows at the molecular level how alipase achieves kinetic resolution. With knowledge of the
energetic parameters and structural information, an attempt can be made to tailor lipases for other transformations. Because of the importance of such reactions, the
targeted design of enzyme catalysts has developed into
an ever more important topic for the synthesis of chiral
building blocks in new drugs.
5.5 Differences in the Activity
of Enantiomers
Flora and fauna are known for their symmetry. Consider
the face, the arms and legs, the ribs, or an orchid ower.
The exceptions, such as the shell of asnail, are rare or, as
in the case of the ounder, occur only under special evolutionary conditions. The internal organs of vertebrates
are partially paired and partially asymmetrically oriented.
At the molecular level, there is no corresponding symmetry: optically active building blocks predominate. All
specic interaction partners of biologically active molecules are chiral. Enzymes and receptors are composed of
l-amino acids. Nucleic acids are built on abackbone of
d
-ribose or d-deoxyribose units. Most naturally occur-
ring sugars have ad-conguration. Important vitamins,
. • Dierences in the Activity of Enantiomers
. Fig. 5.10 Upper row Shown is aphosphorous transition state an-
alogue 5.18 for the lipase with the S-amine. The crystal structure and
MD (molecular dynamics) simulations indicate that it is less-rigidly
xed in the transition state and rarely adopts the geometry with an
H-bond (purple line) to histidine (on the lower edge of the binding
pocket) that is necessary for the reaction to occur. Lower row The
relevant complex with the transition state analogue 5.17 of the faster-reacting R-amine is shown. This substrate is highly restricted in the
binding pocket. Its methyl group (above right) is embedded in asmall
hormones, and second messengers exist in an optically
homogeneous form. Accordingly, enantiomers of an
optically active ligand are expected to have different effects. This has been demonstrated in many thousands of
examples. In most cases, enantiomers show signicant
differences in potency and quality of action.
According to EverhardusJ. Ariëns from the University of Nijmegen, the Netherlands, biologically active enantiomers are called eutomers and inactive enantiomers
are called distomers. The quotient of the two afnities or
effects is dened as the eudismic ratio, and the logarithm
of this value is called the eudismic index. It should be
niche in the binding pocket. This substrate exclusively adopts the geometry with the H-bond to histidine. This orientation is required for
a successful substrate reaction. Therefore, the R-amine 5.17 reacts
with the enzyme faster. (7 https://sn.pub/GLcA6h)
noted that this value must be determined on extremely
pure compounds. As little as 1% eutomer impurity in
acompletely inactive distomer can simulate 1% relative
activity in the distomer!
The more the activities of enantiomers in aracemic
pair differ, the more the eudismic ratio diverges from1.
Examples of this are given in compounds 5.19–5.22
(. Fig. 5.11). A eudismic ratio of 500,000 was measured for an inhibitor of a chloride ion transporter. In
this case, the chemists pulled out all the stops to purify
the less potent enantiomer. Theoretically, a compound
with nanomolar potency should yield even higher values.