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Chirality and its Influence on Pharmaceuticals

Amanda Wong

Dr. Jackson

Saint Mary’s College of California

Fall 2016 Honors Contract

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Introduction to Chirality

Stereochemistry is a subject in organic chemistry focused on the three-dimensional

arrangement of atoms and its influence in chemical reactions. It was first introduced by Jacobus

van’t Hoff and Joseph Achille Le Bel in the mid-1870s. They proposed that the four bonds

attached to the central carbon were directed toward the corners of the tetrahedron (Carey and

Guilianio).

An isomer is a molecule that shares the same atomic mass as another but differs in

structural arrangement. While constitutional isomers differ in atomic connectivity, stereoisomers

maintain atomic connectivity but differ in their spatial arrangement of atoms. Spatial

arrangement is specific in how atoms are situated in space around the carbon backbone.

The concept of isomers is related to chirality, one of the most important topics in organic

chemistry. The word itself, “chiral,” is from the Greek word “cheir” meaning “handedness.” A

chiral molecule is not superimposable on its mirror image. And contrastingly, an achiral, a

molecule that is superimposable on its mirror image. Our left and right hands are often used to

illustrate the concept of chirality since our non-superimposable mirror images of each other.

The property of chirality is common among organic compounds. Chiral molecules are

most commonly formed when four different substituents are bonded to the central carbon, also

known as an asymmetric carbon, stereogenic carbon, chirality center, or stereocenter (He,

Nguyen, and Pham-Huy). Other atoms, such as sulfur, phosphorus and nitrogen, act as an

asymmetric center, and are called omeprazole, cyclophosphamide and methaqualone,

respectively.

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FIGURE 1: Example of enantiomers

Enantiomers are stereoisomers that are related as an object and to its non-superimposable

mirror image. Enantiomers are either classified as levorotary (l-isomer) or dextrorotary (d-

isomer).

FIGURE 2: Comparison of the L-isomer and D-isomer

Since enantiomers are determined by their 3-dimensional spatial arrangement of

substituents around a chiral center in the molecule, a system was used to help identify each of

them. This is known as the Cahn-Ingold- Prelog (CIP) convention, which assigns priorities to

substituent groups. In general, the basic rule is that substituents of the higher atomic number

precede the substituents with lower atomic number (He, Nguyen, and Pham-Huy). In order to

determine whether the enantiomer is a rectus (R) or right conformation or a sinister (L) or left

conformation, the atom is drawn so that the lowest priority substituent is below the plane of the

picture. Then, the substituents are counted from highest priority to lowest priority (higher atomic

number to lower atomic number). For example, bromine is of higher priority that fluorine. Once

all the substituents are labeled according to their priority, an arrow is drawn from lowest to

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highest priority. If the counting is in a clockwise direction, or right, the configuration of the

molecule is R, or rectus. Contrastingly, if the counting is in a counter-clockwise direction, or left,

the configuration of the molecule is S, or sinister. Racemic mixtures, mixtures that contain equal

amounts of both enantiomers, are indicated as R,S (He, Nguyen, and Pham-Huy).

FIGURE 3: Assigning Stereochemistry to Enantiomers

Not all enantiomers have one chiral center. Molecules which have two or more chiral

centers are known as diastereomers. A diastereomer compound with two chiral centers will have

four isomers. Contrastingly to enantiomers which have the same physical and chemical

properties, such as melting and boiling points, pKa, and solubility, diastereomers do not have the

same properties, which makes their biological activities far different. Enantiomers can form

diastereomers during chiral separation (He, Nguyen, and Pham-Huy).

In 1848, Louis Pasteur successfully separated the isomers of sodium ammonium tartarate

(C4H16NNaO10). During the process, he noted that these two isomeric crystals had different

physical properties and later concluded that they had different abilities in rotating plane polarized

light. This property is known as optical activity. Since enantiomers have the same physical

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properties and chemical formula, except for the direction in which they rotate plane polarized

light, these isomers differ in their optical activity. It is for this reason why enantiomers are often

referred to as optical isomers. The diagram below illustrates how optical activity is measured

using a polarimeter.

FIGURE 4: Structure and functioning of a polarimeter

When visible light is present, electromagnetic waves oscillate in many different planes,

perpendicular to the direction in which the light is travelling. When electric and magnetic fields

are randomly orientated in all directions of the planes, non-polarized light is present. When the

non-polarized light is passed through a filter, oscillations become oriented in one direction,

called plane polarized light. This is because only light waves that are oscillating at a specific

plane can be transmitted. If the sample is able to rotate the plane of polarized light, it is

considered “optically active.” An optically active sample contains a chiral substance and one

major enantiomer that is present in excess over the other. All pairs of enantiomers rotate plane-

polarized light to an equal and opposite degree. A compound that rotates plane polarized light to

the right is + (d-isomer) and compounds that rotate light to the left is – (l-isomer). Jean-Baptiste

Biot also discovered that the more molecules encountered by the light, the greater the amount of

rotation. Thus, the degree of rotation is dependent on the molecule, the concentration, and the

sample path length (Autschbach).

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Introduction to Pharmaceuticals

Pharmaceuticals are major elements of modern day medicine. Ensuring that all

pharmaceutical products are safe and effective allows doctors and other prescribers to use them

rationally.

The importance of chiral drugs in the drug industry cannot be understated. There are

approximately 2000 prescription drugs and over-the-counter drugs are used in everyday life, with

about 56% of these drugs consisting of chiral molecules (Carey and Guiliano). 88% of these

drugs are racemic mixtures, mixtures that contain left and right handed enantiomers of a chiral

molecule. Most of the pharmaceuticals that come from a natural source are chiral, usually

containing only one enantiomer. For example, all natural amino acids are levorotatory while all

carbohydrates (sugars) are dextrorotatory (He, Pham-Huy, Nguyen). However, recently organic

chemists have been synthesizing more and more chiral drugs in its enantiomerical pure form

(Carey and Guiliano).

Even though enantiomers of chiral drugs have the same connectivity of atoms within the

molecule, they have significant differences in pharmacology. Only certain enantiomers are

biologically active, which explains why it is crucial to separate two enantiomers when chiral

drugs are being synthesized. Due to the high degree of chiral recognition with enzymes that takes

place during biological processes, it is almost certain that enantiomers of a chiral drug will elicit

different effects or intensity of response. If the incorrect enantiomer is used for synthesis of a

chiral drug, there will be different interactions between the proteins, enzymes, enzyme receptors

and other catalysts, which could influence the activity of the enantiomer.

There are three main stages in understanding the biological effect of drugs. The first stage

is called the initial receptor differentiation phase, where a variety of drugs have different

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affinities and tissue specificity due to the differentiation of the receptors. The next stage, also

known as the absorption, distribution, metabolism, and excretion phase, is where the type of

specific bioavailability is determined. The last stage is where the drug interacts with the

molecular site of action. This is the stage in which the therapeutic effect is actually elicited

(Crossley). The biological systems of humans have chiral drug receptor areas. This requires

specific chiral molecules to be bound. The recognition of a chiral drug by a specific drug

receptor was first introduced by Easson and Stedman. They proposed that an enantiomer of a

chiral drug can interact with multiple binding sites. If the substituents on the enantiomer are not

in the correct arrangement in space, the enantiomer will be inactive. In the figure below, the

three substituents (A, B, and C) of the active enantiomer are able to interact with the three

receptor binding sites (a, b, and c) on the drug. The alignment of the substituents on the

enantiomer is crucial to the interactions and binding with the regions on the receptor. This fitting

interaction determines whether or not there will be an active response. Only a particular

enantiomer has the complementary shape to fit and bind into a receptor site to produce the active

response (McConathy, Owens).

FIGURE 5: Relationship between enantiomers and the drug binding site.

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The three-dimensional spatial arrangement of the molecule can prevent certain

enantiomers from producing a biological effect at the binding site of the receptor. In cases where

the part of the molecule that contains the chiral center does not interact with the region on the

target, the enantiomers can display very similar or the same biological effect (McConathy,

Owens).

The active enantiomer is known as a eutomer, and has a higher pharmacological activity

compared to the inactive enantiomer, known as a distomer (He, Nguyen, and Pham-Huy). This

property, known as enantioselectivity, is when one enantioimer is expressed exclusively over the

other. This means that the biological structure of an enzyme receptor has an affinity for one

enantiomer over the other one. Ultimately, the response that results from the interaction between

the enantiomer and its surrounding environment determines the effectiveness of the chiral drug.

In the food industry, different chiral forms, left-handed and right-handed forms, have

different properties. For example, the left-handed limonene smells like lemons while the right-

handed limonene smells like oranges. Another example is asparagine. While one form is

asparagine is used as a food and beverage sweetener, the other form has an extremely bitter taste

(Jasco).

In some cases, one form of an enantiomer may result with a similar effect compared to

the other. For example, the painkiller ibuprofen results with one active enantiomer, (S)-

ibuprofen, and a totally inactive enantiomer, (R)- ibuprofen. In this racemic mixture, ibuprofen

contains exactly 50% pure and 50% inert ingredients. The (S)-enantiomer is more frequently sold

as a racemic mixture since the (R)- enantiomer is also useful since it is converted to its active

(S)-enantiomer. Even though the (R)-enantiomer cannot inhibit cyclooxygenase at different

concentrations like the (S)-enantiomer, the (R)-enantiomer becomes involved in lipid

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metabolism pathways and can be incorporated into triglycerides and fatty acids. The (S)-

enantiomer does not play a role in these metabolic reactions. However, the (R)-enantiomer

undergoes metabolic inversion to yield the (S)-enantiomer of ibuprofen. The racemic ibuprofen

has been used as an anti-inflammatory and analgesic agent for over three decades. But, the

inversion of enantiomers is not immediate and varies depending on the individual. The kinetics

of this enantiomer inversion may also differ depending on the dosage of the drug given to the

patient. For example, the inversion appears to decrease when the racemic drug is given to a

patient experiencing severe pain (Evans).

FIGURE 6: Structures of the Ibuprofen enantiomers.

Another example of an enantiomer is naproxen. (S)-naproxen is a very common pain

reliever and nonsteroidal anti-inflammatory drug (NSAID), often found as Aleve in drug stores.

It is known to relieve fevers, swelling and stiffness cause by migraines, osteoarthritis, kidney

stones, menstrual cramps, and tendinitis. In order to decrease pain and inflammation, (S)-

naproxen binds to the active site on the enzyme called cyclooxygenase. This inhibits the

synthesis of prostaglandins at the site of injury. However, (R)-naproxen does not inhibit the

cyclooxygenase enzyme due to its arrangement. In fact, the R enantiomer of naproxen is a liver

toxin (Buchanan, Butt, Howard-Lock, Kean, Lock, Rischke).

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FIGURE 7: Structures of the Naproxen Enantiomers.

Norgestrel (C21H28O2) is another example of a chiral drug. The left handed isomer of

norgestrel is often called Levonorgestrel or L-norgestrel since it bends polarized light in a

counter clockwise direction. Levonorgestrel is a form of progestin that patients use as a

medication to prevent pregnancy. Often times, it is referred to as the “mini-pill” since it does not

contain estrogen. This isomer binds to and activates progesterone receptors, which are

concentrated in target tissues of the female reproductive system, such as the ovary, uterus, breast,

and pituitary gland. Normally, when levonorgestrel is not present, progesterone will activate the

progesterone receptors, signaling that an ovum has been released. When these progesterone

receptors are activated, the target organs change activity from stimulating the release of or

receiving another ovum to supporting a pregnancy and preventing the release of a new ovum.

Thus, Levonorgestrel is used to activate progesterone receptors to prevent future pregnancies.

The other isomer of norgestrel, called D-norgestrel, or the right-handed component, is

biologically inactive and harmless. Because D-norgestrel has no medical use compared to L-

norgestrel, the drug is often found as a racemic mixture. Therefore, even though some

medications contain D-norgestrel, it is often labeled as L-norgestrel since the dextronorgestrel

component of the drug it inert (Lamparxzyk, Nowakowska, Zarzycki)

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FIGURE 8: Molecular Structure of Levonorgestrel

Another drug, penicillamine, has two enantiomers: L-penicillamine and D-penicillamine.

D-penicillamine is a chelating agent, a chemical that binds to and removes ions from a solution.

D-penicillamine treats Wilson’s disease, a genetic disorder of copper metabolism, by binding to

copper and eliminating it through urination excretion. In cystinuria, a hereditary disorder that

forms cysteine stones, D-penicillamine reduces the amount of cysteine secretion by creating a

mixture of disulfide, which is significantly more soluble than cysteine. D-penicillamine also

treats patients who have severe arthritis by reducing the amount of T-lymphocytes and inhibiting

the functioning of macrophages. The L-penicillamine is toxic, however, and can inhibit the

functioning of pyridoxine, known as vitamin B6 (PubChem).

L-penicillamine D-penicillamine

FIGURE 9: Structures of L-penicillamine and D-penicillalmine

However, in other cases, it is crucial that correct version of the enantiomer is available in

the chiral drug because while one enantiomer is highly beneficial to the body, the other

enantiomer can be highly toxic and detrimental. For example, the R-enantiomer and S-

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enantiomer of Thalidomide have dramatically different toxicity effects. The figure below

illustrates the structures of the two enantiomers of Thalidomide.

FIGURE 10: Structures of S-thalidomide and R-thalidomide

While the R-enantiomer is an effective sedative and antinausea drug which helps with

reliving anxiety, putting the patient to sleep, the S- enantiomer of Thalidomide causes serious

teratogenic birth defects. This teratogenic fetus has deficient, redundant, or misplaced body parts.

In 1957, a pharmaceutical company introduced thalidomide to pregnant women. Approximately

2000 pregnant women who took the racemic mixture gave birth to children with phocomelia, a

condition in which the hands or feet are immediately at the shoulder or hip rather than from a

limb (arm or leg). Other birth defects included impairment of vision and hearing, internal

disabilities, a cleft palate, and disfigurement. The mothers who ingested thalidomide were

unaware of that the drug had the ability to pass through the placental wall and affect the

development of the fetus.

Though the action of (S)- thalidomide is not entirely understood today, current scientists

have proposed that the (S)- thalidomide could possibly block the genes that are necessary to code

for essential protein (Yip). Because the human liver has an enzyme that converts the wanted (R)-

thalidomide to (S)- thalidomide, a pure (R)- thalidomide administration would still result in a

racemic mixture.

Current research with thalidomide has been conducted in search for cancer and

inflammatory disease treatments. Researchers hope that certain cancer tumors can be killed

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directly by inhibiting the blood supply to them through thalidomide. Thalidomide can also be

helpful in treating AIDS since research has suggested that thalidomide can help reduce the ulcers

in AID patients (Yip).

Enantiomer Processes and the Importance of Chiral Drugs

Today, the difficulty of controlling which chiral form of a molecule will be produced still

remains. (Jasco). Because chiral molecules have very similar physical properties on the

molecular level, it is difficult to separate and quite expensive.

The process in which enantiomers are separated and quantified is established through an

enantioselective assay (He, Nguyen, and Pham-Huy). One common type of synthesis,

stereoselective synthesis involves producing one isomer predominantly over the other. In

stereospecific synthesis, each isomer creates a different product. Chiral switching is a process

used to transform a racemic drug into a single active enantiomer. This procedure is the most used

in today’s research in order to separate different isomers from each other (He, Nguyen, and

Pham-Huy).

One method of separating enantiomers is using high performance liquid chromatography

(HPLC) and collision detection, or circular dichroism (CD). It is successful in detecting chiral

compounds, such as pharmaceuticals, because the collision detectors and fluorescent detectors

are sensitive to concentration. Even though the fluorescent detectors cannot identify which

enantiomer is present, an optical rotation (OR) detector can distinguish between enantiomers

(Jasco).

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FIGURE 11: Naproxen-sodium chromatograms: Top- CD detector, middle- UV detector, bottom- OR detector

Promoting chiral separation is important because it plays a key role in pharmaceutical

industry and clinical therapeutics. In some cases, racemates can be more beneficial than a single

isomer because the two enantiomers could have complementary effects of each other. Although

many drugs used today are racemates, and contain both enantiomers, it is still important to find a

cost-effective separation method for chiral drugs. This could increase the production of single

drug enantiomers. Single enantiomers are known to have greater selectivity for their biological

targets, despite their limited complexity. Thus, they have lesser adverse drug reactions, and are

better pharmacokinetics than mixtures of enantiomers (McConathy, Owens). The adverse drug

reactions that are happening to one enantiomer can be avoided, allowing patients to be exposed

to less amounts of drug (Aseri, Chhabra, Padmanabhan). Ultimately, the decision to use a

separation technique on a chiral drug is independent of the research and depends on the physical

and chemical properties of the drug itself (He, Nguyen, and Pham-Huy).

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Drug isomerism has helped introduce safer and more effective pharmaceutical drugs.

(Aseri, Chhabra, Padmanabhan). Even though single enantiomers and racemic mixtures are both

used in in drug pharmaceuticals, it is important to be able to distinguish between them since they

will most likely have different dosages, efficacies and side effects (McConathy, Owens).

Whether or not one uses a drug of the single enantiomer of a racemic mixture often depends on

the desired drug effects. For example, if one enantiomer produces the therapeutic effects of the

drug while the other produces the undesired effects, a single enantiomer medication would be

preferred since it would be more effective. One example of this is the single enantiomer forms of

(S)-albuterol, which is used to treat asthma. The decision to pursue a racemic or single

enantiomer drug is still uncertain. However, as research continues, there is a greater probability

that single enantiomer drugs will receive more approval (McConathy, Owens).

Due to the increase usage of pharmaceuticals, researchers are currently performing

research about controlling the stereochemistry of chemical reactions in order to discover new

methods for the synthesis of chiral molecules in its enantiomerically pure form. Many

pharmaceutical companies today are making huge efforts toward examining their existing drugs

and analyzing whether or not synthesis of the drugs as single enantiomers would be most

effective. In order to produce the desired enantiomer, it is crucial that scientists promote chiral

separation and analysis of racemic drugs in the pharmaceutical industry. By doing so, we would

be able to eliminate the undesired enantiomer from the preparation and hopefully find an optimal

treatment for the patient in need.

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References

Autschbach, Jochen. (2016). Optical Rotation and Ellipticity. Wolfram Demonstrations Project.

Print.

Butt, R., Buchanan WW., Howard-Lock H., Kean WF, Lock CJ, Rischke J. (1989). Effect of R

and S enantiomers of naproxen on aggregation and thromboxane production in human

platelets. J Pharm Science. 78 (4):324-7. Print.

Carey, Francis and Guilianio, Robert. (2014). Organic Chemistry. 7 January 2013. Print.

Chhabra N., Aseri, M.L., and Padmanabhan D. (2013). A review of drug isomerism and its

significance. International Journal of Applied and Basic Medical Research. 3(1), 16-18.

Crossley, Roger. (1995). Chirality and the Biological Activity of Drugs. Boca Raton: CRC Press

Inc. Print.

Evans AM. (2001). Comparative Pharmacology of S (+)-ibuprofen and (RS)-ibuprofen. US

National Library of Medicine National Institutes of Health. Print.

Jasco. (2016). Detection of Chiral Drugs Using HPLC with CD Detection. Jasco Incorporation.

Print.

He, Hua, Nguyen, Lien Ai., and Pham-Huy, Chuong. (2006). Chiral Drugs: An Overview.

International Journal of Biomedical Science. 2 (2): 85-100. Print.

Lamparxzyk, Henry, Nowakowska, Joanna, and Zarzycki Pawel K. (1994). Effect of temperature

on separation of norgestral enantiomers by high-performance liquid chromatography.

Elsevier. 668 (2) 413-417. Print.

McConathy, Jonathon and Owens, Michael J. (2003). Stereochemistry in Drug Action. Primary

Care Companion to the Journal of Clinical Psychiatry. 5(2):70-73. Print.

Yip, Ashley. Thalidomide. PBWorks. 12 November 2010.