Transcript Folie 1 - FLI

-2010-
3D Structures of Biological Macromolecules
Chirality
Jürgen Sühnel
[email protected]
Leibniz Institute for Age Research, Fritz Lipmann Institute,
Jena Centre for Bioinformatics
Jena / Germany
Supplementary Material: www.fli-leibniz.de/www_bioc/3D/
Chirality
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and has a nonsuperposable mirror image. The feature that is most often the cause of chirality in molecules is the
presence of a so-called asymmetric carbon atom.
Two mirror images of a chiral molecule are called enantiomers or optical isomers. Pairs of enantiomers
are often designated as "right-" and "left-handed."
Molecular chirality is of interest because of its application to stereochemistry in inorganic
chemistry, organic chemistry, physical chemistry, biochemistry and supramolecular chemistry.
Isomers
Chirality – Asymmetric Carbon Atoms
An asymmetric carbon atom is a carbon atom that is attached to four different atoms or four different
groups of atoms.
Knowing the number of asymmetric carbon atoms, one can calculate the maximum possible number
of stereoisomers for any given molecule as follows:
If n is the number of asymmetric carbon atoms then the maximum number of isomers = 2n.
As an example, malic acid has 4 carbon atoms but just one of them is asymmetric:
An aldopentose with 3 asymmetric carbon atoms has 23 = 8 stereoisomers:
Chirality
Which of the following compounds would form enantiomers because the molecule is chiral?
Chirality - Alanine
Symmetry operation, Symmetry element
s
Reflection, Symmetry plane or Mirror plane
All amino acids are chiral except for glycine.
.
Chirality – Optical Active Compounds
Chiral compounds rotate the plane of polarized light. Each enantiomer will rotate the light in a
different sense, clockwise or counterclockwise. Molecules that do this are said to be optically active.
Chirality – Optical Active Compounds
a - specific rotation
Chirality – Optical Active Compounds
Because many optically active chemicals are stereoisomers, a polarimeter can be used to
identify which isomer is present in a sample – if it rotates polarized light to the left, it is a
levo-isomer, and to the right, a dextro-isomer.
Concentration and purity measurements are especially important to determine product or
ingredient quality in the food & beverage and pharmaceutical industries. Samples that
display specific rotations that can be calculated for purity with a polarimeter include:
Steroids, Diuretics, Antibiotics, Narcotics, Vitamins, Analgesics, Amino Acids,
Essential Oils, Polymers, Starches, Sugars.
Chirality – Molecules of Life
Many biologically active molecules are chiral, including the naturally occurring amino acids (the
building blocks of proteins) and sugars.
In biological systems, most of these compounds are of the same chirality: most amino acids are L and
sugars are D. Typical naturally occurring proteins, made of L amino acids, are known as left-handed
proteins, whereas D amino acids produce right-handed proteins.
Chirality: R/S Naming System
By configuration: R and S.
Each chiral center is labeled as R or S according to a system by which its substituents are each assigned a priority,
according to the Cahn-Ingold-Prelog priority rules (CIP), based on atomic number. If the center is oriented so that the
lowest-priority of the four is pointed away from a viewer, the viewer will then see two possibilities: If the priority of the
remaining three substituents decreases in clockwise direction, it is labeled R (for Rectus), if it decreases in
counterclockwise direction, it is S (for Sinister).
This system labels each chiral center in a molecule (and also has an extension to chiral molecules not involving chiral
centers). Thus, it has greater generality than the d/l system, and can label, for example, an (R,R) isomer versus an (R,S) —
diastereomer.
The R / S system has no fixed relation to the (+)/(−) system. An R isomer can be either dextrorotatory or levorotatory,
depending on its exact substituents.
The R / S system also has no fixed relation to the d/l system. For example, the side-chain one of serine contains a hydroxyl
group, -OH. If a thiol group, -SH, were swapped in for it, the d/l labeling would, by its definition, not be affected by the
substitution. But this substitution would invert the molecule's R / S labeling, because the CIP priority of CH2OH is lower
than that for CO2H but the CIP priority of CH2SH is higher than that for CO2H.
For this reason, the d/l system remains in common use in certain areas of biochemistry, such as amino acid and
carbohydrate chemistry, because it is convenient to have the same chiral label for all of the commonly occurring
structures of a given type of structure in higher organisms. In the d/l system, they are nearly all consistent - naturally
occurring amino acids are nearly all l, while naturally occurring carbohydrates are nearly all d. In the R / S system, they are
mostly S, but there are some common exceptions.
Chirality: Cahn-Ingold-Prelog Priority Rules
1.
Compare the atomic number (Z) of the atoms directly attached to the stereocenter; the group having the
atom of higher atomic number receives higher priority.
2.
If there is a tie, we must consider the atoms at distance 2 from the stereocenter—as a list is made for
each group of the atoms bonded to the one directly attached to the stereocenter. Each list is arranged in
order of decreasing atomic number. Then the lists are compared atom by atom; at the earliest difference,
the group containing the atom of higher atomic number receives higher priority.
3.
If there is still a tie, each atom in each of the two lists is replaced with a sub-list of the other atoms
bonded to it (at distance 3 from the stereocenter), the sub-lists are arranged in decreasing order of
atomic number, and the entire structure is again compared atom by atom. This process is repeated, each
time with atoms one bond farther from the stereocenter, until the tie is broken.
After the substituents of a stereocenter have been assigned their priorities, the molecule is so oriented in space that
the group with the lowest priority is pointed away from the observer. If the substituents are numbered from 1
(highest priority) to 4 (lowest priority), then the sense of rotation of a curve passing through 1, 2 and 3 distinguishes
the stereoisomers. A center with a clockwise sense of rotation is an R or rectus center and a center with a
counterclockwise sense of rotation is an S or sinister center. The names are derived from the Latin for right and left,
respectively.
Chirality: Cahn-Ingold-Prelog Priority Rules
Chirality: Cahn-Ingold-Prelog Priority Rules
4
1
3
1
2
4
1
3
2
1
R/S assignments for several compounds
The hypothetical molecule bromochlorofluoroiodomethane shown in
2
its R-configuration would be a very simple chiral compound. The
priorities are assigned based on atomic number (Z): iodine (Z = 53)
C
> bromine (Z = 35) > chlorine (Z = 17) > fluorine (Z = 9). Allowing
R fluorine (lowest priority) to point away from the viewer the rotation is
3
clockwise hence the R-assignment.
In the assignment of L-serine highest priority is given to the nitrogen
atom (Z = 7) in the amino group (NH2). Both the methylalcohol group
(CH2OH ) and the carboxylic acid group (COOH) have carbon atoms (Z
3
= 6) but priority is given to the latter because the carbon atom in the
C
COOH group is connected to a second oxygen (Z=8) whereas in the
CH2OH group carbon is connected to a hydrogen atom (Z=1). Lowest
priority is given to the hydrogen atom and as this atom points away
2
the viewer the counterclockwise decrease in priority over the
L from
three remaining substituents completes the assignment as S.
The stereocenter in S-carvone is connected to one hydrogen atom
(not shown, priority 4) and three carbon atoms. The isopropene
group has priority 1 (carbon atoms only) and for the two remaining
C
carbon atoms priority is decided with the carbon atoms two bonds
removed from the stereocenter, one part of the keto group (O,O,C
priority 2) and one part of an alkene (H,C,C priority 3). The resulting
counterclockwise rotation results in a S assignment.
Chirality: Cahn-Ingold-Prelog Priority Rules
Chirality: d/l Naming System Naming System
By configuration: d- and l.
An optical isomer can be named by the spatial configuration of its atoms. The d/l system does this by relating the
molecule to glyceraldehyde. Glyceraldehyde is chiral itself, and its two isomers are labeled d and l (typically typeset
in small caps in published work). Certain chemical manipulations can be performed on glyceraldehyde without
affecting its configuration, and its historical use for this purpose (possibly combined with its convenience as one of
the smallest commonly used chiral molecules) has resulted in its use for nomenclature.
In this system, compounds are named by analogy to glyceraldehyde, which, in general, produces unambiguous
designations, but is easiest to see in the small biomolecules similar to glyceraldehyde.
One example is the amino acid alanine, which has two optical isomers, and they are labeled according to which
isomer of glyceraldehyde they come from. On the other hand, glycine, the amino acid derived from glyceraldehyde,
has no optical activity, as it is not chiral (achiral). Alanine, however, is chiral.
The d/l labeling is unrelated to (+)/(−); it does not indicate which enantiomer is dextrorotatory and which is
levorotatory.
Rather, it says that the compound's stereochemistry is related to that of the dextrorotatory or levorotatory
enantiomer of glyceraldehyde—the dextrorotatory isomer of glyceraldehyde is, in fact, the d-isomer. Nine of the
nineteen l-amino acids commonly found in proteins are dextrorotatory (at a wavelength of 589 nm), and d-fructose
is also referred to as levulose because it is levorotatory.
A rule of thumb for determining the d/l isomeric form of an amino acid is the "CORN" rule. The groups:
COOH, R, NH2 and H (where R is a variant carbon chain)
are arranged around the chiral center carbon atom. Sighting with the hydrogen atom away from the viewer, if these
groups are arranged clockwise around the carbon atom, then it is the d-form. If counter-clockwise, it is the l-form.
Chirality: (+)/(-) Naming System Naming System
By optical activity: (+)- and (−).
An enantiomer can be named by the direction in which it rotates the plane of polarized light. If it rotates
the light clockwise (as seen by a viewer towards whom the light is traveling), that enantiomer is labeled (+).
Its mirror-image is labeled (−).
The (+) and (−) isomers have also been termed d- and l-, respectively (for dextrorotatory and levorotatory).
Naming with d- and l- is easy to confuse with d- and l- labeling and is therefore strongly discouraged
by IUPAC.
Chirality – Circular Dichroism
When circularly polarized light passes through an absorbing optically active medium, the speeds between
right and left polarizations differ (cL ≠ cR) as well as their wavelength (λL ≠ λR) and the extent to which they
are absorbed (εL≠εR). Circular dichroismi s the difference Δε ≡ εL- εR.
Usually, the so-called absorbance difference
is measured. It can also be expressed, by applying Beer's law, as:
Where εL and εR are the molar extinction coefficients for LCP and RCP light, C is the molar
concentration and l is the path length in centimeters (cm).
Then
is the molar circular dichroism. This intrinsic property is what is usually meant by the circular dichroism
of the substance. Since Δε is a function of wavelength, a molar circular dichroism value (Δε) must
specify the wavelength at which it is valid.
Chirality – Circular Dichroism
Although the absorbance difference is usually measured, for historical reasons most measurements are
reported in degrees of ellipticity. Molar circular dichroism and molar ellipticity, [θ], are readily interconverted
by the equation:
Methods for estimating secondary structure in polymers, proteins and polypeptides in particular, often
require that the measured molar ellipticity spectrum be converted to a normalized value, specifically a value
independent of the polymer length. Mean residue ellipticity is used for this purpose; it is simply the
measured molar ellipticity of the molecule divided by the number of monomer units (residues) in the
molecule.
Secondary structure can be determined by CD spectroscopy in the "far-UV" spectral region (190-250 nm).
At these wavelengths the chromophore is the peptide bond, and the signal arises when it is located in a
regular, folded environment. Alpha-helix, beta-sheet, and random coil structures each give rise to a
characteristic shape and magnitude of CD spectrum.
Like all spectroscopic techniques, the CD signal reflects an average of the entire molecular
population. Thus, while CD can determine that a protein contains about 50% alpha-helix, it cannot
determine which specific residues are involved in the alpha-helical portion.
Chirality : Poly-Lysine CD Spectrum
Chirality : Property Differences of Stereoisomers
Two chiral objects that are mirror images of each other behave identically in achiral
environments. Therefore, enantiomers can only be distinguished in chiral environments.
Enantiomers have identical physical properties in almost every regard except one: their
ability to rotate plane- polarized light, or optical activity. When plane-polarized light is
passed through a solution containing chiral compounds, the plane is rotated by a number
of degrees depending on the nature of the molecules in solution. Enantiomers have equal
but opposite optical rotations.
Chirality : Property Differences of Stereoisomers - Thalidomide
Thalidomide is a sedative drug that was prescribed to pregnant women, from 1957 into the
early 60's. It was present in at least 46 countries under different brand names. "When
taken during the first trimester of pregnancy, Thalidomide prevented the proper growth of
the foetus, resulting in horrific birth defects in thousands of children around the world".
Why? The Thalidomide molecule is chiral. There are left and right-handed Thalidomides,
just as there are left and right hands. The drug that was marketed was a 50/50 mixture.
One of the molecules, say the left one, was a sedative, whereas the right one was found
later to cause foetal abnormalities. "The tragedy is claimed to have been entirely avoidable
had the physiological properties of the individual thalidomide [molecules] been tested
prior to commercialization."
Chirality : Property Differences of Stereoisomers - Aspartame
Aspartame is a sweetening agent that is more than a hundred times sweeter
than sucrose. And yet, the mirror image molecule is bitter. "(S)-carvone
possesses the odor perception of caraway while [the mirror image molecule]
(R)-carvone has a spearmint odor [2]."These examples are just the tip of the
iceberg. DNA, proteins, amino acids, sugars are all chiral. Mirror image amino
acids are called L- and D-aminoacids. Human proteins are exclusively built
from L-aminoacids. The origin of this fundamental dissymmetry is still
mysterious. When interacting, molecules recognize each other just as your
right hand distinguishes another right hand from a left when you shake hands.
This is why mirror image molecules, like mirror image Thalidomides, so often
have radically different fates in our bodies.Drug synthesis is an enormous
worldwide market. As a consequence, issues related to chirality have
gradually pervaded chemical research. This background is to be kept in mind
when appreciating the importance of chirality, whether in science or in
everyday life.
Chiral (Asymmetric) Synthesis
Asymmetric
synthesis,
also
called
chiral
synthesis,
enantioselective
synthesis or stereoselective synthesis, is organic synthesis that introduces one or more
new and desired elements of chirality. This is important in the field of pharmaceuticals
because the different enantiomers or diastereomers of a molecule often have different
biological activity.
Chirality must be introduced to the substance first. Then, it must be maintained. Usually,
chiral products are formed in racemic 50%/50% mixtures.
These mixtures can be separated by physico-chemical methods, for example by chiral
chromatography.
Chiral (Asymmetric) Synthesis
The oldest asymmetric synthesis is the enantioselective decarboxylation of the malonic acid 2ethyl-2-methylmalonic acid mediated by brucine (forming the salt) as reported by Willy
Marckwald in 1904.
One method is the usage of metal ligand complexes derived from chiral ligands. This method
was pioneered by William S. Knowles and Ryōji Noyori (Nobel Prize in Chemistry 2001). Knowles
in 1968 replaced the achiral triphenylphosphine ligands in Wilkinson's catalyst by the chiral
phosphine ligands P(Ph)(Me)(Propyl), thus creating the first asymmetric catalyst.