Transcript PROPERTIES OF ENANTIOMERS IN FOOD

PROPERTIES OF ENANTIOMERS
IN FOOD
DİLVİN ÖZKAN
• Enantiomers can
clearly have different
biological effects. The
importance of this
with drugs was
tragically shown with
the drug thalidomide
in the last century.
1962 photo of a baby born to a mother who
had taken thalidomide while pregnant; the
baby has an extra appendage connected to
the foot and the malformation of the right
arm
• One enantiomer of the drug relieved morning
sickness in pregnant women; the other
enantiomer caused several defects in the
limbs of the unborn child.
• Foods, too, can have different effects. The Dform of vitamin C, for example, has no
biological activity.
• Different enantiomeric forms may vary in their
tastes, odour and toxicity.
• Most naturally occuring sugars exist in the Dform, whereas most naturally occurring amino
acids are in the L- form.
• D- amino acids tend to taste sweet, whereas Lforms often have no taste.
• Both caraway and dill seeds and spearmint
contain carvone, but the L- (+) form of carvone
in caraway and dill seeds tastes very different
from the D- (-) form of carvone in spearmint.
Carvone is a member of a family
of chemicals called terpenoids.
Carvone is found naturally in
many essential oils, but is most
abundant in the oils from seeds of
caraway (Carum carvi) and dill.
• Smells and tastes can appear different people,
because the olfactory receptors also contain
chiral receptor molecules, which can interact
differently with the enantiomeric molecules in
food.
• The natural flavour of raspberries is due to Rα- ionone, whereas synthetic raspberry
flavourings contain both R- and S- isomers.
Other synthetically made foods often contain
a racemic mixture of both enantiomers.
• The ionones are a series of closely related chemical substances that
are part of a group of compounds known as rose ketones. Ionones
are aroma compounds found in a variety of essential oils, including
rose oil. beta-Ionone is a significant contributor to the aroma of
roses, despite its relatively low concentration, and is an important
fragrance chemical used in perfumery. The ionones are derived
from the degradation of carotenoids.
• Orange and lemon peel each contain different
enantiomers of a compound called limonene.
The (+) enantiomer has the smell
characteristic of oranges and the (-) isomer
gives the characteristic smell of lemons.
• Limonene is a colourless liquid hydrocarbon classified
as a cyclic terpene possessing a strong smell of
oranges. It is used in chemical synthesis as a precursor
to carvone and as a renewably-based solvent in
cleaning products.
Interaction of chiral molecules with
polarized light
• Each enantiomer of a chiral structure reacts differently
with polarized light.
• Polarized light is shown above. It is light were only one
plane of polarization is chosen by a polarization filter
(slit).
• When this light is travelling through a chiral substance,
the plane of rotation will change.
• In the example above, R-(-)-carvone has a angle of
rotation of -72°, S-(+)-carvone one of +72 °.
A polaroid filter allows light through only if the light is
polarized at the same angle as the filter.
Why Polarized Light Is Affected
• So why do chiral molecules affect only polarized
light, and not unpolarized?
• Well, they do affect unpolarized light, but since the
rays have no particular orientation to one another,
the effect can not be observed or measured.
• We observe the polarized light rays being rotated
because we knew their orientation before passing
through the chiral substance, and so we can
measure the degree of change afterwards.
• What happens is this; when light passes through
matter, e.g. a solution containing either chiral or
achiral molecules, the light is actually interacting
with each molecule's electron cloud, and these very
interactions can result in the rotation of the plane of
oscillation for a ray of light.
Why Polarized Light Is Affected
• The direction and magnitude of rotation depends on
the nature of the electron cloud, so it stands to
reason that two identical molecules possessing
identical electron clouds will rotate light in the exact
same manner.
• This is why achiral molecules do not exhibit optical
activity.
• In a chiral solution that is not a racemic mixture,
however, the chiral molecules present in greater
numbers are configurationally equivalent to each
other, and therefore each possesses identical
electron clouds to its molecular twins.
• As such, each interaction between light and one of
these 'majority' molecule's electron clouds will result
in rotations of identical magnitude and direction.
Why Polarized Light Is Affected
• When these billions of billions of interactions are
summed together into one cohesive number, they do
not cancel one another as racemic and achiral
solutions tend to do - rather, the chiral solution as a
whole is observed to rotate polarized light in one
particular direction due to its molecular properties.
Enantiomers
• It is just such specificity that accounts for the optical
isomerism of enantiomeric compounds.
• Enantiomers possess identical chemical structures (i.e. their
atoms are the same and connected in the same order), but
are mirror images of one another.
• Therefore, their electron clouds are also identical but
actually mirror images of one another and not
superimposable.
• For this reason, enantiomeric pairs rotate light by the same
magnitude (number of degrees), but they each rotate plane
polarized light in opposite directions.
• If one chiral version has the property of rotating polarized
light to the right (clockwise), it only makes sense that the
molecule's chiral mirror image would rotate light to the left
(counterclockwise).
• Equal amounts of each enantiomer results in no rotation.
Mixtures of this type are called racemic mixtures, and they
behave much as achiral molecules do.
Difference between D and L
• Chiral (Greek, cheir = hand) molecule is not
superimposible on its mirror image while in
other case achiral molecule is superimposible
on its mirror image.
• The chirality of the molecule in nearly all
cases is appropriate to the presence of a
single chiral (asymmetric atom).
• A chiral atom is any tetrahedral atom with four
unlike groups attached to it and is also known
as chiral centre.
• Enantiomorphs exist only in case of chiral
molecules.
• The terms chiral and achiral are also used to
designate dissymmetric and non-disymmetric
molecules respectively.
Optical Activity of D and L
• The majority of the physical and chemical
properties of the optical isomers are equal.
• Since the occurrence of optical isomerism is
caused by the asymmetry of the molecules
the discovery of the difference in the
properties can be measured only when an
asymmetric measuring instrument is used.
• The most common of such tool is planepolarized light.
• Plane polarized light is defined as the light,
whose vibrations happen in a single plane
only is known as plane polarized light and
the phenomenon is known as polarization.
Optical Activity of D and L
• When plane polarized light is passed throughout
certain substances or their solutions its plane of
polarization is rotated moreover towards right
(clockwise) or towards left (anti-clockwise) by a
certain angle.
• The substances which rotate or turn the plane of
polarization of the plane polarized light are known as
optically active and the phenomenon is referred to
as optical activity.
• Those substances which rotate the plane polarized
light to right are called dextro-rotatory indicated by
the sign‘d’ or (+) and those which rotate to the left
are called levorotatory indicated by the sign ‘l’ or (-).
Optical Activity of D and L
• Fig 1: Example for d and l molecule of
C3H6O3 (Glyceraldehyde)
Difference between D and L
The extent of rotation depends on the following factors:
• Nature of the substance.
• Wavelength of the light used.
• Concentration of the solution (if the substance is taken in
solution).
• Thickness of the layer or length of path through which
polarized light passes.
• Nature of the solvent (for solutions only).
• Temperature of measurements.
• The apparatus used to measure the optical activity is
known as polarimeter and has been schematically
represented by the diagram.
• The optical inactivity could also arise due to the
presence of equal amounts of the dextro and levo forms
when the compound is prepared or obtained by mixing
the d and l forms in equal molecular proportions.
REFERENCES
• http://en.wikibooks.org/wiki/Organic_Chem
istry/Chirality/Optical_activity