Food Chemistry
Chapter 17 in Green / Damjii
F.9: Texture
Homework
• Read F9– Texture pp. 488-490
• Do Qs 43-50
• on p 494
F.9.1: Explain the three different conventions
used for naming the different enantiomeric
forms.
Types of Isomerism – II cont.
• Stereoisomers – Enantiomers
(aka Optical Isomers)
• Requires a molecule with a chiral center
– An atom (usu C) with four different groups attached to it
– aka an asymmetric carbon atom
• Two forms are mirror images of one another that
cannot be superimposed on each other (like gloves)
achiral
chiral
24.2
Types of Isomerism – II cont.
• Stereoisomers – Enantiomers
• Common example are amino acids
– only glycine does not have an enantiomeric form
Types of Isomerism – II cont.
• Stereoisomers – Enantiomers
• Common example are amino acids
– only glycine does not have an enantiomeric form - why?
Types of Isomerism – II cont.
Thalidomide
• a drug once prescribed to counteract pregnancy-related morning
sickness, is an effective sedative as the R enantiomer (left side), but
the S enantiomer (right side) is a potent teratogen (it causes birth
defects).
Source - http://www.answers.com/topic/asymmetric-synthesis-1
Types of Isomerism – II cont.
• Stereoisomers – Enantiomers
• have different effects on polarised light
• said to be optically active in a polarimeter
• if a mixture has both enantiomers it is said to be a
racemic mixture and is not optically active
F.9.1: Explain the three different conventions
used for naming the different enantiomeric
forms.
CONVENTION 1:
– dextrorotary [ + or (d) ] – clockwise rotating
enantiomer; positive specific rotation value
– laevorotatory [ - or (l) ] – counter clockwise rotating
enantiomer; negative specific rotation value
Does differentiate between the 2 enantiomers… but does
NOT indicate absolute configuration (spatial
arrangement).
F.9.1:
Explain the three different conventions used for
naming the different enantiomeric forms.
CONVENTION 2:
D, L (no relation to d and l )
– older convention… used mostly for sugars and amino
acids
– Most naturally occurring sugars are in the D-form !
(and taste sweet)
– Most amino acids are in the L-form ! (and are
tasteless)
– Enantiomers that don’t occur naturally are typically
NOT metabolized by our bodies.
F.9.1:
Explain the three different conventions used for
naming the different enantiomeric forms.
CONVENTION 2: D, L (no relation to d and l )
– For sugars… absolute configuration related to
glyceraldehyde
•
•
•
•
Locate chiral center
Orient aldehyde group away from you
If OH is on right, the sugar is DIf OH is on left, the sugar is L-
– Most naturally occurring sugars are in
the D-form ! (and taste sweet)
F.9.1:
Explain the three different conventions used for
naming the different enantiomeric forms.
CONVENTION 2: D, L (no relation to d and l )
– For amino acids… use “CORN” rule
•
•
•
Locate chiral center and orient C-H bond away from you
If the groups COOH, R, NH2 are arranged clockwise around
the chiral carbon the amino acid is the D-form
If the groups COOH, R, NH2 are arranged counterclockwise
around the chiral carbon the amino acid is the L-form
– Most amino acids are in the L-form !
(and are tasteless)
F.9.1:
Explain the three different conventions used for
naming the different enantiomeric forms.
CONVENTION 3:
R, S
– Newer convention… used for most other
compounds
– aka CIP system – named after originators: Cahn,
Ingold, and Prelog
F.9.1:
Explain the three different conventions used for
naming the different enantiomeric forms.
SYSTEM 3: R,S


Locate chiral center
Rank atoms bonded to the chiral carbon in order of increasing
atomic number. (One through four, one being the group of highest
priority… highest atomic number).



H < C < N < O < F < Cl < Br
If there are two or more atoms with same atomic number the
second atoms are used to rank the substituents… then the third…
etc.
Orient the molecule so that the lowest ranking (4) substituent
points away from you.


If the other three substituents decrease in a clockwise direction, it is the
R-enantiomer
If the other three substituents decrease in a counterclockwise direction,
it is the S-enantiomer
F.9.1:
Explain the three different conventions used for
naming the different enantiomeric forms.

Orient the molecule so that the lowest ranking substituent points away from you.
 If the other three substituents decrease in a clockwise direction, it is the Renantiomer
 If the other three substituents decrease in a counterclockwise direction, it is
the S-enantiomer
F.9.1:
Explain the three different conventions used for
naming the different enantiomeric forms.

Orient the molecule so that the lowest ranking substituent points away from you.
 If the other three substituents decrease in a clockwise direction, it is the Renantiomer
 If the other three substituents decrease in a counterclockwise direction, it is
the S-enantiomer
F.9.1:
Explain the three different conventions used for
naming the different enantiomeric forms.
(S)-(+)-lactic acid (left) and
(R)-(–)-lactic acid (right)
F.9.1:
Explain the three different conventions used for
naming the different enantiomeric forms.
Enantiomers of
citalopram (antidepressant).
The top is (S)citalopram
The bottom is (R)citalopram
F.9.1:
Explain the three different conventions used for
naming the different enantiomeric forms.
This molecule
(on left side)
needs to be reoriented so the
C-H bond is
facing away
from us

Enantiomers of mecoprop, 2-(4-chloro-2-methylphenoxy)propanoic acid – a
herbicide
F.9.2: Distinguish between the properties of
the different enantiomeric forms of
stereoisomers found in food.
• Natural flavors tend to be pure enatiomers –
biosynthesis is stereospecific
• Synthetic flavors tend to be racemic mixtures
Alpha-ionone (found in raspberries)
Natural = R-alpha-ionone
• Stereoisomers – Enantiomers
• Usually have similar physical properties
– except when they interact with other optically active
substances – which happens often in the human body
• Usually have similar chemical properties
– except when they interact with other optically active
substances – which happens often in the human body
– general, only one enantiomer of a drug, agrochemical
(herbicide, pesticide), flavoring agent, or other molecule
(when asymmetric) has the desired biological effect,
while the other enantiomer has very different effects or,
at least, places a metabolic burden on the body.
Types of Isomerism – II cont.
amino acid asparagine - taste
• one form tastes bitter, the other tastes sweet
• each of the two enantiomers binds differently
to chemoreceptors in the tongue.
Source - http://www.answers.com/topic/asymmetric-synthesis-1
Carvone
forms two mirror image forms or enantiomers:
•
•
R-(–)-carvone smells like spearmint
(happens to be laevorotary)
S-(+)-carvone, smells like caraway seeds.
– each of the two enantiomers binds differently to
chemoreceptors in the nose
Limone
forms two mirror image forms or
enantiomers:
• R + (d) enantiomer - smells like orange
• S – (l) enantiomer - smells like lemon
(R)-(+)-(E)-alpha-ionone
(S)-(-)-(E)-alpha-ionone –
•
•
•
•
• woody
• cedar wood like
• fresh juicy greenish flavor(aroma)
violet-like
fruity
raspberry-like
flowery
( Yamamoto et al., 2009)
( Yamamoto et al., 2009)