Food Chemistry Chapter 17 in Green / Damjii

Homework

• Read F9– Texture pp. 488-490• Do Qs 43-50• on p 494

F.9: Texture

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)

24.2

achiral chiral

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

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 D-• If 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.

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 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 R-

enantiomer If the other three substituents decrease in a counterclockwise direction, it is

the S-enantiomer

1

2

3

4 4 3

2

1

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 (anti-depressant).

The top is (S)-citalopram

The bottom is (R)-citalopram

1

4

1

4

2

F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.

Enantiomers of mecoprop, 2-(4-chloro-2-methylphenoxy)propanoic acid – a herbicide

11

2

2

3

4

3

4

This molecule (on left side) needs to be re-oriented so the C-H bond is facing away from us

• 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

F.9.2: Distinguish between the properties of the different enantiomeric forms of stereoisomers found in food.

• 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

Carvoneforms 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

Limoneforms two mirror image forms or enantiomers: • R + (d) enantiomer - smells like orange• S – (l) enantiomer - smells like lemon

(R)-(+)-(E)-alpha-ionone

• violet-like• fruity• raspberry-like• flowery

( Yamamoto et al., 2009)

(S)-(-)-(E)-alpha-ionone –

• woody• cedar wood like• fresh juicy greenish flavor(aroma)

( Yamamoto et al., 2009)