STRUCTURE OF THALIDOMIDE OCOC COCO NCH OCOC NH H2CH2CC O C H2H2

STRUCTURES OF LIMONENE H3CH3C H C CH 2 CH 3 and CH 3 H C CH 2 H3CH3C

STRUCTURE OF VITAMIN A (all – trans - retinol) CH 3 H3CH3C CH C C CH 2 OH CH 3 STRUCTURE OF all – trans - RETINAL CH 3 CH CH C CH CH CH C CH C H CH 3 CH 3 O H 3 C CH 3

STRUCTURE OF AMINO ACIDS All amino acids have the general formula:- N C C H R O H H OH NOTE:- when R = H the amino acid GLYCINE (aminoethanoic acid) does not have a chiral centre and hence does not give rise to enantiomers.

STRUCTURE OF SALBUTAMOL H2CH2C HO CH OH CH 2 NHC CH 3 C OH CHIRAL CENTRE

Properties of enantiomers Thalidomide One isomer was used as a nausea suppressant for pregnant women (morning sickness) however the other isomer is a growth suppressant and led to babies being born without fully developed arms or legs.

Properties of enantiomers (continued) Limonene Enantiomers can taste differently due to the chiral nature of the taste bud receptors in the tongue. This can also affect the fragrence where one isomer of limonene smells of oranges and the other of lemons.

Salbutamol One of the enantiomers of salbutamol is 70 times more effective in the treatment of asthma and is used in the United States (i.e. the L form) however in the U.K. a racemic mixture is used. It is used mainly in the form of a nebuliser or inhaler as a relief for bronchospasm.

Amino acids The majority of D-amino acids are sweet while most L-amino acids are bitter. However, the D-form of proline is bitter and the L-form is borderline between bitter and sweet and this is thought to be due to the cyclic nature of the amino acid. The two dicarboxylic amino acids namely, glutamic and aspartic are both sour.

Retinal and vision All-trans-retinol which is vitamin A cannot be synthesised by the body and must be obtained through the diet. All-trans-retinol is converted to 11-cis retinol in two steps, both involving enzymes. Initially the alcohol group is oxidised to an aldehyde, then the double bond between C-11 and C-12 is isomerised from a trans to a cis configuration.

Retinal and vision (continued) The photosensitive molecule involved in vision is called rhodopsin which consists of a large protein called opsin, joined to 11-cis retinal. When a photon of light falls on rhodopsin the molecule absorbs the energy and the cis double bond between C-11 and C-12 in the retinal is temporarily converted into a single bond which swivels through 180 o and the double bond reforms in the trans configuration again.Thus the retinal has changed from a curved to a straight molecule i.e. the energy in the photon has been converted into atomic motion.

Retinal and vision (continued) These rapid movements of the retinal are transferred to the protein and from there into the lipid membrane and nerve cells to which it is attached. This generates nerve impulses which travel along the optic nerve to the brain and we perceive them as visual signals i.e. sight. The free all-trans-retinal is then converted back into the cis-form, reattaches to another opsin ready for the next photon to begin the process again.

Colour vision In order to see colour we have three types of visual receptors, sensitive to red, green and blue light but again the molecule involved is 11-cis-retinal. The different wavelength sensitivities are due to small variations in side groups attached to the opsin protein.