Stereochemistry Of Biomolecules

Question 0

Naming enantiomers using R and S

Answer 0

1,2,3 Clockwise, group 4 back, R
Counterclockwise back S
Clockwise Front, S
Counterclockwise front R

Question 1

Constitutional isomers
Conformations
Stereoisomers

Answer 1

Different sequence of atoms

Staggered (60 degrees) vs eclipsed (0 degrees) can be converted to each other by rotation around C-C bond

Stereoisomers 
    Chiral carbons (R/S) 
    Double bond (CIS/trans) 

Rotation around a double bond does not occur because that would break the pi cloud formed by overlap of p orbitals. CIS/trans cannot intercom very

Question 2

Polarimeter d vs l for whole molecules

Answer 2

Polarimeter is a device that analyzes rotation of plane-polarized light by asymmetric molecules. Light leaving a source includes waves oriented in many directions. The polarizer only transmits those waves whose planes are oriented in a particular direction. The light is called plane polarized light and this can be rotated by molecules containing one or more chiral carbons. Molecules causing rotation in the direction marked + are called dextrorotatory (d) and molecules rotating to the left are l. ONLY MOLECULES WITH CHIRAL CARBONS

Question 3

Why polarimeter doesn’t work for R S

Answer 3

For a molecule with only one asymmetric carbon and a particular configuration it is not very easy to predict whether a sample of R enantiomers will cause right or left rotation of light of a particular wavelength. Some may cause d or l.

R and S do have equal but opposite effects.

d and l describe one whole molecule while R,S,D,L will describe each individual asymmetric carbon

Question 4

Bio sugars: D at chiral carbon furthest from c=o

Answer 4

Arrangement of four groups in d-glyceraldrhyde was called D arrangement and arrangement of four groups in l was called L arrangement.

Question 5

Aldoses and ketoses

Answer 5

Two main class of sugars

Aldoses contain aldehyde group
All biologically occurring aldoses resemble D-glyceraldehyde
D, L systems ony apply to asymmetric carbon furthest from carbonyl group

Ketoses contain ketone group

Question 6

Molecules having several chiral carbons

Answer 6

Each chiral carbon can be R or S.

2^n
N= number of asymmetric carbons in molecule

Question 8

Biological amino acids: L at backbone carbon

Answer 8

D, L used for ONLY naming chiral carbon occurring in backbone of amino acids.

Biological amino acids are L at the backbone carbon (19/20 of the amino acds) and biological sugars are D at the asymmetric carbon farthest away from the C=O group.

Question 9

d vs l, R vs S and D vs L DO NOT correlate with each other

Answer 9

18 of the 19 L amino acids are S. Cysteine is the one example of a L amino acid which is R. The sidechain has a sulfur which gives it a higher priority.

Question 10

Bonds in the alpha helix

Answer 10

Helical structure found in many proteins.

When a string of amino acids twists in a repetitive pattern that repeats over and over at each successive amino acid in the string, that protein generates a helix. Many H bonds are seen joining backbone NH groups to backbone C=O groups.

Question 11

Stereospecific binding

Answer 11

For a chiral substrate molecule having two forms (R and S) only one of those forms will be able to make all three potential contacts with the enzyme surface.

Question 12

Stereoisomers: thalidomide and aspartame

Answer 12

Aspartame: L is sweet while D is bitter

Thalidomide: birth defect caused by wrong isomer