Lec 5 Ravi

Question 1

What is circular dichroism?

Answer 1

Circular dichroism occurs when a molecule differentially absorbs left and right circularly polarised light

Question 2

Light produced by natural sources are…

Answer 2

unpolarised

Question 3

What is unpolarised light?

Answer 3

Unpolarised light waves are randomly orientated around the beam axis.

Question 4

What are polarized filters made of?

Answer 4

Polarization filters are made of long chains of organic molecules, arranged in parallel to each other.

Question 5

How do polarizing filters polarise light?

Answer 5

When light passes through the filter, the filter absorbs components of electric fields that are parallel to the direction in which the organic molecules are arranged.

Question 6

What is polarised light?

Answer 6

light that comes out would have its electric field oscillating along one direction = polarised.

Question 7

How do polarising sunglasses work?

Answer 7

Polaroid sunglasses make use of polarizing filters. The light coming from glares off the roads or from a water surface is polarized.
Since polaroid sunglasses have a polarizing filter, when you look at a glare through these sunglasses, a significant portion of the light coming from the “glare” is unable to travel through the sunglasses and reach your eyes.

Question 8

What is linerarly polarised light?

Answer 8

• just up and down or side to side light

Question 9

By using a second filter in a linear polariser, what does that do?

Answer 9

causes a quarter wave retardation, the light becomes circularly polarised

Question 10

What is a quarter-wave retardation plate?

Answer 10

• a sheet of birefringent (double refracting) material 1 of thickness such that horizontally and vertically polarized light entering in phase will emerge from the retardation plate 1/4 of a wavelength out of phase.

Question 11

What is circularly polarised light?

Answer 11

• spirals round in a circle

- can be left or right handed depending on whether the tip is travelling clockwise or anti-clockwise.

Question 12

Elliptically polarized light is…

Answer 12

If a molecule differentially absorbs left and right circularly polarized light ….
L & R beams have different amplitudes
The resulting light is elliptically polarised

Question 13

Requirements for a molecule to create circular dichroism

Answer 13

• Circular dichroism occurs when a molecule differentially absorbs left and right circularly polarised light.
• Ability to absorb light (chromophore)
• Asymmetric / contain CHIRAL residues
• Contain a peptide bond
• Contain aromatic residues in asymmetric environments
• Contain DNA bases in asymmetric environments

Question 14

Properties of a chiral molecule?

Answer 14

Chiral (handedness) molecules have pairs of optical isomers or enantiomers
Non-superimposable mirror images
Called L and D enantiomers

Question 15

Many biomolecules are…

Answer 15

chiral

• E.g. most amino acids are L-enantiomers
• E.g. most monosaccharides are D-enantiomers

Question 16

Example of why chirality is important for structure, shape and functional properties of biomolecules?

Answer 16

E.g. L-amino acids result in right handed alpha-helices in proteins, D-amino acids result in left handed alpha-helices.

Question 17

How do enantiomers of an optically active molecule interact with left and right polarised light?

Answer 17

• L: Laevorotation (rotates to the left)
• D: Dextrorotation (rotates to the right)
• A 50:50 mixture of the enantiomers does not rotate the plane of the light at all.