Topic 19 · page 11 of 13

Amino acids and polarised light

Most amino acids have a chiral carbon, so a single enantiomer rotates the plane of plane-polarised monochromatic light. Glycine has no chiral carbon and does not, and a racemic mixture shows no net rotation.

12 min read19.11(ii)

Read first: Amino acids as acids and bases

Which amino acids are chiral 19.11(ii)

A carbon atom with four different groups attached is a chiral centre. A molecule with one chiral centre exists as two enantiomers: mirror images that cannot be superimposed (Topic 15).

In a 2-amino acid, RCH(NH₂)COOH, the carbon in the middle carries H, NH₂, −COOH\ce{-COOH} and R.

The two enantiomers of alanine, 2-aminopropanoic acid. The central carbon has four different groups: an amino group, a carboxylic acid group, a methyl group and H. The two molecules are mirror images that cannot be superimposed. Wedge bonds come towards the viewer and dashed bonds go away.
The two enantiomers of alanine. Reflect one in a mirror and you get the other.

Every 2-amino acid in proteins other than glycine has this chiral centre.

Plane-polarised light 19.11(ii)

Ordinary light vibrates in all planes at right angles to its direction of travel. A polariser lets through only the light that vibrates in one plane, which gives plane-polarised light. Monochromatic light has a single wavelength (one colour), such as the yellow light of a sodium lamp.

A racemic mixture has equal amounts of the two enantiomers. Their rotations cancel, so there is no net rotation.

Experiment: the polarimeter 19.11(ii)

A polarimeter. Monochromatic light from a sodium lamp passes through a polariser, which leaves light vibrating in one plane. The light then passes through a tube of amino acid solution. A chiral amino acid rotates the plane by an angle alpha. The analyser is turned until the light is brightest again, and the angle turned is the rotation. The angle drawn is illustrative.
A polarimeter. The analyser is turned to find the new plane; the angle turned is the rotation.

PracticalMeasuring optical rotation

  1. Shine monochromatic light (a sodium lamp) through a polariser. The light leaving it is plane-polarised.
  2. Pass the light through a tube of the amino acid solution.
  3. Look through the analyser, a second polariser that can be turned. With the sample absent (or a non-chiral sample), the analyser is set so that the light is brightest.
  4. Turn the analyser until the light is brightest again. The angle it has been turned through is the rotation.
  5. A chiral amino acid gives a non-zero angle. A non-chiral one gives an angle of zero.

Monochromatic light is used because the angle of rotation depends on the wavelength. With white light, each colour would rotate by a different angle and no single plane would be found.

Worked exampleInterpreting readings

Four solutions of 0.10 mol dm⁻³ are measured in the same tube (illustrative data). What does each reading show?

Sample Rotation
A +8.5°
B 0°
C −8.5°
mixture of equal volumes of A and C 0°
  1. A rotates the plane clockwise (positive) so it is optically active: one enantiomer of a chiral amino acid, such as alanine.
  2. C rotates by the same angle the other way, so it is the other enantiomer.
  3. The 1 : 1 mixture of A and C is a racemic mixture: the rotations cancel.
  4. B could be a non-chiral amino acid such as glycine, or a racemic mixture of a chiral one. A single reading cannot tell which.

Check yourself

  1. Draw 2-aminopropanoic acid and mark the chiral centre.
  2. Explain why glycine is not optically active.
  3. What is a racemic mixture? What reading does it give in a polarimeter, and why?
  4. State two features of the light used in a polarimeter and explain why each is needed.
  5. Describe how to measure the optical rotation of an alanine solution.
  6. A solution of one enantiomer rotates the light by +12.0°. What would you expect for the other enantiomer at the same concentration, and for an equal mixture of the two?

Answers cover these until you have tried the questions

  1. CH₃–C*H(NH₂)–COOH, with the central carbon (carbon 2) marked: it bonds to H, CH₃, NH₂ and −COOH\ce{-COOH}.
  2. Its central carbon bonds to two H atoms, so it does not have four different groups. It has no chiral centre, so it has no enantiomers and does not rotate plane-polarised light.
  3. A mixture of equal amounts of two enantiomers. It gives no net rotation (0°), because the equal and opposite rotations cancel.
  4. The light is plane-polarised so that a change of plane can be detected, and monochromatic because the angle of rotation depends on the wavelength, so one wavelength gives one definite angle.
  5. Pass monochromatic light through a polariser, then a tube of the solution, then an analyser. Turn the analyser until the light is brightest again. The angle turned is the rotation.
  6. The other enantiomer: −12.0°. The equal mixture (a racemate): 0°.