Answers to questions on Biological pigments

1. (a) In both acidic and alkaline solution cyanidin contains extensive conjugation. The delocalised pi electrons absorbs energy in the visible region of the spectrum as they are excited (to an empty orbital of higher energy, which is usually a non-bonding orbital) and
the colour observed is the transmitted complementary colour.

(b) There is greater conjugation in the structure of cyanidin in the alkaline solution (due to the ketone group) so it absorbs visible light energy of lower energy/lower frequency/longer wavelength. This means that it absorbs orange light and transmits the complementary colour of blue, whereas in the acidic solution it absorbs green light (which is higher in energy than orange) and transmits red.

2. (a) Porphyrin contains extensive conjugation of double bonds (i.e. alternate double and single bonds ) involving the delocalisation of pi electrons so absorbs visible light as electrons are excited to higher energy levels.

(b) Chlorophyll absorbs light in both the low (red) and high (blue) regions of the visible spectrum so transmits green light which lies in the middle of the visible spectrum.

(c) Two of nitrogen atoms can lose hydrogen ions so that they can form ionic bonds with metal ions and also they can use their non-bonding electrons to form coordinate compounds (i.e. act as ligands) with the metal ions.

3. (a) The interaction between haemoglobin and gaseous oxygen molecules is an example of cooperative binding. As the oxygen binds it alters the shape of the haemoglobin molecule, which increases its affinity to molecular oxygen. It reaches a maximum when all the active sites are saturated.

(b) Fetal haemoglobin is structurally different to adult haemoglobin and can bind oxygen more efficiently. This allows oxygen to diffuse from adult blood to fetal blood across the placenta and for the fetal blood to release a greater proportion of oxygen to developing tissues in the foetus even at relatively low partial pressures of oxygen. (c) Changing the pH and increasing the temperature can both cause structural changes to the haemoglobin making it less able to bind to oxygen.

4. (a) R_f is short for retention factor and is equal to the distance travelled by the sample (in this case the amino acid) divided by the distance travelled by the solvent (or eluent).

(b) Because the R_f values for all the 22 possible amino acids have not been determined the amino acids cannot be identified unequivocally. However it seems likely that the protein contains four different amino acids (unless two or more or them have identical R_f values). Three of them are likely to be cysteine and isoleucine and either histidine or lysine. It is possible that the mixture does in fact contain both histidine and lysine. The mixture also contains at least one amino acid that is not in the above list. It may also contain another amino acid that is not on the list but that has the same R_f value as one that is on the list.

(c) The TLC can be run again using a different ratio of components in the eluent. This will alter the polarity of the eluent and produce different R_f values. This will help to determine whether the mixture does only contain four different amino acids and also whether it contains either histidine or lysine or possibly both.

Answers to Biological pigments questions