Questions on Biological pigments

1. Cyanidin is an anthocyanin found in berries and other fruits. The diagram below shows the structure of cyanidin in acidic solution and in alkaline solution.

(a) Explain why cyanidin is coloured in both acidic and alkaline solution.

(b) Explain why the colour of cyanidin changes from red in acidic solution (pH < 3) to blue when it is in alkaline solution (pH > 11).

2. Many biological pigments (e.g. haem and chlorophyll) contain porphyrins. The basic structure of porphyrin is shown below.

(a) The name porphyrin comes form the Greek word porphyros meaning purple. Explain why pophyrins are coloured compounds.

(b) The UV-vis absorption spectrum of chlorophyll a (which is present in plant leaves) is shown below.

Explain why leaves are green.

(c) Explain how porphyrins are able to form complexes with metal ions such as Fe²⁺ in haem and Mg²⁺ in chlorophyll.

3. The graph below shows how the percentage saturation of haemoglobin with oxygen varies according to the partial pressure of oxygen.

(a) Explain why the affinity of haemoglobin for oxygen increases as the partial pressure of oxygen is increased until it reaches a maximum.

(b) Explain the significance of fetal haemoglobin having a higher affinity for oxygen than adult haemoglobin.

(c) Suggest why a change in pH and an increase in temperature can affect the affinity of haemoglobin for oxygen.

4. Part of a protein was hydrolysed to release the individual amino acids contained in the protein. Thin layer chromatography was carried out using a silica gel plate in a solvent of butan-1-ol, ethanoic acid and water in the ratio of 3:1:1 by volume. The plate was spotted with individual samples of the amino acids: alanine, cysteine, glycine, histidine, isoleucine, and lysine and the sample from the protein. After elution the plate was developed using a spray of ninhydrin and the R_f values measured. The following results were obtained.

(a) Define the term R_f.

(b) Analyse the results to determine the possible composition of the hydrolysed protein.

(c) Design a separate experiment, using only the existing chemicals, to help to clarify the composition of the hydrolysed protein further.

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