chapter 5

Question 1

Which structural changes occur during heat-set gelling of a globular protein solution?
Explain these changes in terms of molecular interactions

Answer 1

• when the protein solution is heated, the proteins will denature
• because delta H decreases and the term -TS becomes more negative, delta G is more negative (denaturation)
• at high temperatures, hydrophobic interactions are increasing in strength -> aggregation of denatured proteins
• when these are present in concentrations high enough to create overlap, a gel is formed

Question 2

how come that at low ionic strength and a low pH the gel is a transparent gel

Answer 2

we have strong electrostatic repulsion between the protein molecules. When the solution is heated, the proteins self-
assemble into thin rods or semi flexible chains. Because the thickness of these chains is smaller than the wavelength of visible light, they do not scatter light, and the gel is therefore transparent

Question 3

why do the gels become more turbid when ionic strength is increased?

Answer 3

When the ionic strength
is increased, the repulsion decreases and branched or even random
aggregates are formed. The characteristic size of these aggregates is
bigger than the wave length of light, and therefore they scatter light.

Question 4

At a pH very far from the isoelectric point, a globular protein can denature without
heating. Explain this phenomenon in terms of molecular interactions, and discuss its
effect on the viscosity of the protein solution.

Answer 4

Far from the isoelectric point the globular protein is highly charged.
Electrostatic repulsion between the charges in the hydrophilic shell of the protein can cause the compact structure of the protein to unfold.
Since the structure of the protein is less compact after denaturation, the viscosity of
its solution will increase

Question 5

How does the viscosity of a solution of a random coil protein vary with pH? Explain your answer in terms of molecular interactions.

Answer 5

At the IEP the amount of positive charged and negative charged molecules will attract each other and the protein will be small which leads to a low viscosity.
Moving away from the IEP they start to repel each other due to their electrostatic repulsion. The protein will denature and take up more space, leading to a higher viscosity