A protein molecule in its folded native state has one favored conformation, but when it is denatured it becomes a random coil, with very many possible conformations.
What must be the sign of the change in entropy (S) for the change native---> denatured
Would this be positive, since the protein is becoming more disordered?
Will the contribution of the change in S to the free energy change be positive or negative?
Would this be negative since a negative change in G indicates a spontaneous reaction, hence more disorder?
What requirement does this impose if proteins are to be stable structures?
I'm not sure about this answer
I think your answer to #1 is correct.
I think your answer to #2 is correct but I don't necessarily agree with your wording. I think the delta S indicates a more negative free energy; I don't know that I agree with the reverse of that.
I think the answer to #3 is that the protein must not be denatured (must remain in its native state).
In order to determine the sign of the change in entropy (ΔS) when a protein goes from its folded native state to a denatured random coil state, we need to consider the level of disorder in each state.
In the folded native state, the protein molecule has one favored conformation, which represents a more ordered arrangement. In the denatured state, however, the protein becomes a random coil with many possible conformations, indicating a higher level of disorder.
Since entropy is a measure of the degree of disorder in a system, the change in entropy (ΔS) for the native to denatured transition would be positive. So, your understanding that the protein becoming more disordered leads to a positive change in entropy is correct.
Now, let's consider the contribution of the change in entropy (ΔS) to the free energy change (ΔG). The equation for the Gibbs free energy change (ΔG) is ΔG = ΔH - TΔS, where ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.
Since we established that the change in entropy (ΔS) is positive, when this value is multiplied by the temperature (T), the result will also be positive. Thus, the contribution of the change in entropy (ΔS) to the free energy change (ΔG) will be negative, as the positive ΔS value leads to a subtractive term in the ΔG equation. Therefore, your understanding that the contribution of ΔS to ΔG is negative is accurate.
Finally, the requirement for proteins to maintain stability as structures is that their folded native states have lower free energy (ΔG) compared to their denatured states. Since a negative ΔG value indicates a spontaneous reaction, a stable protein would have a negative overall ΔG. This means that the protein's native state is more energetically favorable and more stable than its denatured state. The stability requirement for proteins ensures that they maintain their functional structures and perform their biological functions effectively.