In proteins, the alcohol group of serine is generally more difficult to deprotonate than the thiol group of cysteine.
Serine and cysteine respectively:
In the literature, the explanation given is that the sulfur atom is larger than the oxygen atom, and therefore the resultant negative charge is stabilised over a larger surface.
But I am confused. Oxygen is a more electronegative atom than sulfur and therefore can more stably carry a negative charge. How are these two assertions compatible?
- oxygen can carry negative charge more easily because it is more electronegative, and
- sulfur can carry negative charge more easily because it is larger?
Clearly, they cannot both be true and in the case of these two amino acids, the size of the sulfur prevails over the electronegativity of the oxygen. Why is that? Why in some circumstances is it the other way around?