What is the pKa value of aspartate's R-group?

The pKa value of aspartate's R-group, which contains a carboxylic acid functional group, is approximately 3.9. This value indicates that the side chain of aspartate can donate a proton (H+) at physiological pH, making it negatively charged at this pH level. This negative charge plays a critical role in the function of proteins, particularly in enzyme active sites and in protein-protein interactions, as it can engage in hydrogen bonding and electrostatic interactions.

At pH levels below the pKa, the carboxylic acid group is more likely to be protonated and hence exists primarily in its neutral form. However, at pH levels above the pKa, the group will predominantly exist in its deprotonated form, which is important for the overall charge and reactivity of the amino acid in various biochemical contexts.

The choices provided include values that may correspond to other amino acids or functional groups. However, the value of approximately 3.9 distinctly pertains to aspartate, confirming its unique role in biochemical processes as influenced by its side chain's acidity.