Figure . H-bonding of aspartic acid NH controls the pKa shift. Schematic representation of H-bonding interaction of aspartic acid NH bond in Fmoc-AVD(NMe), Fmoc-GGD, and Fmoc-AVD assemblies. pKa change of aspartic acid residue depends strongly on the H-bonding interaction strength of the aspartic acid NH-bond, i.e., the stronger the H-bonding interaction, the higher the pKa value.