Calculating the net charge of amino acids and proteins from Henderson-Hasselbalch equations and pKa values is a foundational chemical principle taught in introductory biochemistry.
The claim states a fundamental biochemical relationship: the ionization state and net charge of amino acid residues can be determined directly from the pH, the pKa values, and the Henderson-Hasselbalch relation. Because this is a matter of basic definition and textbook calculation taught in standard chemistry curricula, it represents common knowledge and does not require citation of specific research papers.