In this study, the relation between the number of positive (Y) or negative (X) charges of nucleic acid and the pH is derivedas Y=∑(i=1- 3)[H^+]~n~i/([H^+]+K~b~i) and X=∑(i=1-5)K~a~im~i/([H^+]+k~a~i) respectively. The two relation are also applicable to the proteins. The pI formula of nucleotide, various amino acids and bases can be derived from the relations. This shows that the relations are right and reliable. The net charges of Creatin Kinase at various pH calculated from the relations are confirmed by pretipitation of protein with surfactant, and electrophoresis. The variation of the number of charges of human DNA, silkworm DNA and β-lactoglobulia with pH is calculated.

Key words: NUCLEIC ACIDS, PROTEIN, ELECTRIC CHARGE NUMBER