Proteins are made up of amino acids, which are molecules with a carbon center bound to a hydrogen, amino group, carboxyl group, and a "R" group. The R group vareis between amino acids. The chemistry of the functional groups is what determines the properties of ionization. Some R groups are ionizable, while others are not. Some R groups are polar and uncharged, nonpolar, or charged. In a high pH environment, the common functional groups are usually deprotonated, which means the amino group is neutral and the carboxyl group has a negative charge. On the other hand, in a low pH environment, the common functional groups are usually protonated, meaning the amino group has a positive charge and the carboxyl group is neutral. However, at the neutral physiological pH, the amino group is usually protonated while the carboxyl group is deprotonated. This is how pH affects the surface charge of a protein. In low pH or acidic environments, the surface charge is more positive since the functional groups are protonated. In high pH or basic environments, the surface charge is more negative since since the functional groups of the amino acid are deprotonated. Some R groups ionize, meaning they have a conjugate acid and base form. This means that some amino acids have three ionizable groups instead of just two. This can be determined through titration, which is when a strong base is added to an acid. At a low pH, all the groups are protonated. As base is added and the pH increases, groups begin to deprotonate. Inflection points are visible, which are when enough base is added to react with half of the acid. Then at the equivalence point, enough base has been added to completely deprotonate the acid. Based on how many inflection and equivalence points there are, one can determine the number of ionizable groups present and determine the identity of the amino acid present in solution.
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