At an intermediate pH value, an amino acid acts as a dipolar ion, or zwitterion, which has no net charge. This pH is the isoelectric point (pI), which varies according to the R group involved.

• 在pH值适中的时候,氨基酸可作为偶极离子,或两性离子,它不带电。这个pH值叫等电点,它随着R基团的不同而不同。