Final answer:
Amino acids cannot exist in a neutral form in nature due to ionization, which leads to the formation of zwitterions at physiological pH levels, where they exhibit both negative and positive charges.
Step-by-step explanation:
The neutral form of amino acids cannot exist in nature primarily because of ionization. Amino acids, when dissolved or in a physiological environment, exist predominantly as zwitterions, which are molecules that contain both a negatively charged group and a positively charged group. When amino acids are in a solution, such as in our bodies, they do not remain neutral due to the tendency of the amino group to donate a proton (becoming positively charged), and the carboxyl group to accept a proton (becoming negatively charged). This dual nature allows amino acids to act as buffers in biological systems, maintaining pH by either accepting or donating protons in response to changes in the environment.
The zwitterionic form of amino acids is more stable and represents how most amino acids exist at physiological pH. Moreover, it is due to this ionization that amino acids have high melting points, behaving more like inorganic salts rather than organic amines or acids in solid form.