1. Although the isoelectric points of dissolved cystine, tyrosine, and aspartic acid molecules lie at widely differing pH values, the isoelectric points of the surfaces of these substances in the crystalline state are all near pH 2.3. This was found to be true in solutions of hydrochloric acid and in acetate buffers of approximately constant ionic strength.

2. When suspended in gelatin, tyrosine and cystine crystals adsorb the protein and attain a surface identical in behavior with gelatin-coated quartz or collodion particles.

3. Aluminum ions at low concentrations reduce the electric mobilities of tyrosine crystals to zero in a manner analogous to their effect on other surfaces.

4. Alkyl benzene droplets also have their electric mobility reduced to zero at low pH values but, unlike the amino acids, a change in sign was never noticed.

5. The mobility of tyrosine crystals is independent of crystal length between 2โ€“100ยต. Below this size the mobilities are decreased.

6. These results are discussed in connection with the concept of the general definition of the isoelectric point and the behavior of certain insoluble proteins such as wool and silk fibroin.

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