The uncommon amino acid selenocysteine has an r group with the structure - ch2-seh (pka " 5). In an aqueous solution, ph = 7.0, selenocysteine would:

In a pH 7-solution Selenocysteine has lost the proton bond to Selenium (Se-)

Explanation:

The amino acid-pka is the pH value where the proton in HSe changes from free to a bond with Selenium:

-at pHs lower than 5: Selenium is protonated (HSe)

- at pHs higher than 5: Selenium has a negative charge and the proton has been released into the solution. This is what happens in water (pH 7).

Besides, at pH 7, the carboxyl group (COOH) has probably lost its proton (H) : and has a negative charge (COO-); and the amino group is not protonated yet (this happens at lower pHs) : the group is NH2. Then, the complete amino acid has two negative charges.