What is the structure of glycine at pH 5?

1 Answer
Mar 30, 2017

Glycine will be almost entirely in the neutral form.

Explanation:

The structure of glycine is #"H"_3 stackrel("+")("N")"CH"_2"COO"^"-"#.

Glycine has two #"p"K_text(a)# values: #"p"K_text(a1) = 2.34# for the carboxylic acid and #"p"K_text(a2) = 9.60# for the amine.

At the isoelectronic point (#"p"I#), the structure is electrically neutral.

#"p"I = ("p"K_text(a1) + "p"K_text(a2))/2 = (2.34 + 9.60)/2 = 11.94/2 = 5.97#

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Below #"pH 5.97"#, glycine will have a net positive charge, while above #"pH 5.97"#, glycine will have a net negative charge.

Within #"±1.5 pH"# unit of #"pI"#, (i.e. from #"pH 4.5 to 7.5"#) glycine exists almost entirely in the neutral form (I calculated a net charge of +0.004 at #"pH 5"#).

I used Wolfram Alpha to calculate the net charge on glycine as a function of #"pH"#.

Here's what I got.

Q vs pH