?

Biochemistry

Free Version

Upgrade subject to access all content

Difficult

Determining Overall Peptide Charge at Different pH Values

BIOCHM-OXXJBB

The overall net charge on a peptide will vary depending upon several factors, including the pH of the surrounding environment. Each free amino acid contains an ionizing alpha-amino and alpha-carboxyl group and several amino acids contain an R-group which ionizes, depending upon the pH.

Use the image of amino acid structures (no names) and pK ${ (=pK }_{ a }{ ) }$ values to determine the overall (net) charge on the peptide DGRTNLEF at the indicated pH.

It may be helpful to first draw out the peptide with each amino acid fully protonated.

Dancojocari. "Amino Acids." Wikimedia Commons. N.p., Jan. 2010. Web. 08 Aug. 2016. (Modified by M. Rumpho-Kennedy)

Please note that in general, a functional group is considered to be ionized when the pH exceeds the pK value by one unit. Likewise, a functional group is considered to be fully protonated if the pH is more than one unit below the associated pK value. When the pH = pK, about 50% of the functional groups are ionized; therefore, the overall charge on that functional group will be the average of the two values and equal to +/- 0.5. Keep this in mind when summing up all of the charges on the amino acids at the different pH values.

Identify ALL of the following which correctly pair the net charge on the peptide with the given pH.

Select ALL that apply.

A

At pH = 5.5, the net charge will equal 0.

B

At pH = 8, the net charge will equal +1.

C

At pH = 1, the net charge will equal +3.

D

At pH = 2, the net charge will equal +2.5.

E

At pH = 12, the net charge will equal -2.5.