Question
Aspartame is formed from the two amino acids aspartic acid (Asp) and phenylalanine (Phe).
Chromatography is used in the analysis of proteins in the food and pharmaceutical industry.
a. Draw the structure of the dipeptide Asp-Phe using section 33 of the data booklet.
b(i)Describe, using another method, how a mixture of four amino acids, alanine, arginine, glutamic acid and glycine, could be separated when placed in a buffer solution of $\mathrm{pH} 6.0$.
b(iiß) Suggest why alanine and glycine separate slightly at $\mathrm{pH} 6.5$.
$\mathrm{b}$ (iiifalculate the ratio of $\left[\mathrm{A}^{-}\right]:[\mathrm{HA}]$ in a buffer of $\mathrm{pH} 6.0$ given that $\mathrm{p} K_{\mathrm{a}}$ for the acid is 4.83 , using section 1 of the data booklet.
▶️Answer/Explanation
Markscheme
a.
amide link (eg, $\mathrm{CONH})$
correct order and structures of amino acids
NOTE: Accept a skeletal formula or a full or condensed structural formula.
Accept zwitterion form of dipeptide.
Accept CO-NH but not CO-HN for amide link.
$\mathrm{b}(\mathrm{i})$ Any three of:
«gel» electrophoresis «technique»
OR
mixture «in buffer solution» placed on gel/paper
voltage/potential «difference» applied
amino acids move differently «depending on $\mathrm{pH} /$ isoelectric point»
compare/measure distances travelled/ $R_{\mathrm{f}}$ values
NOTE: Accept “mixture placed on plate covered with polyacrylamide “gel» OR “mixture put in a gel «medium»”.
b(iidifferent sizes/molar masses/chain lengths «so move with different speeds»
NOTE: Do not accept “different side-chains/R-groups/number of carbons”.
\begin{gathered}
\mathrm{b}(\mathrm{iiii}) 6.0=4.83+\log \frac{\left[\mathrm{A}^{-}\right]}{[\mathrm{HA}]} » \\
\text { «log } \frac{\left[\mathrm{A}^{-}\right]}{[\mathrm{HA}]}=1.17 » \\
\text { «[ } \left.\mathrm{A}^{-}\right]:[\mathrm{HA}]=» 14.8: 1 \\
\text { NOTE: Accept “15:1”. } \\
\text { Do not accept 1:14.8. }
\end{gathered}
\begin{aligned}
& \mathrm{b}(\mathrm{iii}) 6.0=4.83+\log \frac{\left[\mathrm{A}^{-}\right]}{[\mathrm{HA}]} » \\
& « \log \frac{\left[\mathrm{A}^{-}\right]}{[\mathrm{HA}]}=1.17 » \\
& \text { «[ } \left.\mathrm{A}^{-}\right]:[\mathrm{HA}]=» 14.8: 1 \\
& \text { NOTE: Accept “15:1”. } \\
& \text { Do not accept 1:14.8. }
\end{aligned}