Amino Acids and Peptides: Zwitterions and the Peptide Bond · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Charged ends and stiff links

Science · Chemistry · ages 21-22
Name ______________________   Date ____________

  1. In strongly acidic solution, what is the net charge of an amino acid?

    • Negative
    • Neutral
    • Positive

  2. In neutral water, what happens inside a single amino acid molecule?

    • It takes a proton from the solvent
    • It hands a proton from its carboxyl group to its amino group
    • It sheds both charged groups

  3. The peptide bond favours the trans geometry.

    Circle one:   True   False

  4. Glutamic acid carries a net negative charge at pH 7. Which way does it drift in an electric field?

    • Toward the positive electrode
    • Toward the negative electrode
    • It stays exactly put

  5. The two pKa values bracketing the neutral form are 2.2 and 9.4. Type the isoelectric point.

    Answer: ______________

  6. During peptide synthesis, why is every other reactive group protected?

    • They would slow the solvent down
    • They would freeze the backbone flat
    • They would join the reaction

  7. Why does each coupling begin by activating the carboxyl group?

    • The acid and amine do not condense on their own
    • Water must be frozen out first
    • The side chain must stay charged

  8. A student finds the isoelectric point of an amino acid with an acidic side chain by averaging its lowest and highest pKa. What is the mistake?

    • An average is the wrong operation
    • The two values must bracket the neutral form
    • Only one pKa value may be used
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Charged ends and stiff links W1-mt_cXdg1yjq-n-s1

  1. Positive · Excess acid protonates every group, so the molecule becomes a cation.
  2. It hands a proton from its carboxyl group to its amino group · The proton moves within the molecule, leaving a positive end and a negative end.
  3. True · Partial double character locks the linkage, and the trans shape wins.
  4. Toward the positive electrode · Each residue moves toward the electrode opposite to its own net charge.
  5. 5.8 · Add the two values and halve the sum: (2.2 + 9.4) / 2 = 5.8.
  6. They would join the reaction · Protection keeps all groups except the chosen pair out of the coupling.
  7. The acid and amine do not condense on their own · Activation wakes the carboxyl group so the chosen amine can attack it.
  8. The two values must bracket the neutral form · You must average the two values around the zero-charge form, which both sit on the acidic side here.
Worksheet · LightMySky