Notes on Titration Curves, pKa, and Amino Acids (Lecture Summary)

Titration curves, pKa, and amino acid ionization

• Key ideas from the lecture

  • Titration as pH is raised (base environment) drives deprotonation of ionizable groups on amino acids and peptides.
  • The first group to give up a proton is typically the carboxyl group at the C-terminus (alpha-carboxyl), with a low pKa around ~2–3. In the transcript, this is shown as pKa ≈ 2.3.
  • At low pH, groups are protonated. As pH increases and reaches a pKa, buffering occurs and the curve flattens around that pKa. The midpoint of the horizontal (buffering) region corresponds to the pKa value.
  • The segment where the curve appears horizontal is described in the transcript as the buffering region around each pKa. The “middle point” of that horizontal region is the pKa. After all protons are lost from that group, further pH increase produces little change (a vertical-like rise is described when buffering is exhausted).
  • After the carboxyl groups, other groups (such as amines) begin to deprotonate as pH rises. In the example, amines deprotonate around pH ≈ 9.6 (a high pKa region for amine groups), with a 50/50 point at that pH.
  • In a titration of amino acids with multiple ionizable groups, you can have multiple pKa values corresponding to different ionizable sites (e.g., N-terminus, C-terminus, and side chains). The overall charge of the molecule changes as different groups lose or gain protons.
  • The extremes of pH give extreme charges: at very low pH (acidic conditions) the molecule is positively charged; at very high pH (basic conditions) it is negatively charged. The transcript walks through this with an amino acid that has two carboxyl groups (the C-terminus and a side-chain carboxyl) and one amino group.

• Henderson–Hasselbalch context (relevant formula)

  • For a given ionizable group, the relationship between pH, pK_a, and the ratio of ionized to unionized forms is:
  • $pH = pK_a + \log\left(\frac{[A^-]}{[HA]}\right)$
  • The pH at which [A^-] = [HA] is pH = pK_a (i.e., the 50/50 point).

• About pKa values and ionizable groups

  • Carboxyl groups (COOH) have low pKa values and tend to lose a proton first as pH increases.
  • Amine groups (NH3+) have higher pKa values and lose a proton later as pH increases.
  • Side-chain carboxyl groups (e.g., Aspartic acid COOH) have their own pKa (e.g., ≈ 3.9 for Aspartate side-chain COOH).
  • Side-chain amine groups exist for certain residues (e.g., Lys, Arg) with high pKa values (Lys ~10.5, Arg ~12.5) or histidine with a pKa around physiological range (≈6.0) due to the imidazole ring.
  • Tyrosine and cysteine can be ionizable but are context-dependent and often considered only at higher-level (grad) biochemistry studies; in this class, cysteine and tyrosine are not the focus for ionization under standard conditions.

• Specific examples discussed in the lecture

  • Aspartic acid (Asp, D) has two ionizable carboxyl groups relevant here:
  • Alpha-carboxyl group (C-terminus) pKa ≈ 2.2
  • Side-chain carboxyl group pKa ≈ 3.9
  • Alpha-amino group pKa ≈ 9–10 (noted as a higher-pKa amine in the discussion; exact value can vary with environment)
  • Therefore, in order as pH increases: alpha-COOH (≈2.2) deprotonates first, then side-chain COOH (≈3.9), then the amino group (≈9–10).
  • The transcript notes that the end carboxyl group (COO−) is generally the first to lose its proton, followed by the side-chain carboxyl, with amine deprotonation occurring later.
  • The general trends mentioned for other residues:
  • Arginine: very high pKa for its side-chain guanidinium group (≈12.5) – remains protonated (positive) until very basic pH.
  • Histidine: has a unique pKa around physiological pH due to the imidazole side chain; described as “weird” in the lecture because it can buffer around neutral pH.
  • Cysteine: often not ionized in this basic class talk; would require special conditions for deprotonation (thiolate formation).
  • Tyrosine: phenolic OH can be ionizable under certain conditions (grad-level topic); in standard biochem teaching this is not a focus as it’s less readily deprotonated under physiological conditions.

• Peptide bond formation (dehydration synthesis)

  • When forming a peptide bond, remove the hydroxyl (OH) from the carboxyl group and a hydrogen from the amine to form a bond:
  • $\text{R-COOH} + \text{H-NH-R'} \rightarrow \text{R-CO-NH-R'} + \text{H}_2\text{O}$
  • The peptide bond is an amide bond; it is the linkage between the carbonyl carbon of one amino acid and the amide nitrogen of the next.
  • This bond can be hydrolyzed under appropriate conditions (water can break the bond in a reaction catalyzed by enzymes or acid/base conditions).

• Orientation of a peptide and basic labeling

  • The “front” of the peptide is the N-terminus (where the free amino group is located).
  • The “start” is the N-terminus, and the “end” (C-terminus) bears the free carboxyl group.
  • Each amino acid in a peptide contributes a side chain or “rock” as described; structure and charge depend on pH and the pKa values of ionizable groups on each residue.

• Practical exercise approach (as described in the lecture)

  • You may be given several amino acids and asked to draw their titration curves and determine the net charge at various pH values.
  • Because these amino acids have multiple ionizable groups (often 3 major sites: N-terminus, C-terminus, and a side-chain group), the curve will have multiple pKa points corresponding to those groups.
  • To determine net charge at a given pH, count the protonation state of each ionizable group:
  • N-terminus: protonated (+1) when NH3+; deprotonates to neutral NH2 at high pH.
  • Carboxyl groups: protonated (neutral) as COOH; deprotonated (-1) as COO− at higher pH.
  • Side-chain groups: depend on their own pKa values (e.g., Asp side-chain COO− contributes -1 when deprotonated).
  • Example charge check (illustrative, following the lecture's scenario):
  • At very low pH: the molecule has a net +1 charge (only the amine group is positively charged: NH3+).
  • At pH ~2.2 (after alpha-carboxyl deprotonates): net charge becomes 0 (NH3+ plus one COO−, others protonated).
  • At pH ~3.9 (Asp side-chain COO− deprotonates): net charge becomes −1 (extra negative from side-chain COOH).
  • At high pH (e.g., pH ~14): both carboxyl groups are deprotonated (−2 total) and the amino group is neutral (NH2), giving net charge ≈ −2.
  • A note on pKa planning: in the transcript, a value around 2.2 is cited for the terminal carboxyl, around 3.9 for the Asp side-chain carboxyl, and around 9.5–9.6 for amine deprotonation; actual values depend on the specific residue and environment.

• Quick recap of connections and implications

  • Titration curves illuminate how amino acids and proteins change charge with pH, which in turn affects solubility, folding, and enzyme activity.
  • The pKa values of ionizable groups determine buffering behavior and the pH ranges where amino acids act as buffers.
  • Understanding the order of deprotonation helps predict the net charge of peptides at different pH levels, which is crucial for protein structure and interactions.

• Practical tips for exam-style questions

  • Remember the common order for a simple amino acid with an N-terminus, C-terminus, and side-chain carboxyl group (e.g., Asp/Glu):
  • First: alpha-carboxyl (pKa ≈ 2–3)
  • Then: side-chain carboxyl (pKa ≈ 3–5 for Asp/Glu)
  • Then: amine groups (pKa ≈ 9–11 for main chain amino; side-chain amines can be higher, e.g., Lys ~10.5, Arg ~12.5)
  • For a given pH, compute net charge by adding +1 for protonated amine groups, −1 for deprotonated carboxyl groups, and adjusting for side chains.
  • Use the midpoint of a horizontal region on a titration curve to identify pKa (the 50/50 state for protonated vs deprotonated forms).

• Quick example calculation (abstracted from the lecture’s values)

  • At pH = 1 (very acidic): +1 overall charge (NH3+ is present; carboxyl groups are COOH).
  • At pH = 14 (very basic): −2 overall charge (carboxyl groups are COO−; amino group is NH2, neutral).
  • At pH ≈ 7 (near physiological pH): assume two carboxyl groups are deprotonated and the amino group is still protonated; overall charge ≈ −1 (example when C-terminus and side-chain COO− are −1 each, N-terminus still +1).
  • Remember these numbers are schematic; exact charges depend on the specific amino acid and its side chains.