1.1 Amino Acids, Ionization & Isoelectric Points

Introduction to Amino Acids & Stereochemistry

Amino acids are the fundamental monomeric building blocks of peptides, polypeptides, and functional proteins. Every standard alpha-amino acid consists of a central alpha-carbon ($\text{C}_\alpha$) covalently bonded to four distinct chemical species:

1. An alpha-amino group ($-\text{NH}_3^+$ or $-\text{NH}_2$)
2. An alpha-carboxyl group ($-\text{COOH}$ or $-\text{COO}^-$)
3. A hydrogen atom ($-H$)
4. A distinctive variable side chain (R group)

Except for glycine—where the R group is a second hydrogen atom—the alpha-carbon is attached to four distinct substituents, making it a chiral center (stereocenter). Consequently, 19 of the 20 standard amino acids are optically active and exist as pairs of enantiomers. In human biology and eukaryotic protein synthesis, ribosomal translation exclusively incorporates L-amino acids. On a standard Fischer projection drawn with the carboxyl group at the top, the alpha-amino group of an L-amino acid projects to the left.

Under the Cahn-Ingold-Prelog (CIP) absolute configuration system, almost all L-amino acids correspond to the (S)-configuration. The sole exception among the chiral amino acids is cysteine, which possesses an (R)-configuration. Cysteine is an exception because its sulfur-containing thiol side chain ($-\text{CH}_2\text{SH}$) takes priority over the alpha-carboxyl group ($-\text{COOH}$) under CIP priority rules. Glycine, having two identical hydrogen atoms on its alpha-carbon, is achiral (neither R nor S).

Additionally, two standard amino acids—isoleucine and threonine—possess a second chiral center located on their beta-carbon atom ($C_\beta$). As a result, isoleucine and threonine can form four possible stereoisomers (two pairs of enantiomers), though only one specific diastereomer is incorporated into biological proteins.

   L-Amino Acid (Fischer Projection)          (S)-Absolute Configuration
              COOH                                       COOH
               |                                          |
         H2N --+-- H                                H2N --+-- H  (S-config)
               |                                          |
               R                                          R

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Structure & Classification of the 20 Standard Amino Acids

Mastery of all 20 standard proteinogenic amino acids—including their full names, three-letter abbreviations, single-letter codes, side-chain chemical structures, and physical properties—is mandatory for the MCAT. The amino acids are divided into five distinct functional classes:

1. Nonpolar, Aliphatic Side Chains

• Glycine (Gly, G): Side chain is $-H$. Smallest amino acid, achiral, confers immense conformational flexibility to polypeptide chains, acts as a structural helix breaker.
• Alanine (Ala, A): Side chain is a methyl group ($-\text{CH}_3$). Hydrophobic, chemically inert in non-catalyzed reactions.
• Valine (Val, V): Side chain is an isopropyl group ($-\text{CH}(\text{CH}_3)_2$). Branched-chain amino acid (BCAA), strongly hydrophobic, drives hydrophobic core collapse.
• Leucine (Leu, L): Side chain is an isobutyl group ($-\text{CH}_2\text{CH}(\text{CH}_3)_2$). Branched-chain amino acid, highly hydrophobic, abundant in transmembrane domains.
• Isoleucine (Ile, I): Side chain is a sec-butyl group ($-\text{CH}(\text{CH}_3)\text{CH}2\text{CH}3$). Possesses two chiral centers ($C\alpha$ and $C\beta$), isomer of leucine.
• Proline (Pro, P): Side chain is a cyclic pyrrolidine ring ($-\text{CH}_2\text{CH}_2\text{CH}_2-$) bonded covalently back to the alpha-amino nitrogen, forming a secondary amine (imino acid). Its rigid ring constrains backbone dihedral angles, introducing kinks in alpha-helices and forming turns in beta-sheets.
• Methionine (Met, M): Side chain is a nonpolar thioether ($-\text{CH}_2\text{CH}_2\text{S}\text{CH}_3$). Serves as the universal initial amino acid in eukaryotic translation (encoded by the AUG start codon).

2. Aromatic Side Chains

• Phenylalanine (Phe, F): Side chain is a benzyl group ($-\text{CH}_2\text{C}_6\text{H}_5$). Strongly nonpolar and hydrophobic.
• Tyrosine (Tyr, Y): Side chain is a para-hydroxybenzyl group ($-\text{CH}_2\text{C}_6\text{H}4\text{OH}$). Polar ionizable hydroxyl group ($\text{p}K{a,R} \approx 10.5$), absorbs ultraviolet light at 280 nm, key target for protein phosphorylation by tyrosine kinases.
• Tryptophan (Trp, W): Side chain is a bulky double-ring indole group ($-\text{CH}_2\text{indole}$). Largest side chain, weakly polar, absorbs UV light strongly at 280 nm (used for spectrophotometric protein quantification).

3. Polar, Uncharged Side Chains

• Serine (Ser, S): Side chain is a primary alcohol ($-\text{CH}_2\text{OH}$). Forms hydrogen bonds, primary site for O-linked glycosylation and phosphorylation by serine/threonine kinases.
• Threonine (Thr, T): Side chain is a secondary alcohol ($-\text{CH}(\text{OH})\text{CH}_3$). Possesses two chiral centers, forms hydrogen bonds, target for phosphorylation.
• Cysteine (Cys, C): Side chain is a thiol / sulfhydryl group ($-\text{CH}2\text{SH}$, $\text{p}K{a,R} \approx 8.3$). Under oxidizing conditions (such as the endoplasmic reticulum lumen or extracellular fluid), two cysteine sulfhydryl groups oxidize to form a covalent disulfide bond (yielding cystine).
• Asparagine (Asn, N): Side chain is an amide ($-\text{CH}_2\text{CONH}_2$). Forms hydrogen bonds, site of N-linked glycosylation in the ER.
• Glutamine (Gln, Q): Side chain is an amide ($-\text{CH}_2\text{CH}_2\text{CONH}_2$). Primary nitrogen carrier in circulation, non-ionizable.

Critical MCAT High-Yield Rule: Asparagine (Asn, N) and Glutamine (Gln, Q) contain amide functional groups. Amide nitrogens are resonance-stabilized by the adjacent carbonyl group and do NOT gain or lose protons across physiological pH ranges. Do not confuse amide side chains with acidic carboxylate side chains (Asp, Glu).

4. Acidic (Negatively Charged at pH 7) Side Chains

• Aspartate / Aspartic Acid (Asp, D): Side chain is a beta-carboxyl group ($-\text{CH}2\text{COOH}$, $\text{p}K{a,R} \approx 3.9$). Conjugate base is aspartate.
• Glutamate / Glutamic Acid (Glu, E): Side chain is a gamma-carboxyl group ($-\text{CH}_2\text{CH}2\text{COOH}$, $\text{p}K{a,R} \approx 4.3$). Conjugate base is glutamate.
• At physiological pH (7.4), both side-chain carboxyl groups are deprotonated ($-\text{COO}^-$), conferring a -1 net negative charge.

5. Basic (Positively Charged at pH 7) Side Chains

• Histidine (His, H): Side chain is an aromatic imidazole ring ($\text{p}K_{a,R} \approx 6.0$). Because its $\text{p}K_a$ is near physiological pH (7.4), histidine shuttles between protonated (+1) and deprotonated (0) states, making it an exceptional catalytic acid-base residue in enzyme active sites.
• Lysine (Lys, K): Side chain is a primary aliphatic epsilon-amino group ($-\text{(CH}_2)_4\text{NH}3^+$, $\text{p}K{a,R} \approx 10.5$). Positively charged (+1) at physiological pH, common site for acetylation, methylation, and ubiquitination.
• Arginine (Arg, R): Side chain is a guanidino group ($-\text{(CH}_2)_3\text{NHC}(\text{NH}_2)2^+$, $\text{p}K{a,R} \approx 12.5$). Resonance-stabilized positive charge (+1), remains protonated across almost all physiological pH ranges.

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Acid-Base Chemistry, Ionization States & Zwitterions

Amino acids are amphoteric compounds capable of acting as either Brønsted-Lowry acids (proton donors) or Brønsted-Lowry bases (proton acceptors) depending on environmental pH. The quantitative protonation state of each ionizable group is governed by the Henderson-Hasselbalch equation:

$\text{pH} = \text{p}K_a + \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right)$

From this relationship, three fundamental rules dictate ionization behavior:

1. When $\text{pH} < \text{p}K_a$: The solution is more acidic than the group's affinity constant. The protonated species $[\text{HA}]$ predominates ($> 50%$ protonated).
2. When $\text{pH} > \text{p}K_a$: The solution is more basic than the group's affinity constant. The deprotonated species $[\text{A}^-]$ predominates ($> 50%$ deprotonated).
3. When $\text{pH} = \text{p}K_a$: Exactly $50%$ of the group is protonated and $50%$ is deprotonated ($[\text{HA}] = [\text{A}^-]$). This condition represents maximal acid-base buffering capacity.

Every free amino acid contains at least two ionizable groups:

• Alpha-carboxyl group ($-\text{COOH}$): $\text{p}K_{a1} \approx 2.0 - 2.4$ (average $\approx 2.2$)
• Alpha-amino group ($-\text{NH}_3^+$): $\text{p}K_{a2} \approx 9.0 - 10.5$ (average $\approx 9.4$)

At physiological pH ($\approx 7.4$), environmental pH is well above $\text{p}K_{a1}$ (deprotonating the carboxyl group to a negatively charged carboxylate, $-\text{COO}^-$) and well below $\text{p}K_{a2}$ (keeping the amino group protonated as a positively charged ammonium, $-\text{NH}_3^+$). A dipolar molecule containing both a positive charge and a negative charge while maintaining an overall net electrical charge of zero is called a zwitterion.

   Low pH (< 2)              Physiological pH (~7.4)           High pH (> 10)
    Cation (+1)                  Zwitterion (0)                  Anion (-1)
       COOH                           COO-                          COO-
        |                              |                             |
  H3N+--+--H                    H3N+--+--H                    H2N --+--H
        |                              |                             |
        R                              R                             R

Seven amino acids possess a third ionizable group located on their side chain ($\text{p}K_{a,R}$):

$\text{Asp (3.9)} < \text{Glu (4.3)} < \text{His (6.0)} < \text{Cys (8.3)} < \text{Tyr (10.5)} < \text{Lys (10.5)} < \text{Arg (12.5)}$

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Isoelectric Point (pI) Calculation Principles & Worked Examples

The isoelectric point (pI) is the precise pH at which the average net electrical charge of an amino acid or peptide population is exactly zero. At $\text{pH} = \text{pI}$, molecules carry no net charge, exhibit minimal solubility in aqueous solvent, and do not migrate toward either electrode in an electric field (such as during isoelectric focusing electrophoresis).

1. Neutral Amino Acids (Non-Ionizable Side Chain)

For diprotic neutral amino acids (such as Alanine, Valine, Serine, or Glycine), the zwitterionic species exists between $\text{p}K_{a1}$ and $\text{p}K_{a2}$. The pI is simply the arithmetic average of the two $\text{p}K_a$ values:

$\text{pI} = \frac{\text{p}K_{a1} + \text{p}K_{a2}}{2}$

Worked Example: Alanine has $\text{p}K_{a1} = 2.34$ and $\text{p}K_{a2} = 9.69$. $\text{pI} = \frac{2.34 + 9.69}{2} = \frac{12.03}{2} = 6.015$

2. Acidic Amino Acids (Aspartate, Glutamate)

Acidic amino acids possess a side-chain carboxyl group. Below $\text{p}K_{a1}$, the net charge is $+1$. Deprotonation of the alpha-carboxyl group at $\text{p}K_{a1}$ yields the net zero zwitterion. Deprotonation of the side-chain carboxyl group at $\text{p}K_{a,R}$ yields a $-1$ charge, and final deprotonation of the alpha-amino group at $\text{p}K_{a2}$ yields a $-2$ charge. The net zero zwitterion lies between $\text{p}K_{a1}$ and $\text{p}K_{a,R}$. Calculate pI by averaging the two lowest $\text{p}K_a$ values:

$\text{pI} = \frac{\text{p}K_{a1} + \text{p}K_{a,R}}{2}$

Worked Example: Glutamate has $\text{p}K_{a1} = 2.19$, $\text{p}K_{a,R} = 4.25$, and $\text{p}K_{a2} = 9.67$. $\text{pI} = \frac{2.19 + 4.25}{2} = \frac{6.44}{2} = 3.22$

3. Basic Amino Acids (Histidine, Lysine, Arginine)

Basic amino acids possess a positively charged side chain. Below $\text{p}K_{a1}$, the net charge is $+2$. Deprotonation of the alpha-carboxyl group at $\text{p}K_{a1}$ yields a $+1$ charge. Deprotonation of the side chain at $\text{p}K_{a,R}$ (or alpha-amino) yields the net zero zwitterion, and final deprotonation at $\text{p}K_{a2}$ yields a $-1$ charge. The net zero zwitterion lies between $\text{p}K_{a,R}$ and $\text{p}K_{a2}$. Calculate pI by averaging the two highest $\text{p}K_a$ values:

$\text{pI} = \frac{\text{p}K_{a,R} + \text{p}K_{a2}}{2}$

Worked Example 1: Lysine has $\text{p}K_{a1} = 2.18$, $\text{p}K_{a2} = 8.95$, and $\text{p}K_{a,R} = 10.53$. $\text{pI} = \frac{8.95 + 10.53}{2} = \frac{19.48}{2} = 9.74$

Worked Example 2: Histidine has $\text{p}K_{a1} = 1.82$, $\text{p}K_{a,R} = 6.00$, and $\text{p}K_{a2} = 9.17$. $\text{pI} = \frac{6.00 + 9.17}{2} = \frac{15.17}{2} = 7.585$

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Titration Curves of Diprotic vs Triprotic Amino Acids

A titration curve plots solution pH against the added molar equivalents of strong base (e.g., $\text{NaOH}$). Analyzing a titration curve reveals key thermodynamic features:

• Buffering Regions: Flat horizontal plateaus centered at each $\text{p}K_a$ value (spanning $\text{pH} = \text{p}K_a \pm 1$). Within these regions, added base converts $[\text{HA}]$ to $[\text{A}^-]$ with minimal change in pH. At $\text{pH} = \text{p}K_a$, buffering capacity is maximal.
• Equivalence Points: Steep vertical inflection points representing complete deprotonation of a specific ionizable species.
• Isoelectric Point (pI): The equivalence point corresponding to 100% conversion into the net zero zwitterionic form.

Diprotic Titration Curve (e.g., Glycine)

Starting at $\text{pH} < 1$, glycine exists as a $+1$ cation ($^+\text{H}_3\text{N}-\text{CH}_2-\text{COOH}$).

1. First Buffering Region ($\text{p}K_{a1} = 2.34$): Adding $0.5$ equivalents of $\text{OH}^-$ yields a 1:1 mixture of cation (+1) and zwitterion (0).
2. First Equivalence Point ($\text{pI} = 5.97$): At $1.0$ equivalent of $\text{OH}^-$, glycine is 100% zwitterion ($^+\text{H}_3\text{N}-\text{CH}_2-\text{COO}^-$).
3. Second Buffering Region ($\text{p}K_{a2} = 9.60$): Adding $1.5$ equivalents of $\text{OH}^-$ yields a 1:1 mixture of zwitterion (0) and anion (-1).
4. Second Equivalence Point: At $2.0$ equivalents of $\text{OH}^-$, glycine is 100% anion ($\text{H}_2\text{N}-\text{CH}_2-\text{COO}^-$).

  pH 12 +---------------------------------------------------------+
        |                                                / (2.0 eq, -1 anion)
  pH 10 |                                    +--pKa2--+ / (1.5 eq, pKa2=9.6)
        |                                   / (pKa2 buffer)
   pH 6 |                        +--pI--+  / (1.0 eq, pI=5.97 zwitterion)
        |                       / (equivalence point)
   pH 2 |           +--pKa1--+ / (0.5 eq, pKa1=2.34)
        |  /-------/ (pKa1 buffer)
   pH 0 +---------------------------------------------------------+
        0.0        0.5        1.0        1.5        2.0  Equivalents OH-

Triprotic Titration Curve (e.g., Glutamate vs Histidine)

Triprotic amino acids possess three buffering plateaus centered at $\text{p}K_{a1}$, $\text{p}K_{a,R}$, and $\text{p}K_{a2}$.

• For Glutamate (acidic), the pI inflection point occurs at $1.0$ equivalent of base between $\text{p}K_{a1}$ ($2.19$) and $\text{p}K_{a,R}$ ($4.25$), yielding $\text{pI} = 3.22$.
• For Histidine (basic), the pI inflection point occurs at $2.0$ equivalents of base between $\text{p}K_{a,R}$ ($6.00$) and $\text{p}K_{a2}$ ($9.17$), yielding $\text{pI} = 7.59$.

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Peptide Bond Synthesis, Resonance & Proteolytic Hydrolysis

Condensation / Dehydration Reaction

Amino acids polymerize into peptides through the formation of peptide bonds (amide linkages). A peptide bond forms via a nucleophilic condensation (dehydration) reaction where the nucleophilic alpha-amino nitrogen of one amino acid attacks the electrophilic alpha-carboxyl carbon of another amino acid, releasing one molecule of water ($\text{H}_2\text{O}$).

$\text{Amino Acid}_1 + \text{Amino Acid}_2 \longrightarrow \text{Dipeptide} + \text{H}_2\text{O}$

Peptide bond formation is endergonic ($\Delta G > 0$) and requires energy input (ATP/GTP hydrolysis) during ribosomal translation.

     H   O                 H   H   O                H   O   H   H   O
     |   ||                |   |   ||               |   ||  |   |   ||
 H2N-C---C---OH   +   H----N---C---C---OH  ---> H2N-C---C---N---C---C---OH + H2O
     |                         |                    |       |   |
     R1                        R2                   R1      H   R2
   Amino Acid 1              Amino Acid 2                Peptide Bond

Resonance Stabilization & Planar Amide Backbone

The peptide bond possesses two primary resonance structures:

1. A major neutral contributor with a $\text{C}=\text{O}$ double bond and $\text{C}-\text{N}$ single bond.
2. A dipolar minor contributor with a single-bonded negatively charged oxygen ($-\text{C}-\text{O}^-$) and a double-bonded positively charged nitrogen ($-\text{C}=\text{N}^+$).

Due to resonance delocalization of the lone pair of electrons on the nitrogen atom, the $\text{C}-\text{N}$ peptide bond exhibits ~40% partial double-bond character. As a consequence:

• The $\text{C}-\text{N}$ bond length ($1.32\text{ \AA}$) is significantly shorter than a standard $\text{C}-\text{N}$ single bond ($1.49\text{ \AA}$).
• Free rotation around the peptide bond is restricted at physiological temperatures.
• The six atoms of the peptide backbone unit ($C_{\alpha 1}$, $C=O$, $N-H$, $C_{\alpha 2}$) lie within a single rigid, coplanar plane.
• To minimize steric overlap between bulky side chains, peptide bonds almost exclusively adopt the trans configuration.

Proteolytic Hydrolysis

While peptide bonds are kinetically stable at physiological pH (with a half-life of over 400 years in pure water), they are thermodynamically unstable ($\Delta G < 0$ for hydrolysis). Cells utilize specialized proteases (peptidases) to catalyze sequence-specific peptide bond cleavage:

• Trypsin: Cleaves peptide bonds specifically at the carboxyl side of positively charged basic residues (Lysine, Arginine), provided the following residue is not Proline.
• Chymotrypsin: Cleaves peptide bonds specifically at the carboxyl side of bulky aromatic residues (Phenylalanine, Tryptophan, Tyrosine), provided the following residue is not Proline.