Biochemistry Study Notes: Amino Acid Structure, Properties, Titration, and Classification

Fundamental Structure and Properties of Amino Acids

Amino acids serve as the fundamental monomeric building blocks of peptides and proteins, which are polymers of amino acid residues. Every standard amino acid shares a common structural backbone centered around a central carbon atom designated as the alpha-carbon ($C_\alpha$).

The alpha-carbon is covalently bonded to four distinct chemical substituents:

1. An amino group ($-\text{NH}_3^+$ in its protonated form, or $-\text{NH}_2$ in its unprotonated form).

2. A carboxylate or carboxylic acid group ($-\text{COOH}$ in its protonated form, or $-\text{COO}^-$ in its deprotonated form).

3. A single hydrogen atom ($-H$).

4. A distinctive side chain or $R$ group ($-R$), which varies among different amino acids and imparts unique chemical character.

The specific side chain ($R$ group) differentiates one amino acid from another, determining its physical and chemical properties. For instance, glycine and alanine possess identical backbone structures but differ entirely in their side chains: glycine contains a hydrogen atom ($-H$), whereas alanine contains a methyl group ($-\text{CH}_3$).

Amino acids vary widely across six key physical and chemical parameters:

1. Size: Ranging from a single hydrogen atom to bulky bicyclic ring structures.

2. Shape: Variable spatial geometries and steric bulks of side chains.

3. Charge: Positively charged, negatively charged, or neutral at physiological pH.

4. Hydrogen-bonding capability: Presence of polar functional groups ($-\text{OH}$, $-\text{SH}$, $-\text{CONH}_2$) that act as hydrogen bond donors or acceptors.

5. Hydrophobicity: Nonpolar aliphatic and aromatic side chains that avoid aqueous solvent versus polar or charged side chains that interact favorably with water.

6. Chemical reactivity: Reactive functional groups capable of forming covalent linkages, undergoing redox reactions, or participating in general acid-base catalysis.

At physiological pH (approximately $\text{pH} = 7.0 - 7.4$), amino acids exist primarily as dipolar ions known as zwitterions. In a zwitterion, the carboxyl group is fully deprotonated ($-\text{COO}^-$) and carries a $-1$ charge, while the amino group is fully protonated ($-\text{NH}_3^+$) and carries a $+1$ charge. These equal and opposite charges cancel each other out internally, resulting in an overall net charge of zero ($0$). Because they contain both acidic and basic functional groups capable of donating or accepting protons ($H^+$), amino acids are amphoteric substances and exhibit characteristic titration curves.

Stereochemistry and Chirality of Amino Acids

A molecule is defined as chiral (or optically active) if its central carbon atom is bonded to four completely different atoms or functional groups. A chiral molecule cannot be superimposed on its mirror image, giving rise to two non-superimposable stereoisomers termed enantiomers. These enantiomers are designated as the $L$ (levorotatory) and $D$ (dextrorotatory) stereoisomers based on their spatial configuration relative to glyceraldehyde.

With a single exception, all standard amino acids found in proteins possess a chiral alpha-carbon ($C_\alpha$) because $C_\alpha$ is attached to four distinct substituents ($-\text{NH}_3^+$, $-\text{COO}^-$, $-H$, and $-R$).

The sole exception among the 20 standard amino acids is glycine. In glycine, the side chain ($R$ group) is a second hydrogen atom ($-H$). Because the alpha-carbon is bonded to two identical hydrogen atoms, it lacks four distinct substituents and is therefore achiral (optically inactive). Consequently, glycine does not exist as $L$ or $D$ stereoisomers.

In living biological systems, proteins are constructed exclusively from $L$--amino acids. Conversely, biological sugars utilized in carbohydrates exist almost exclusively in the $D$--configuration.

The stereospecificity of biological molecules has critical implications in pharmacology and enzyme biochemistry:

1. Enzyme Active Site Stereospecificity: Active sites of enzymes are three-dimensional, chiral environments. An enzyme functions as a stereospecific "lock" that accepts only the correctly configured stereoisomer "key."

2. Synthetic vs. Enzymatic Reactions: Standard organic synthesis reactions conducted in a laboratory flask produce a 50:50 mixture of both enantiomers, known as a racemic mixture. In contrast, enzymatic reactions in biological systems produce exclusively one specific stereoisomer.

3. Pharmacological Implications:

   • Ibuprofen: Only one specific enantiomer of ibuprofen possesses anti-inflammatory action by inhibiting cyclooxygenase enzymes. The alternative enantiomer is pharmacologically inactive.

   • Thalidomide: Prescribed widely in Europe during the early 1960s as a mild sedative to treat morning sickness in pregnant women. The drug was administered as a racemic mixture. While one enantiomer functioned as an effective sedative, the mirror-image enantiomer acted as a potent teratogen, causing severe congenital limb malformations when taken during the first trimester of pregnancy.

Classification and Structural Classes of the 20 Standard Amino Acids

The 20 standard amino acids are classified into five distinct categories based on the chemical nature, polarity, and charge of their side chains ($R$ groups). Each amino acid is designated by a full name, a three-letter abbreviation, and a single-letter code.

Nonpolar, Aliphatic R Groups

This group consists of hydrophobic amino acids with nonpolar, aliphatic hydrocarbon side chains. Their side chains range from mildly hydrophobic to strongly hydrophobic. In globular proteins folded in aqueous environments, these residues tend to cluster together inside the hydrophobic interior core of the protein, away from contact with water.

1. Glycine (Gly, G):

   • Formula/Structure: $R = -H$

   • Molecular Weight ($M_r$): $75$

   • $pK_1\,(-\text{COOH}) = 2.34$, $pK_2\,(-\text{NH}_3^+) = 9.60$, $pI = 5.97$

   • Hydropathy Index: $-0.4$; Occurrence in proteins: $7.2\%$

   • Key Features: Smallest amino acid, achiral, provides high conformational flexibility to polypeptide backbones.

2. Alanine (Ala, A):

   • Formula/Structure: $R = -\text{CH}_3$

   • Molecular Weight ($M_r$): $89$

   • $pK_1 = 2.34$, $pK_2 = 9.69$, $pI = 6.01$

   • Hydropathy Index: $1.8$; Occurrence in proteins: $7.8\%$

3. Proline (Pro, P):

   • Formula/Structure: Pyrrolidine ring structure where the aliphatic side chain loops back and covalently bonds to the alpha-amino nitrogen atom.

   • Molecular Weight ($M_r$): $115$

   • $pK_1 = 1.99$, $pK_2 = 10.96$ (or $10.6$), $pI = 6.48$

   • Hydropathy Index: $-1.6$; Occurrence in proteins: $5.2\%$

   • Key Features: Secondary amino (imino) acid, introduces rigid bends and breaks into secondary protein structures like alpha-helices.

4. Valine (Val, V):

   • Formula/Structure: $R = -\text{CH}(\text{CH}_3)_2$ (branched-chain)

   • Molecular Weight ($M_r$): $117$

   • $pK_1 = 2.32$, $pK_2 = 9.62$, $pI = 5.97$

   • Hydropathy Index: $4.2$; Occurrence in proteins: $6.6\%$

5. Leucine (Leu, L):

   • Formula/Structure: $R = -\text{CH}_2\text{CH}(\text{CH}_3)_2$ (branched-chain)

   • Molecular Weight ($M_r$): $131$

   • $pK_1 = 2.36$, $pK_2 = 9.60$, $pI = 5.98$

   • Hydropathy Index: $3.8$; Occurrence in proteins: $9.1\%$

6. Isoleucine (Ile, I):

   • Formula/Structure: $R = -\text{CH}(\text{CH}_3)\text{CH}_2\text{CH}_3$ (branched-chain)

   • Molecular Weight ($M_r$): $131$

   • $pK_1 = 2.36$, $pK_2 = 9.68$ (or $9.60$), $pI = 6.02$

   • Hydropathy Index: $4.5$; Occurrence in proteins: $5.3\%$

   • Key Features: Contains a second chiral center at the beta-carbon atom.

7. Methionine (Met, M):

   • Formula/Structure: $R = -\text{CH}_2\text{CH}_2\text{S}\text{CH}_3$ (thioether)

   • Molecular Weight ($M_r$): $149$

   • $pK_1 = 2.28$, $pK_2 = 9.21$, $pI = 5.74$

   • Hydropathy Index: $1.9$; Occurrence in proteins: $2.3\%$

   • Key Features: Contains a sulfur atom within a nonpolar thioether linkage. The sulfur atom is non-reactive under normal physiological conditions. Radioactive sulfur-35 ($^{35}\text{S}$) is widely utilized in laboratory research for isotopic metabolic labeling of synthesized proteins.

A common memory aid for nonpolar, aliphatic amino acids is the acronym GAP V LIM (Glycine, Alanine, Proline, Valine, Leucine, Isoleucine, Methionine).

Aromatic R Groups

Aromatic amino acids possess aromatic ring side chains. These side chains represent the most hydrophobic functional groups in proteins and are predominantly buried inside the interior core of folded proteins.

1. Phenylalanine (Phe, F):

   • Formula/Structure: Phenyl ring attached to a methylene carbon ($R = -\text{CH}_2\text{C}_6\text{H}_5$)

   • Molecular Weight ($M_r$): $165$

   • $pK_1 = 1.83$, $pK_2 = 9.13$, $pI = 5.48$

   • Hydropathy Index: $2.8$; Occurrence in proteins: $3.9\%$

2. Tyrosine (Tyr, Y):

   • Formula/Structure: Phenolic hydroxyl group attached to a phenyl ring ($R = -\text{CH}_2\text{C}_6\text{H}_4\text{OH}$)

   • Molecular Weight ($M_r$): $181$

   • $pK_1 = 2.20$, $pK_2 = 9.11$, $pK_R = 10.07$, $pI = 5.66$

   • Hydropathy Index: $-1.3$; Occurrence in proteins: $3.2\%$

   • Key Features: Dual classification. Classified under aromatic R groups due to its benzene ring, but also classified under polar uncharged groups due to its ionizable phenolic hydroxyl group ($pK_R = 10.07$), which can form hydrogen bonds.

3. Tryptophan (Trp, W):

   • Formula/Structure: Bicyclic indole ring attached to a methylene carbon

   • Molecular Weight ($M_r$): $204$

   • $pK_1 = 2.38$ (or $2.83$), $pK_2 = 9.39$, $pI = 5.89$

   • Hydropathy Index: $-0.9$; Occurrence in proteins: $1.4\%$

Polar, Uncharged R Groups

These amino acids possess side chains that are hydrophilic and capable of forming hydrogen bonds with water or other polar molecules, but carry zero net electrical charge at neutral physiological pH.

1. Serine (Ser, S):

   • Formula/Structure: Primary hydroxyl group ($R = -\text{CH}_2\text{OH}$)

   • Molecular Weight ($M_r$): $105$

   • $pK_1 = 2.21$, $pK_2 = 9.15$, $pI = 5.68$

   • Hydropathy Index: $-0.8$; Occurrence in proteins: $6.8\%$

2. Threonine (Thr, T):

   • Formula/Structure: Secondary hydroxyl group ($R = -\text{CH}(\text{OH})\text{CH}_3$)

   • Molecular Weight ($M_r$): $119$

   • $pK_1 = 2.11$ (or $2.63$), $pK_2 = 9.62$ (or $10.43$), $pI = 5.87$

   • Hydropathy Index: $-0.7$; Occurrence in proteins: $5.9\%$

   • Key Features: Contains a second chiral carbon atom at the beta-position.

3. Cysteine (Cys, C):

   • Formula/Structure: Sulfhydryl or thiol group ($R = -\text{CH}_2\text{SH}$)

   • Molecular Weight ($M_r$): $121$

   • $pK_1 = 1.96$ (or $1.71$), $pK_2 = 10.28$ (or $10.78$), $pK_R = 8.18$ (or $8.33$), $pI = 5.07$

   • Hydropathy Index: $2.5$; Occurrence in proteins: $1.9\%$

   • Key Features: Weakly acidic thiol group ($pK_R \approx 8.2$) that can undergo oxidation to form covalent disulfide bonds.

4. Asparagine (Asn, N):

   • Formula/Structure: Amide side chain derived from aspartate ($R = -\text{CH}_2\text{CONH}_2$)

   • Molecular Weight ($M_r$): $132$

   • $pK_1 = 2.02$, $pK_2 = 8.80$, $pI = 5.41$

   • Hydropathy Index: $-3.5$; Occurrence in proteins: $4.3\%$

5. Glutamine (Gln, Q):

   • Formula/Structure: Amide side chain derived from glutamate ($R = -\text{CH}_2\text{CH}_2\text{CONH}_2$)

   • Molecular Weight ($M_r$): $146$

   • $pK_1 = 2.17$, $pK_2 = 9.13$, $pI = 5.65$

   • Hydropathy Index: $-3.5$; Occurrence in proteins: $4.2\%$

Positively Charged (Basic) R Groups

Also referred to as basic amino acids. Their side chains are strongly hydrophilic, contain basic nitrogenous functional groups, accept protons ($H^+$), and carry a net positive charge at physiological pH ($\text{pH} \approx 7$). Each basic amino acid possesses an ionizable side chain characterized by a $pK_R$.

1. Lysine (Lys, K):

   • Formula/Structure: Primary amino group located at the epsilon-position ($R = -\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{NH}_3^+$)

   • Molecular Weight ($M_r$): $146$

   • $pK_1 = 2.18$, $pK_2 = 8.95$, $pK_R = 10.53$ (or $10.79$), $pI = 9.74$

   • Hydropathy Index: $-3.9$; Occurrence in proteins: $5.9\%$

2. Histidine (His, H):

   • Formula/Structure: Imidazole ring attached to a methylene group

   • Molecular Weight ($M_r$): $155$

   • $pK_1 = 1.82$, $pK_2 = 9.17$, $pK_R = 6.00$, $pI = 7.59$

   • Hydropathy Index: $-3.2$; Occurrence in proteins: $2.3\%$

   • Key Features: Ionizable imidazole ring with $pK_R = 6.00$

3. Arginine (Arg, R):

   • Formula/Structure: Guanidino group attached to a propyl chain ($R = -\text{CH}_2\text{CH}_2\text{CH}_2\text{NH}-\text{C}(=\text{NH}_2^+)\text{NH}_2$)

   • Molecular Weight ($M_r$): $174$

   • $pK_1 = 2.17$, $pK_2 = 9.04$, $pK_R = 12.48$, $pI = 10.76$

   • Hydropathy Index: $-4.5$; Occurrence in proteins: $5.1\%$

Negatively Charged (Acidic) R Groups

Also referred to as acidic amino acids. Their side chains contain carboxylic acid functional groups that lose protons ($H^+$) at physiological pH, leaving a net negative charge ($-1$). They are extremely hydrophilic and possess an ionizable side chain $pK_R$

1. Aspartate / Aspartic Acid (Asp, D):

   • Formula/Structure: Carboxylate group attached to a methylene carbon ($R = -\text{CH}_2\text{COO}^-$)

   • Molecular Weight ($M_r$): $133$

   • $pK_1 = 1.88$ (or $2.09$), $pK_2 = 9.60$ (or $9.82$), $pK_R = 3.65$ (or $3.86$), $pI = 2.77$

   • Hydropathy Index: $-3.5$; Occurrence in proteins: $5.3\%$

2. Glutamate / Glutamic Acid (Glu, E):

   • Formula/Structure: Carboxylate group attached to an ethylene chain ($R = -\text{CH}_2\text{CH}_2\text{COO}^-$)

   • Molecular Weight ($M_r$): $147$

   • $pK_1 = 2.19$, $pK_2 = 9.67$, $pK_R = 4.25$, $pI = 3.22$

   • Hydropathy Index: $-3.5$; Occurrence in proteins: $6.3\%$

Special Cases: Cysteine Oxidation and Histidine Ionization

Special Case #1: Cysteine Oxidation and Disulfide Bond Formation

Cysteine contains a polar, ionizable sulfhydryl or thiol group ($-\text{SH}$). The thiol group of cysteine is nucleophilic and chemically reactive.

When two cysteine residues are brought into proximity in an oxidizing environment, their sulfhydryl groups undergo a reversible oxidation reaction. In this process, the two cysteine molecules lose two protons ($2 H^+$) and two electrons ($2 e^-$) to form a covalent sulfur-sulfur linkage known as a disulfide bond ($-\text{S}-\text{S}-$).

The resulting dimeric oxidized molecule formed by cross-linking two cysteine residues is called cystine.

The oxidation-reduction equilibrium equation is written as:

$2\,\text{Cysteine} \rightleftharpoons \text{Cystine} + 2 H^+ + 2 e^-$

Disulfide bonds play a vital role in stabilizing the tertiary and quaternary structures of secreted proteins, extracellular enzymes, and antibodies by cross-linking polypeptide chains.

Special Case #2: Histidine Ionization near Physiological pH

The side chain of histidine contains a five-membered heterocyclic imidazole ring. The nitrogens in the imidazole ring can bind or release a proton ($H^+$).

The side chain ionization constant of histidine is $pK_R = 6.00$. Because this value is close to physiological pH ($\text{pH} \approx 7.0 - 7.4$), small changes in ambient microenvironment or pH induce a change in histidine's ionization state.

At physiological pH, histidine exists in dynamic equilibrium between its protonated positively charged form ($+1$) and its unprotonated neutral zwitterionic form ($0$). Under normal resting physiological conditions, the neutral zwitterionic form is slightly more abundant. Because of its near-neutral $pK_R$, histidine acts as an effective biological buffer near physiological pH and serves as a proton shuttle or catalytic acid-base residue in many enzyme active sites.

Acid-Base Chemistry, Ionization States, and Net Charge

Understanding amino acid behavior in aqueous solution requires precise chemical definitions:

• Ionizable Group: A functional group capable of gaining or losing a proton ($H^+$), thereby changing its electrical charge.

• Protonated State: The form of a molecule or functional group that holds or has bound a proton ($H^+$).

• Deprotonated State: The form of a molecule or functional group that has lost or released a proton ($H^+$).

• Net Charge: The overall net electrical charge calculated by summing all individual formal positive ($+$) and negative ($-$) charges across all functional groups in the molecule.

All standard amino acids possess at least two ionizable functional groups:

1. The alpha-carboxylic acid group ($-\text{COOH}$), governed by ionization equilibrium constant $pK_1 \approx 2.0$

2. The alpha-amino group ($-\text{NH}_3^+$), governed by ionization equilibrium constant $pK_2 \approx 9.0 - 10.0$

Amino acids with ionizable side chains (cysteine, tyrosine, lysine, histidine, arginine, aspartate, glutamate) possess a third ionization constant designated as $pK_R$

Deprotonation Sequence with Increasing pH

As the pH of a solution increases from strongly acidic to strongly basic, the concentration of hydrogen ions ($[H^+]$) decreases, causing the amino acid to undergo sequential loss of protons. Functional groups deprotonate in order of their acidity, with the lowest $pK_a$ group releasing its proton first.

1. At Low pH (Strongly Acidic Environment):

   • High concentration of ambient protons ($[H^+]$).

   • Amino acids are fully protonated.

   • Carboxylic acid group: $-\text{COOH}$ (charge = $0$).

   • Amino group: $-\text{NH}_3^+$ (charge = $+1$).

   • Structural formula: $\text{NH}_3^+-\text{CHR}-\text{COOH}$.

   • Net Charge: $(+1) + 0 = +1$

2. At Intermediate pH (Increasing pH, $pK_1 < \text{pH} < pK_2$):

   • As pH rises above $pK_1$, the alpha-carboxylic acid group loses its proton first ($pK_1 \approx 2.0 < pK_2 \approx 9.6$) because it is a stronger acid.

   • Carboxylic acid group deprotonates to carboxylate: $-\text{COOH} \rightarrow -\text{COO}^- + H^+$ (charge = $-1$).

   • Amino group remains fully protonated: $-\text{NH}_3^+$ (charge = $+1$).

   • Structural formula: $\text{NH}_3^+-\text{CHR}-\text{COO}^-$.

   • Net Charge: $(+1) + (-1) = 0$

   • This electrically neutral form carrying opposite formal charges is the zwitterion.

3. At High pH (Strongly Basic Environment, $\text{pH} > pK_2$):

   • As pH rises above $pK_2$, the alpha-amino group loses its proton: $-\text{NH}_3^+ \rightarrow -\text{NH}_2 + H^+$ (charge = $0$).

   • Carboxylate group remains deprotonated: $-\text{COO}^-$ (charge = $-1$).

   • Structural formula: $\text{NH}_2-\text{CHR}-\text{COO}^-$.

   • Net Charge: $0 + (-1) = -1$

Core Rule of Net Charge Dynamics

As pH increases, an amino acid progressively loses protons ($H^+$) and its net charge shifts monotonically to more negative values:

$\text{Net Charge: } +1 \longrightarrow 0 \longrightarrow -1$

Conversely, at low pH, high ambient $[H^+]$ drives the equilibrium toward the fully protonated positive species.

Amino Acid Titration Curves and Buffer Regions

A titration curve is generated by measuring the pH of an amino acid solution as strong base (such as $\text{OH}^-$ equivalents) is sequentially added. Titration curves directly reveal the $pK_a$ values of ionizable groups and define the buffering zones of the amino acid.

Buffer Regions and Maximum Buffering Capacity

A buffer region is a flat zone on a titration curve where the addition of base or acid produces minimal changes in solution pH.

• Buffer regions extend approximately $\pm 1$ pH unit around each $pK_a$ value ($\text{pH} = pK_a \pm 1$).

• Maximum buffering capacity occurs precisely at the midpoint of titration, where $\text{pH} = pK_a$. At this point, equal concentrations of the protonated donor and deprotonated acceptor species exist in solution.

Titration Curve of Glycine

Glycine is a diprotic amino acid possessing two ionizable groups ($pK_1 = 2.34$ and $pK_2 = 9.60$).

• Starting Point ($0$ equivalents of $\text{OH}^-$ added): Glycine exists entirely in its fully protonated form ($\text{NH}_3^+-\text{CH}_2-\text{COOH}$), with a net charge of $+1$

• First Buffer Region (centered at $\text{pH} = pK_1 = 2.34$):

  • As $\text{OH}^-$ is added, the $-\text{COOH}$ group deprotonates according to:     $-\text{COOH} \rightleftharpoons -\text{COO}^- + H^+$

  • At $0.5$ equivalents of $\text{OH}^-$, $\text{pH} = 2.34$. An equimolar concentration of $+1$ cation and $0$ zwitterion is present.

• First Inflection Point / Isoelectric Point ($1.0$ equivalent of $\text{OH}^-$ added):

  • At $\text{pH} = 5.97$, the $-\text{COOH}$ group is completely deprotonated into $-\text{COO}^-$, while the $-\text{NH}_3^+$ group remains fully protonated.

  • Glycine exists $100\%$ in the neutral zwitterionic form ($\text{NH}_3^+-\text{CH}_2-\text{COO}^-$), yielding a net charge of $0$

• Second Buffer Region (centered at $\text{pH} = pK_2 = 9.60$):

  • Continued addition of $\text{OH}^-$ deprotonates the amino group:     $-\text{NH}_3^+ \rightleftharpoons -\text{NH}_2 + H^+$

  • At $1.5$ equivalents of $\text{OH}^-$, $\text{pH} = 9.60$. An equimolar concentration of $0$ zwitterion and $-1$ anion is present.

• End Point ($2.0$ equivalents of $\text{OH}^-$ added):

  • At high pH ($\text{pH} > 11$), glycine is fully deprotonated into $\text{NH}_2-\text{CH}_2-\text{COO}^-$, with a net charge of $-1$

Isoelectric Point (pI) Determination and Calculations

The isoelectric point ($pI$) is defined as the specific pH at which an amino acid or protein carries no net electrical charge ($\text{Net Charge} = 0$), existing entirely as a neutral zwitterion.

General Procedure for Calculating pI

To calculate the $pI$ of any amino acid (including triprotic amino acids with ionizable side chains):

1. List all ionization equilibria in order of increasing $pK_a$ values ($pK_1 < pK_R < pK_2$).

2. Trace the net electrical charge of the predominant chemical species at each stage, from fully protonated to fully deprotonated.

3. Locate the specific neutral species that possesses a net charge of zero ($0$).

4. Identify the two $pK_a$ values that bracket this neutral species.

5. Calculate the mathematical average of these two bounding $pK_a$ values using the formula:

$pI = \frac{pK_a^{(A)} + pK_a^{(B)}}{2}$

Critical Caution: Never select random $pK_a$ values to average. Always determine the net charge state of every species first, locate the zero net charge form, and average the two $pK_a$ values on either side of that zero charge species.

Calculation Example 1: Diprotic Amino Acid (Glycine)

• Ionization equilibria:   $\text{Form (+1)} \xrightarrow{pK_1 = 2.34} \text{Form (0)} \xrightarrow{pK_2 = 9.60} \text{Form (-1)}$

• The neutral species ($0$) is bounded by $pK_1$ and $pK_2$

• Calculation:

$pI = \frac{pK_1 + pK_2}{2} = \frac{2.34 + 9.60}{2} = \frac{11.94}{2} = 5.97$

Calculation Example 2: Basic Triprotic Amino Acid (Histidine)

Histidine possesses three ionizable groups: $pK_1 = 1.82$ (\alpha$-\text{COOH}), pK_R = 6.00$(imidazole side chain), and$pK_2 = 9.17$($\alpha$-\text{NH}_3^+).

• Step 1: Determine charge states across ionization stages:

  • Fully protonated state ($\text{pH} < 1.82$):

  • \alpha$-\text{COOH} = 0, Imidazole-$H^+ = +1$, \alpha$-\text{NH}_3^+ = +1

  • Net Charge = $+2$

  • Stage 1 ($pK_1 = 1.82$):

  • \alpha$-\text{COOH} deprotonates to \alpha$-\text{COO}^- (charge = $-1$).

  • Net Charge = $(+1) + (+1) + (-1) = +1$

  • Stage 2 ($pK_R = 6.00$):

  • Imidazole side chain loses its proton (charge = $0$).

  • Net Charge = $(+1) + (-1) + 0 = 0$. (Neutral Zwitterion Formed).

  • Stage 3 ($pK_2 = 9.17$):

  • \alpha$-\text{NH}_3^+ deprotonates to \alpha$-\text{NH}_2 (charge = $0$).

  • Net Charge = $0 + (-1) + 0 = -1$

• Step 2: Identify bounding pKa values for zero net charge:

  • The neutral species ($\text{Net Charge} = 0$) exists between $pK_R = 6.00$ and $pK_2 = 9.17$

• Step 3: Calculate pI:

$pI = \frac{pK_R + pK_2}{2} = \frac{6.00 + 9.17}{2} = \frac{15.17}{2} = 7.585 \approx 7.59$

Calculation Example 3: Acidic Triprotic Amino Acid (Glutamic Acid)

Glutamic acid possesses three ionizable groups: $pK_1 = 2.19$ (\alpha$-\text{COOH}), pK_R = 4.25$($\gamma$-\text{COOH} side chain), and $pK_2 = 9.67$ (\alpha$-\text{NH}_3^+).\n\n\n\n- **Step 1: Determine charge states across ionization stages**:\n - Fully protonated state (\text{pH} < 2.19):\n - \alpha$-\text{COOH} = 0$,$\gamma$-\text{COOH} = 0$,$\alpha$-\text{NH}_3^+ = +1\n - Net Charge = +1\n - Stage 1 (pK_1 = 2.19):\n - \alpha$-\text{COOH}$deprotonates to$\alpha$-\text{COO}^-$(charge =$-1).\n - Net Charge = (+1) + (-1) + 0 = 0. (Neutral Zwitterion Formed).\n - Stage 2 (pK_R = 4.25):\n - Side chain \gamma$-\text{COOH}$deprotonates to$\gamma$-\text{COO}^-$(charge =$-1).\n - Net Charge = (+1) + (-1) + (-1) = -1\n - Stage 3 (pK_2 = 9.67):\n - \alpha$-\text{NH}_3^+$deprotonates to$\alpha$-\text{NH}_2$(charge =$0).\n - Net Charge = 0 + (-1) + (-1) = -2\n\n- **Step 2: Identify bounding pKa values for zero net charge**:\n - The neutral species (\text{Net Charge} = 0$) exists between$pK_1 = 2.19$and$pK_R = 4.25\n\n- **Step 3: Calculate pI**:\n\npI = \frac{pK_1 + pK_R}{2} = \frac{2.19 + 4.25}{2} = \frac{6.44}{2} = 3.22\n\n# Formation of Peptide Bonds and Peptide Nomenclature\n\n### Peptide Bond Formation Mechanism\n\nAmino acids join together covalently via **peptide bonds** (substituted amide linkages) to build linear polypeptide chains and functional proteins.\n\nA peptide bond is formed through a **condensation reaction** (also termed a dehydration synthesis reaction). In this reaction:\n\n1. The alpha-carboxyl group (-\text{COOH}$or$-\text{COO}^-$) of one amino acid reacts with the alpha-amino group ($-\text{NH}_2$or$-\text{NH}_3^+) of an adjacent amino acid.\n2. A molecule of water (\text{H}_2\text{O}) is eliminated.\n3. A covalent amide linkage (-\text{C}(=\text{O})-\text{NH}-) is synthesized between the carbonyl carbon of the first amino acid and the amino nitrogen of the second.\n\n\n\nThe condensation reaction equation is expressed as:\n\n\text{Amino Acid}_1 + \text{Amino Acid}_2 \rightleftharpoons \text{Dipeptide} + \text{H}_2\text{O}\n\n### Terminology and Chain Length Classifications\n\n- **Amino Acid Residue**: When individual amino acids combine via peptide bonds, the elements of water (\text{H}_2\text{O}) are lost. The remaining unit within the chain is termed an amino acid *residue*.\n- **Dipeptide**: Two amino acid residues joined by a single peptide bond.\n- **Tripeptide**: Three amino acid residues joined by two peptide bonds.\n- **Oligopeptide**: A short polymer composed of a few amino acid residues (typically 2$to$20).\n- **Polypeptide**: A continuous, unbranched peptide chain containing between 2$and$100$amino acid residues ($2 - 100 residues).\n- **Protein**: A macromolecular polypeptide complex containing more than 100$amino acid residues ($>100 residues).\n- *Usage Note*: In biochemical contexts, the terms *peptide*, *polypeptide*, and *protein* are frequently used interchangeably depending on structural complexity.\n\n# Practice Questions and Self-Assessment\n\n### Question 1\nWhat is the one-letter code for the amino acid Tryptophan?\n- a) T\n- b) W\n- c) F\n- d) K\n\n**Answer**: **b) W**\n*Explanation*: The single-letter code for Tryptophan is **W**. Option a) T corresponds to Threonine; option c) F corresponds to Phenylalanine; option d) K corresponds to Lysine.\n\n### Question 2\nWhich amino acid has the one-letter code K?\n- a) Arginine\n- b) Glycine\n- c) Lysine\n- d) Kerosine\n\n**Answer**: **c) Lysine**\n*Explanation*: The single-letter code **K** stands for Lysine. Arginine is represented by **R**, Glycine by **G**, and kerosine is a hydrocarbon fuel, not an amino acid.\n\n### Question 3\nWhich of the following structured amino acids is acidic, and how do you recognize it?\n- Structure A: Side chain contains -\text{CH}_2\text{COO}^-\n- Structure B: Side chain contains -\text{CH}_2\text{OH}\n- Structure C: Side chain contains -\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{NH}_3^+\n\n**Answer**: **Structure A (Aspartate / Aspartic Acid)**\n*Explanation*: Structure A represents an acidic amino acid. It is recognized by inspecting the side chain (R$group), which contains an ionizable carboxyl group ($-\text{COO}^-$) that releases a proton ($H^+$$) at physiological pH, resulting in a net negative charge. Structure B is polar uncharged (Serine), and Structure C is basic positively charged (Lysine).