Amino Acids and Peptides

01 / UNDERSTAND & APPLY

1. An amino acid is more than an amino group plus an acid

A common α-amino acid has an amino group and a carboxyl group associated with the same α carbon, represented in an un-ionized bookkeeping form as H₂N–CH(R)–COOH. R distinguishes the side chain. Glycine has R=H and therefore no α-carbon stereocentre; alanine has R=CH₃ and does. Proline has its nitrogen incorporated into a ring and is a secondary amine rather than the usual primary amine.

Most amino-acid residues incorporated by ribosomal protein synthesis have the L configuration. L does not mean levorotatory and does not automatically mean S under the CIP rules: cysteine is the familiar L/R exception because sulfur changes priority. D amino acids occur in nature, including bacterial cell-wall chemistry, so “nature contains only L amino acids” is too broad.

Side chains can be nonpolar, polar uncharged, acidic or basic, but charge depends on pH. Aspartate and glutamate usually have negatively charged side-chain carboxylates near neutral pH, whereas lysine and arginine usually carry positive side-chain charges. Histidine’s imidazole can change protonation near biologically relevant pH, making it useful in acid–base catalysis.

Essential amino acids are those that humans must obtain in adequate amounts from dietary sources because synthesis is insufficient. Nonessential means the body can ordinarily synthesize them, not that they are unnecessary. This classification is nutritional, whereas acidic/basic/polar classifications are chemical; the two should not be confused.

02 / UNDERSTAND & APPLY

2. Zwitterions explain amphoteric behaviour

In water near the intermediate pH range, a simple amino acid is predominantly H₃N⁺–CH(R)–COO⁻. This zwitterion contains opposite formal charges while its total charge is zero when the side chain is uncharged. Zero net charge is not the same as having no charged groups. Ionic interactions help explain why many amino acids are crystalline solids with relatively high melting/decomposition temperatures.

At sufficiently low pH, carboxylate accepts a proton and the main form is H₃N⁺–CH(R)–COOH, net +1. At sufficiently high pH, ammonium loses a proton and the main form is H₂N–CH(R)–COO⁻, net −1. Intermediate states coexist at equilibrium; dominant does not mean that every molecule has exactly the same protonation at every instant.

The zwitterion is amphiprotic: its ammonium can donate H⁺ and its carboxylate can accept H⁺. Amino acid plus acid/base reactions are normally proton-transfer changes, not destruction of the carbon skeleton. The two dissociation steps have different pKa values because different groups lose protons.

For side chains with additional ionizable groups, include those charges as well. A lysine molecule with two protonated amino groups and one carboxylate has net +1, even though part of it resembles the familiar neutral zwitterion. Writing the actual groups is safer than declaring all amino acids neutral at pH 7.

03 / UNDERSTAND & APPLY

3. Find the pKa pair that surrounds zero charge

The isoelectric point pI is the pH at which the ensemble-average net charge is zero. For a simple diprotic amino acid with no ionizable side chain, pI≈(pKa₁+pKa₂)/2 in the usual dilute activity approximation. It is an average of pKa values, not an average of Ka values, and it is not necessarily pH 7.

For an acidic or basic side chain, first list successive net charges as protons are lost. Select the two pKa values immediately bracketing the zero-charge form. Blindly averaging the α-carboxyl and α-ammonium pKa values can give the wrong result. Protein pI calculations are more complex because many groups and their environments contribute.

Below pI, the mean charge is positive; above pI it is negative. In an idealized electrophoresis comparison, a positively charged amino acid migrates toward the negative electrode and a negatively charged one toward the positive electrode. At pI there is no net electrophoretic drift in that simple model, although molecules still diffuse.

Charge also affects solubility and aggregation. Many proteins are least soluble near pI because electrostatic repulsion between molecules is reduced, but pI alone does not determine every protein’s solubility. Ionic strength, temperature, shape and solvent interactions also matter.

04 / UNDERSTAND & APPLY

4. Quantify protonation rather than guessing

For one resolved acid–base step, pH=pKa+log₁₀([base form]/[acid form]) under a dilute concentration approximation. The base form is the species after losing the relevant proton, even if that species is a neutral zwitterion rather than an anion. Label the pair before inserting a ratio.

Near a pKa, both adjacent forms are appreciably present and buffering is effective. Far from a pKa, one form dominates and that pair has limited buffering capacity. A pI between widely separated pKa values is not automatically the point of maximum buffer capacity.

In an ideal titration starting with a fully protonated amino acid with no ionizable side chain, two equivalents of strong base per mole are needed to remove the two relevant protons. Half-equivalence points correspond approximately to pKa values, while the first equivalence region is dominated by the zwitterion.

Overlapping ionizations, activity effects and changing pKa values in proteins can defeat a simple independent-group approximation. Numerical exercises here explicitly supply model pKa values rather than implying that free-amino-acid constants apply unchanged to every peptide.

05 / UNDERSTAND & APPLY

5. Build a directional chain with amide bonds

A peptide bond is the amide linkage –C(=O)–NH– between residues; proline can give an amide nitrogen without an N–H. In a formal condensation, the carboxyl group of one amino acid and amino group of the next lose H₂O. Actual cellular peptide synthesis uses activated intermediates and energy-coupled machinery; free amino acids do not spontaneously build proteins efficiently simply by being mixed in water.

A linear unbranched chain of n residues has n−1 peptide bonds and formally loses n−1 water molecules relative to the free amino-acid sum. The N terminus has the free α-amino end and the C terminus the free α-carboxyl end, subject to possible modifications. Conventional sequences are written N→C.

Thus Gly–Ala and Ala–Gly have the same amino-acid composition but different connectivity. Composition analysis alone cannot determine sequence. If a chain is cyclic, branched or crosslinked, the simple n−1 counting rule must be checked against its topology.

Amide resonance gives the peptide C–N bond partial double-bond character and a roughly planar peptide unit, restricting rotation. Most peptide bonds adopt trans geometry, but cis bonds occur, especially before proline. Flexibility mainly comes from rotations around other backbone single bonds; restricted amide rotation does not make the whole protein rigid.

06 / UNDERSTAND & APPLY

6. Hydrolysis, end groups and sequence evidence

Hydrolysis adds water across peptide bonds and ultimately releases amino acids under suitable acid, base or enzyme-catalyzed conditions. Protonation states of the products depend on the medium. Complete hydrolysis gives composition; selective cleavage and overlapping fragments provide sequence information.

A peptide’s amino and carboxyl end groups can ionize, while internal peptide nitrogens are far less basic than free amines because their lone pairs are delocalized into carbonyl groups. Side-chain ionizations remain relevant. Counting every nitrogen as a free basic amino group greatly overestimates positive charge.

Ninhydrin is associated with free amino groups and many amino acids give a purple product, while proline gives a different colour response. The biuret test concerns coordination involving multiple peptide linkages in alkaline copper chemistry; one isolated amino acid is not a positive protein-like biuret result. These are qualitative concepts, not unsupervised test instructions.

Sequence deductions require enough independent information. If complete hydrolysis gives one glycine and one alanine, Gly–Ala and Ala–Gly remain possible. Identification of glycine as the N-terminal residue resolves that two-residue case, provided the peptide is linear and unmodified.

07 / UNDERSTAND & APPLY

7. Four levels of structure answer different questions

Primary structure specifies the amino-acid sequence and, when fully described, covalent connectivity such as disulfide bridges. A sequence change can alter folding or function even when overall composition changes only slightly. Peptide bonds preserve the backbone order during ordinary conformational changes.

Secondary structure describes recurring local backbone arrangements, notably α helices and β sheets. Backbone C=O and N–H hydrogen bonds stabilize these patterns. In a typical α helix the carbonyl of residue i hydrogen-bonds to the N–H of residue i+4; side chains project outward rather than forming the helix’s main hydrogen-bond ladder.

β sheets consist of extended strands aligned alongside one another. Adjacent strands may run parallel or antiparallel in N→C direction, with hydrogen bonds between their backbones. Strands can belong to distant regions of one chain or to different chains; a β sheet does not itself prove that the protein has several subunits.

Tertiary structure is the overall three-dimensional organization of one polypeptide, stabilized by the hydrophobic effect, hydrogen bonds, ionic interactions, dispersion forces and sometimes covalent disulfides. Quaternary structure describes how two or more polypeptide subunits assemble. A folded single-chain protein has tertiary structure but no obligatory quaternary level.

08 / UNDERSTAND & APPLY

8. Folding and denaturation are not the same as hydrolysis

Folding brings a chain into a functional ensemble of conformations. In water, burying many nonpolar side chains reduces unfavourable exposure to solvent, while polar interactions organize compatible contacts. The hydrophobic effect is a solvent-dependent free-energy phenomenon, not a new covalent bond between hydrophobic groups.

Two cysteine thiols can be oxidized to a disulfide, R–S–S–R. Such a bridge can stabilize a loop within a chain or link separate chains. Methionine contains a thioether rather than a thiol and does not simply substitute for cysteine in this reaction. Breaking a disulfide by reduction changes covalent crosslinking but does not itself hydrolyze the peptide backbone.

Heat, extreme pH or certain solvents can disrupt interactions sustaining higher-order structure, causing denaturation and loss of activity. Ordinary denaturation usually leaves peptide-bond sequence intact. Harsh or prolonged chemical treatment may also cause covalent changes; “denaturation never changes any covalent bond” is therefore too absolute.

Renaturation may occur if conditions permit refolding and aggregation or chemical damage has not prevented recovery. It is not guaranteed. Enzyme activity depends on three-dimensional placement of functional groups, so unchanged amino-acid sequence does not mean unchanged function after denaturation. Fibrous structural proteins and soluble globular proteins emphasize different architectures, but both depend on sequence and interactions.

09 / UNDERSTAND & APPLY

9. A reliable solution sequence

For acid–base questions, draw the ionizable groups, order their pKa values, add charges and then identify pI or the relevant equilibrium ratio. For peptide calculations, specify chain topology, terminal modifications and whether free-amino-acid or residue masses were supplied.

For structure questions, ask whether the statement concerns sequence, local backbone pattern, one chain’s fold or several subunits. Separate peptide bonds, hydrogen bonds and disulfides, since each has different consequences when disrupted.

All worked examples and practice here are original. They connect quantitative acid–base chemistry, covalent connectivity and protein structure rather than treating biochemical names as a memorization list.