Unit 7: Purines, Pyrimidines & Nucleic Acid Biochemistry

Nucleic acids store and transmit genetic information using heterocyclic nitrogenous bases derived from two parent aromatic scaffolds: purine (fused pyrimidine-imidazole bicycle) and pyrimidine (six-membered diazine).

The Canonical Bases

1. Purines (Fused 9-Membered Bicycle):

• Adenine (A): 6-Aminopurine ($C_5H_5N_5$)
• Guanine (G): 2-Amino-6-oxopurine ($C_5H_5N_5O$)

2. Pyrimidines (6-Membered Monocycle):

• Cytosine (C): 4-Amino-pyrimidin-2(1H)-one ($C_4H_5N_3O$)
• Uracil (U, in RNA): Pyrimidine-2,4(1H,3H)-dione ($C_4H_4N_2O_2$)
• Thymine (T, 5-methyluracil, in DNA): 5-Methylpyrimidine-2,4(1H,3H)-dione ($C_5H_6N_2O_2$)

Aromaticity and UV Absorption

Purines and pyrimidines possess extensive delocalized $\pi$-systems obeying Hückel's $(4n+2)\pi$-electron rule ($10\pi$-electrons for purine, $6\pi$-electrons for pyrimidine). The electronic transitions ($\pi \to \pi^*$) give rise to strong ultraviolet absorption spectra with characteristic absorption maxima around $\lambda_{\text{max}} \approx 260\text{ nm}$ ($\epsilon \sim 7,000 - 15,000\text{ M}^{-1}\text{cm}^{-1}$).

Prototropic Tautomerism and Mutational Implications

Under physiological conditions, bases exist in dynamic prototropic equilibria:

• Keto (Lactam) vs Enol (Lactim): Guanine, thymine, and uracil exist predominantly ($>99.99\%$) in the keto (lactam) tautomeric form.
• Amino vs Imino: Adenine and cytosine exist predominantly in the amino tautomeric form.

Transient shifts to the rare enol or imino tautomers (equilibrium constant $K_T \sim 10^{-4} - 10^{-5}$) alter the hydrogen-bonding donor/acceptor pattern:

• The rare imino form of adenine pairs aberrantly with cytosine instead of thymine ($A^* \cdot C$).
• The rare enol form of thymine pairs with guanine ($T^* \cdot G$).

These transient tautomeric shifts represent an intrinsic chemical source of spontaneous transition mutations during DNA replication.

Thermodynamic Nearest-Neighbor Matrix for DNA Duplex Stability in 1.0 M Na+

The stability of a DNA double helix is calculated by summing pairwise nearest-neighbor interactions ($\Delta H^\circ$ in $\text{kJ/mol}$, $\Delta S^\circ$ in $\text{J/(mol}\cdot\text{K)}$):

| Sequence Step ($5^\prime \to 3^\prime / 3^\prime \to 5^\prime$) | Enthalpy $\Delta H^\circ$ ($\text{kJ/mol}$) | Entropy $\Delta S^\circ$ ($\text{J/(mol}\cdot\text{K)}$) | Free Energy $\Delta G^\circ_{298}$ ($\text{kJ/mol}$) | | :--- | :--- | :--- | :--- | | AA / TT | $-33.1$ | $-92.9$ | $-5.4$ | | AT / TA | $-30.1$ | $-85.4$ | $-4.6$ | | TA / AT | $-25.1$ | $-72.4$ | $-3.5$ | | CA / GT | $-35.6$ | $-95.0$ | $-7.3$ | | GT / CA | $-35.1$ | $-93.7$ | $-7.2$ | | CT / GA | $-32.6$ | $-87.9$ | $-6.4$ | | GA / CT | $-34.3$ | $-92.5$ | $-6.7$ | | CG / GC | $-44.4$ | $-113.8$ | $-10.5$ | | GC / CG | $-41.0$ | $-102.1$ | $-10.6$ | | GG / CC | $-33.5$ | $-83.3$ | $-8.7$ | | Helix Initiation (terminal GC) | $+0.4$ | $-11.7$ | $+3.9$ | | Helix Initiation (terminal AT) | $+9.6$ | $+17.2$ | $+4.5$ |

Wilhelm Traube developed the classical, robust synthetic methodology (1900) for constructing purines from simple acyclic precursors, which remains the primary industrial and laboratory route.

The Traube Synthesis of Guanine

1. Condensation to Pyrimidine:

Guanidine condenses with ethyl cyanoacetate in ethanolic sodium ethoxide:

2. Nitrosation:

Treatment with nitrous acid ($\text{HNO}_2$) introduces a nitroso group at the nucleophilic C5 position:

3. Reduction to 4,5-Diamine:

Catalytic hydrogenation or reduction with sodium dithionite ($\text{Na}_2\text{S}_2\text{O}_4$) or ammonium sulfide reduces the nitroso group to an amine, yielding 2,4,5-triamino-6-hydroxypyrimidine.

4. Ring Closure with C1 Donors (Formylation and Imidazole Ring Closure):

Heating the 4,5-diaminopyrimidine with formic acid ($\text{HCOOH}$) or formamide introduces a formyl group ($-\text{CHO}$) at the C5-amino group. Subsequent heating above $200^\circ\text{C}$ or treatment with alkali induces intramolecular cyclodehydration between the formamide carbonyl and the C4 amino group, closing the 5-membered imidazole ring to afford guanine.

Uric acid (2,6,8-trioxypurine, $C_5H_4N_4O_3$) is the primary end-product of purine metabolism in birds, reptiles, and humans. Overproduction or impaired excretion causes hyperuricemia, leading to the deposition of monosodium urate crystals in joints (gout).

Structural Proof through Classical Oxidative Degradation (Baeyer and Emil Fischer)

Uric acid was structurally elucidated by dissecting it with targeted oxidizing agents:

1. Nitric Acid Oxidation (Cleavage of Imidazole Ring):

Treatment of uric acid with concentrated nitric acid ($\text{HNO}_3$) selectively oxidizes and cleaves the five-membered imidazole ring, releasing alloxan (mesoxalylurea) and urea:

Alloxan is pyrimidine-2,4,5,6-tetraone. Reduction of alloxan with $\text{H}_2\text{S}$ yields dialuric acid; condensation of equimolar alloxan and dialuric acid yields the purple ammonium salt murexide, the basis of the diagnostic murexide test for uric acid.

2. Alkaline Potassium Permanganate Oxidation (Cleavage of Pyrimidine Ring):

Oxidation of uric acid with cold alkaline $\text{KMnO}_4$ selectively oxidizes the six-membered pyrimidine ring, cleaving it with loss of $\text{CO}_2$ to yield allantoin ($C_4H_6N_4O_3$):

Further alkaline hydrolysis of allantoin cleaves it into allantoic acid, which hydrolyzes to glyoxylic acid ($\text{CHO-COOH}$) and two equivalents of urea. These two complementary degradations—one isolating the intact pyrimidine ring (alloxan) and the other isolating the intact imidazole derivative (allantoin)—rigorously established the 2,6,8-trioxypurine constitution of uric acid.

Nucleosides and nucleotides are the monomeric subunits of nucleic acids and vital biochemical cofactors (ATP, NAD+, CoA).

Architecture and Nomenclature

• Nucleoside: A purine or pyrimidine base linked via a covalent $\beta$-N-glycosidic bond to the C1' carbon of a pentose sugar:
• Ribose in ribonucleosides (Adenosine, Guanosine, Cytidine, Uridine).
• 2'-Deoxyribose in 2'-deoxyribonucleosides (dA, dG, dC, dT).
• Attachment point: Linked to N9 of purines or N1 of pyrimidines.
• Nucleotide: A nucleoside phosphorylated at one of its sugar hydroxyl groups (typically C5', C3', or C2'):
• E.g., Adenosine 5'-monophosphate (AMP), adenosine 5'-triphosphate (ATP).

Conformational Dynamics: Syn vs Anti

Rotation around the $\beta$-N-glycosidic bond ($\chi$ torsion angle) is restricted by steric interactions:

• Anti Conformation: The bulky Watson-Crick face of the base points away from the pentose sugar ring ($\chi \approx 180^\circ$ to $-120^\circ$).
• Syn Conformation: The base points directly over the furanose ring ($\chi \approx 0^\circ \pm 90^\circ$).

In canonical B-DNA and RNA duplexes, all bases adopt the anti conformation to permit unhindered Watson-Crick base pairing. Left-handed Z-DNA is a notable exception where alternating purines adopt the syn conformation.

Hydrolysis Mechanics: RNA vs DNA

• DNA is highly resistant to alkaline hydrolysis: Because 2'-deoxyribose lacks a 2'-hydroxyl group, the phosphodiester backbone of DNA remains intact in $1\text{ M NaOH}$ for months.
• RNA is rapidly cleaved in alkali: The 2'-hydroxyl group ($-\text{OH}$) is deprotonated by base ($-\text{O}^-$), which attacks the adjacent phosphorus atom in an intramolecular nucleophilic substitution, forming a strained 2',3'-cyclic phosphate intermediate that hydrolyzes randomly to 2'- and 3'-monophosphates, fragmenting the RNA chain within minutes.

In 1953, James Watson and Francis Crick elucidated the double-helical structure of deoxyribonucleic acid (B-DNA), incorporating Rosalind Franklin's Photo 51 X-ray fiber diffraction data and Erwin Chargaff's rules of base equivalence.

Watson-Crick B-DNA Geometric Parameters

1. Right-Handed Antiparallel Strands: Two polynucleotide chains wind around a central axis in a right-handed helix, running in opposite directions ($5^\prime \to 3^\prime$ and $3^\prime \to 5^\prime$).

2. Helical Dimensions:

• Diameter of duplex: $2.0\text{ nm}$ ($20\text{ \AA}$).
• Helical pitch (one complete turn): $3.4\text{ nm}$ ($34\text{ \AA}$).
• Base pairs per turn: $10.5\text{ bp}$ in solution ($10.0\text{ bp}$ in fiber crystal).
• Axial rise per base pair ($h$): $0.34\text{ nm}$ ($3.4\text{ \AA}$).

3. Major and Minor Grooves: Because the glycosidic bonds do not emanate at $180^\circ$ directly opposite each other, helical twisting creates two unequal surface indentations:

• Major Groove: Wide ($12\text{ \AA}$) and deep ($8.5\text{ \AA}$), presenting sequence-specific arrays of hydrogen-bond donors, acceptors, and methyl groups that enable recognition by transcription factors and restriction enzymes.
• Minor Groove: Narrow ($6\text{ \AA}$) and deep ($7.5\text{ \AA}$).

Thermodynamic Stability: Base Stacking vs Hydrogen Bonding

While complementary Watson-Crick base pairing ($\text{A}=\text{T}$ with 2 hydrogen bonds; $\text{G}\equiv\text{C}$ with 3 hydrogen bonds) provides strict genetic specificity, the primary thermodynamic driving force stabilizing the double helix is hydrophobic $\pi$-$\pi$ base stacking:

Aromatic base rings stack planar surfaces at van der Waals contact distance ($3.4\text{ \AA}$), burying hydrophobic surfaces and displacing organized water molecules into bulk solvent.

Advanced Research Monograph: Non-Canonical Nucleic Acid Architectures: G-Quadruplexes & i-Motifs

Beyond the classic Watson-Crick double helix, guanine- and cytosine-rich sequences fold into stable non-canonical four-stranded topologies under physiological conditions:

1. G-Quadruplexes (G4 DNA/RNA):

Sequences containing repetitive guanine tracts ($\text{G}_{\ge 3}\text{N}_{1-7}\text{G}_{\ge 3}\text{N}_{1-7}\text{G}_{\ge 3}\text{N}_{1-7}\text{G}_{\ge 3}$) assemble into G-quartets:

• Four guanine bases arrange in a planar square stabilized by eight cyclic Hoogsteen hydrogen bonds.
• Two to four planar quartets stack atop each other, coordinated to a central monovalent cation ($K^+ \gg Na^+$) positioned in the central channel cavity.
• G-quadruplexes occur with high density at human chromosome telomeres ($(\text{TTAGGG})_n$) and oncogene promoter regions (c-MYC, c-KIT, BCL-2), functioning as natural transcriptional repressors.

2. i-Motif DNA (Intercalated Motifs):

Cytosine-rich sequences fold into four-stranded structures held together by intercalated, hemi-protonated cytosine-cytosine base pairs ($\text{C}\cdot\text{C}^+$) requiring protonation of N3:

• Stable at slightly acidic to neutral pH ($\text{pH } 5.5 - 7.0$).
• Act as molecular pH sensors in living mammalian nuclei, regulating cell-cycle gene expression.

Thermal denaturation (DNA melting) is the reversible, cooperative separation of the double-stranded helix into two random-coil single strands upon heating.

The Hyperchromic Effect

• In native double-stranded DNA, the closely stacked aromatic base pairs align their transition dipoles in parallel arrays, resulting in mutual dipole shielding and hypochromicity (suppression of UV absorption at $260\text{ nm}$).
• Upon denaturation into unstacked single strands, electronic transition dipoles become uncoupled, causing a $30–40\%$ increase in UV absorbance at $260\text{ nm}$, termed the hyperchromic effect.

Melting Temperature ($T_m$)

The melting temperature ($T_m$) is the temperature at which exactly $50\%$ of the helical structure is denatured ($f_{\text{denatured}} = 0.50$).

1. Dependence on GC Content:

Each $\text{G}\equiv\text{C}$ base pair contributes three hydrogen bonds and stronger base stacking compared to two hydrogen bonds in an $\text{A}=\text{T}$ pair. For long DNA duplexes in $1\text{ M Na}^+$ (Marmur-Doty equation):

2. Dependence on Ionic Strength:

The negatively charged phosphate diester groups ($-\text{O}-\text{PO}_2^--\text{O}-$) along the sugar-phosphate backbones repel each other across the minor and major grooves. Divalent ($Mg^{2+}$) and monovalent ($Na^+$) cations screen these electrostatic repulsions, dramatically stabilizing the helix and raising $T_m$:

where $L$ is the length of the duplex in base pairs.

Unlike double-stranded DNA, ribonucleic acid (RNA) molecules typically exist as single strands that fold into intricate three-dimensional tertiary architectures capable of catalytic function (ribozymes).

Structural Diversity of Cellular RNAs

1. Messenger RNA (mRNA): Transmits coding sequence from genomic DNA to the ribosome. Eukaryotic mRNAs feature a $5^\prime\text{-m}^7\text{GpppN}$ cap and a $3^\prime\text{-poly(A)}$ tail.

2. Transfer RNA (tRNA): Molecular adaptors (76–90 nucleotides) translating triplet codons into amino acids.

• Secondary Structure: Classical cloverleaf with acceptor stem, D-loop, anticodon loop, and T$\psi$C-loop.
• Tertiary Structure: Coaxial stacking folds the cloverleaf into an L-shaped architecture with the anticodon at one tip and the 3'-CCA-OH aminoacyl attachment site at the other, $75\text{ \AA}$ apart.

3. Ribosomal RNA (rRNA): Structural and catalytic core of the ribosome. Thomas Steitz and coworkers proved that the peptidyl transferase center of the 50S large subunit is composed purely of 23S rRNA with zero protein side chains within $18\text{ \AA}$, establishing that the ribosome is a ribozyme.

Catalytic RNAs (Ribozymes)

Discovered by Thomas Cech (self-splicing group I intron of Tetrahymena) and Sidney Altman (RNase P), ribozymes utilize coordinated divalent metal ions ($Mg^{2+}$) and active-site nucleobases with shifted $pK_a$ values to catalyze phosphodiester transesterifications and cleavages with rate enhancements exceeding $10^{11}$-fold.

Modern genomics, PCR diagnostics, and synthetic biology rely on the automated chemical synthesis of defined single-stranded DNA and RNA oligonucleotides via solid-phase phosphoramidite chemistry.

The Standard Phosphoramidite Cycle

Synthesized in the $3^\prime \to 5^\prime$ direction on controlled-pore glass (CPG) solid support:

1. Detritylation (Deblocking):

The 5'-dimethoxytrityl (DMT) ether is cleaved with $3\%$ trichloroacetic acid (TCA) in dichloromethane, releasing the orange $\text{DMT}^+$ trityl cation and unmasking the reactive 5'-OH group. Spectrophotometric measurement of $\text{DMT}^+$ at $498\text{ nm}$ provides real-time monitoring of stepwise cycle yields.

2. Coupling (Chain Elongation):

The incoming monomer (a 5'-DMT-2'-deoxynucleoside-3'-O-($\beta$-cyanoethyl-$N,N$-diisopropyl) phosphoramidite) is mixed with 1H-tetrazole (or 5-ethylthiotetrazole):

• Tetrazole protonates the diisopropylamino leaving group.
• The nucleophilic 5'-OH of the solid-supported chain attacks the activated phosphite phosphorus, forming an internucleotide phosphite triester linkage in $>99.5\%$ yield within 60 seconds.

3. Capping:

Unreacted 5'-OH groups ($<0.5\%$) are acetylated using acetic anhydride and 1-methylimidazole (NMI), terminating failed chains and preventing deletion mutations ($N-1$ sequences).

4. Oxidation:

The unstable trivalent phosphite triester ($\text{P}^{\text{III}}$) is oxidized to a stable pentavalent phosphate triester ($\text{P}^{\text{V}}=\text{O}$) using aqueous iodine in pyridine/THF.

Final Deprotection and Cleavage

After assembling the desired sequence:

• The oligonucleotide is cleaved from the CPG support and the cyanoethyl protecting groups on phosphate are removed using concentrated aqueous ammonium hydroxide ($28\%\text{ NH}_4\text{OH}$) at $55^\circ\text{C}$.
• Basic treatment also hydrolyzes the exocyclic amino protecting groups (benzoyl on A and C; isobutyryl on G).
• The crude oligonucleotide is purified by reversed-phase HPLC or polyacrylamide gel electrophoresis (PAGE).