Key Concepts in Water Chemistry, Nucleic Acids, and Proteins

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Water is a fundamental molecule in biochemistry, with unique structural and chemical properties that influence biological systems.

Structure: Water (H2O) is a bent molecule with a bond angle of approximately 104.5°, resulting in a polar molecule.
Hydrogen Bonding: Water forms hydrogen bonds with itself and with other biological functional groups (e.g., alcohols, amines, carbonyls), contributing to its high boiling point and solvent properties.
Proton Donor/Acceptor: Water can act as both a proton donor (acid) and acceptor (base), facilitating acid-base reactions.
Hydrophobicity vs. Hydrophilicity: Hydrophilic compounds interact well with water (often polar or charged), while hydrophobic compounds do not (often nonpolar). Amphipathic molecules contain both hydrophilic and hydrophobic regions.
Solubility: Polar and charged molecules are generally soluble in water; nonpolar molecules are not.
Example: Sodium chloride (NaCl) is hydrophilic and highly soluble in water, while hexane is hydrophobic and insoluble.

Acids and bases ionize in water, affecting pH and biological processes.

pH Scale: Measures the concentration of hydrogen ions;
Kw: The ion product of water, at 25°C.
pH and pOH:
Strong Acids/Bases: Completely ionize in water; pH can be calculated directly from concentration.
Weak Acids/Bases: Partially ionize; characterized by acid dissociation constant () and pKa ().
Polyprotic Species: Acids or bases with more than one ionizable proton, each with its own pKa.
Example: Acetic acid is a weak acid with .

The Henderson-Hasselbalch equation relates pH, pKa, and the ratio of acid/base concentrations.

Equation:
Ionization Curve: Plots fraction of ionized species vs. pH; midpoint corresponds to pKa.
Identification: Weak acids/bases show gradual ionization; strong acids/bases show sharp transitions.
Example: The titration curve of glycine shows two pKa values for its amino and carboxyl groups.

Buffers resist changes in pH and are essential in biological systems.

Buffering Range: Effective within ±1 pH unit of the pKa.
Suitable Buffers: Weak acids/bases with pKa near desired pH.
Polyprotic Species: At each pKa, different species predominate.
Carbonic Acid/Bicarbonate System: Major physiological buffer in blood;
Buffer Preparation: Mix weak acid/base pair or adjust with strong acid/base.
Example: Phosphate buffer is used in many biological experiments.

Nucleic acids are polymers of nucleotides, which are the building blocks of DNA and RNA.

Nucleotides: Consist of a nitrogenous base, a pentose sugar, and a phosphate group.
Di- and Tri-nucleotide Oligos: Formed by phosphodiester bonds between nucleotides.
Watson-Crick Base Pairing: Adenine pairs with Thymine (A-T), Guanine pairs with Cytosine (G-C) via hydrogen bonds.
Double Helical Structure: DNA forms a right-handed double helix stabilized by base pairing and stacking interactions.
Enthalpic and Entropic Contributions: Stability arises from hydrogen bonding (enthalpic) and base stacking (entropic).
Example: The sequence 5'-ATGC-3' forms complementary base pairs with 3'-TACG-5'.

Genetic information is copied and expressed through replication, transcription, and translation.

Replication: DNA is copied to produce identical molecules.
Transcription: DNA is transcribed into RNA.
Translation: RNA is translated into protein.
DNA Sequencing: Methods such as Sanger sequencing determine nucleotide order.
Restriction Enzyme Digestion and Cloning: Enzymes cut DNA at specific sequences, enabling cloning and genetic engineering.
Example: EcoRI recognizes the sequence GAATTC and cuts between G and A.

Proteins are polymers of amino acids, each with unique properties.

Amino Acids: 20 standard amino acids, each with a specific name, three-letter code, one-letter code, and side chain properties.
Peptide Bond: Formed between the carboxyl group of one amino acid and the amino group of another; planar and rigid due to partial double bond character.
Charge States: Ionizable groups (carboxyl, amino, side chains) change charge depending on pH.
Isoelectric Point (pI): The pH at which the net charge of the amino acid is zero; for amino acids with up to three ionizable groups, (for simple cases).
Population Percentage: Calculated using Henderson-Hasselbalch equation for each ionizable group.
Formal Charge: Sum of charges on all ionizable groups at a given pH.
Example: Glutamic acid has a net negative charge at pH 7 due to its side chain carboxyl group.

Several methods are used to analyze and purify proteins based on their properties.

SDS-PAGE: Denatures proteins and separates them by size; SDS imparts uniform negative charge, DTT/BME reduce disulfide bonds.
Native PAGE: Separates proteins by charge and size without denaturation.
Ion-Exchange Chromatography: Separates proteins based on charge.
Size Exclusion Chromatography: Separates proteins based on size.
Affinity Chromatography: Separates proteins based on specific binding interactions.
Example: His-tagged proteins are purified using nickel affinity chromatography.

Technique	Principle	Application
SDS-PAGE	Size separation	Protein molecular weight determination
Native PAGE	Charge/size separation	Protein activity analysis
Ion-Exchange	Charge separation	Purification of charged proteins
Size Exclusion	Size separation	Purification of large/small proteins
Affinity	Specific binding	Purification of tagged proteins

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