Water, Chemistry of Life, and Nucleic Acids: Study Notes

Water and the Chemistry of Life

Atomic Structure and Chemical Bonds

Atoms are the fundamental units of matter, consisting of protons, neutrons, and electrons. The behavior of electrons, especially those in the outermost shell (valence electrons), determines how atoms interact and form bonds.

• Valence Electrons: Electrons in the outermost shell; shared or transferred during bond formation.

• Ions: Atoms that gain or lose electrons become charged (cations if they lose electrons, anions if they gain electrons).

• Electronegativity: The tendency of an atom to attract electrons in a bond. Order: O > N > C ≈ H.

• Polar Covalent Bonds: Electrons are shared unequally, creating partial charges (e.g., O-H in water).

• Nonpolar Covalent Bonds: Electrons are shared equally (e.g., C-H, N≡N).

Example: In water (H2O), the O-H bonds are polar because oxygen is more electronegative than hydrogen.

Hydrogen Bonds and Properties of Water

Water's unique properties arise from its ability to form hydrogen bonds, which are weak attractions between the partial positive charge on hydrogen and the partial negative charge on oxygen in neighboring molecules.

• Hydrogen Bonds: Weak bonds (compared to covalent bonds) but crucial for water's properties.

• High Specific Heat: Water absorbs a lot of energy before changing temperature due to hydrogen bonds.

• High Heat of Vaporization: Large amounts of energy are needed to break hydrogen bonds for evaporation (why sweating cools us).

• Surface Tension: Water molecules stick together at the surface, allowing insects to walk on water.

• Cohesion: Water molecules stick to each other.

• Adhesion: Water molecules stick to other substances (important for capillary action in plants).

• Ice Structure: Hydrogen bonds stabilize in ice, making it less dense than liquid water.

Example: The hexagonal structure of snowflakes is due to hydrogen bonding.

Bonds and Energy

Breaking chemical bonds requires energy input. Covalent bonds are stronger than hydrogen bonds, meaning more energy is needed to break them.

• Bond Strength: Covalent > Hydrogen bonds.

• Heat: Increases molecular motion, leading to more frequent bond breaking and formation.

• Boiling Water: Breaks hydrogen bonds, not covalent bonds.

Solubility: Polar and Non-Polar Molecules

Water is a polar solvent, so it dissolves polar and ionic substances (hydrophilic), but not non-polar substances (hydrophobic).

• Hydrophilic: Water-loving; polar molecules that dissolve in water.

• Hydrophobic: Water-fearing; non-polar molecules that do not dissolve in water and tend to cluster together.

Example: Ethanol (polar) dissolves in water; oil (non-polar) does not.

pH and Acids/Bases

Water can dissociate into H+ and OH- ions. The concentration of H+ ions determines the pH of a solution.

• pH Definition: $pH = -\log_{10}[H^+]$

• Neutral pH: Water has pH 7 (equal H+ and OH-).

• Acidic: pH < 7; more H+ ions.

• Basic: pH > 7; fewer H+ ions (more OH-).

• Biological Importance: Extreme pH disrupts hydrogen bonds in biomolecules.

Example: Adding HCl to water increases H+ concentration, lowering pH.

Summary Table: Hydrogen Bonds vs. Covalent Bonds

Bond Type	Strength	Example	Role in Biology
Covalent	Strong	O-H in H2O	Stabilizes molecules
Hydrogen	Weak	Between H2O molecules	Gives water unique properties

Nucleic Acids

Structure of Nucleotides

Nucleic acids (DNA and RNA) are polymers made of nucleotide monomers. Each nucleotide consists of three parts:

• Phosphate Group

• Pentose Sugar: Deoxyribose (DNA) or Ribose (RNA)

• Nitrogenous Base: Adenine (A), Thymine (T, DNA only), Uracil (U, RNA only), Cytosine (C), Guanine (G)

Phosphate Group: Loses hydrogen ions in water, making nucleotides acidic and solutions with nucleotides have low pH.

Carbon Numbering: The carbons in the sugar are numbered 1' to 5'. The phosphate attaches to the 5' carbon, and the next nucleotide attaches to the 3' carbon.

Polymerization and Polarity

Nucleotides are joined by covalent bonds (phosphodiester bonds) between the 3' OH of one sugar and the 5' phosphate of the next.

• Polarity: Nucleic acid polymers have directionality (5' to 3').

• Backbone: Sugar-phosphate backbone is consistent; bases project from one side.

• RNA vs. DNA: RNA has an extra OH on the 2' carbon; DNA has H.

Base Pairing and Double Helix

DNA is typically double-stranded, with two antiparallel strands held together by hydrogen bonds between complementary bases.

• Base Pairing: A pairs with T (or U in RNA), G pairs with C.

• Hydrogen Bonds: A-T pairs have 2 H-bonds; G-C pairs have 3 H-bonds (G-C pairs are stronger).

• Antiparallel Strands: One strand runs 5' to 3', the other 3' to 5'.

Example: If 35% of DNA is G, then 35% is C, and the remaining 30% is split between A and T (15% each).

Experimental Evidence for DNA Structure

• Chargaff's Rules: %A = %T and %G = %C in DNA from many organisms.

• Rosalind Franklin's X-ray Crystallography: Revealed the double helix structure, width, and periodicity of DNA.

• Watson and Crick: Modeled DNA structure using base pairing rules and Franklin's data.

Applications: DNA Origami

Base pairing rules allow scientists to design and fold DNA into complex 3D shapes for nanotechnology applications.

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strands	Double (usually)	Single (usually)
Function	Genetic storage	Information transfer, catalysis

Key Learning Objectives

• Compare hydrogen and covalent bonds in terms of strength and mechanism.

• Draw and label water molecules, indicating electron distribution, partial charges, and hydrogen bonds.

• Identify polar and nonpolar covalent bonds in biomolecules and predict their interactions with water.

• Define acid, base, and pH; sketch the pH scale and locate strong acids, bases, and water.

• Explain how hydrogen bonding leads to phenomena like adhesion and capillary action.

• Compare DNA and RNA structure, chemical composition, location, and function.

• Define complementary base pairing and its connection to antiparallel DNA strands.

• Use base pairing rules to predict complementary DNA sequences and calculate base percentages.

Additional info: These notes integrate foundational chemistry concepts with their biological applications, focusing on water and nucleic acids as central molecules in life processes.