2.2 Hydrogen Bonds

Hydrogen bonds are the unsung heroes of biology, keeping molecules together like tiny magnets. They're weaker than covalent bonds but stronger than van der Waals forces, making them perfect for life's constant molecular dance.

In this section, we'll see how hydrogen bonds shape water's unique properties and give structure to important biological molecules like proteins and DNA. These bonds are key players in making life work at the molecular level.

Hydrogen Bonds in Biology

Definition and Role in Biological Systems

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• A hydrogen bond is a weak electrostatic attraction between a slightly positive hydrogen atom and a slightly negative atom, usually oxygen or nitrogen
• Hydrogen bonds form when a hydrogen atom covalently bonded to an electronegative atom (oxygen, nitrogen) interacts with another electronegative atom nearby
• In biological systems, hydrogen bonds play a crucial role in maintaining the structure and function of various molecules (proteins, nucleic acids, water)
• Hydrogen bonds are responsible for the cohesion of water molecules, which contributes to the unique properties of water essential for life
• The strength of hydrogen bonds is weaker than covalent bonds but stronger than van der Waals forces, allowing them to be easily formed and broken in biological processes
• Hydrogen bonds help stabilize the secondary and tertiary structures of proteins, enabling them to perform their specific functions
  • Example: Hydrogen bonds between amino acid residues in alpha helices and beta sheets
• In DNA and RNA, hydrogen bonds between complementary base pairs (adenine-thymine, guanine-cytosine) contribute to the stability and specificity of the double helix structure

Importance in Macromolecular Structure

• Hydrogen bonds play a crucial role in the secondary structure of proteins by stabilizing the folding patterns of the polypeptide chain
  • Alpha helices and beta sheets are maintained by hydrogen bonds between amino acid residues
• In the tertiary structure of proteins, hydrogen bonds contribute to the overall three-dimensional shape by forming between amino acid residues that are far apart in the primary sequence
• Hydrogen bonding between the amino and carbonyl groups of the peptide backbone helps maintain the structure and stability of proteins
• In DNA, hydrogen bonds between complementary base pairs on opposite strands contribute to the stability and specificity of the double helix
  • The strength of hydrogen bonding in DNA is sufficient to maintain the double helix structure while still allowing the strands to be separated during replication and transcription
• RNA molecules also rely on hydrogen bonding to maintain their secondary structures (hairpin loops, stem-loops), which are essential for their function
• Hydrogen bonds between sugar residues and between sugar and base molecules contribute to the overall structure and stability of nucleic acids
• In polysaccharides like cellulose, hydrogen bonds between adjacent glucose molecules help maintain the rigid structure of plant cell walls

Properties of Water

Hydrogen Bonding and Water's Unique Properties

• Water molecules form extensive hydrogen bond networks, with each molecule capable of forming up to four hydrogen bonds with neighboring molecules
• Hydrogen bonding in water contributes to its high surface tension, as the molecules at the surface experience a net inward force, creating a "skin" on the surface
  • This allows small insects (water striders) to walk on water
• The cohesive nature of water, caused by hydrogen bonding, allows water to exhibit capillary action, enabling it to move through narrow spaces without the need for external forces
  • Example: Water moving up through the xylem of plants
• Hydrogen bonds in water also contribute to its high specific heat capacity, as energy is required to break these bonds before the water temperature can rise, making water an excellent temperature buffer
  • This helps regulate temperature in living organisms and aquatic environments
• The high heat of vaporization of water is attributed to the energy required to overcome hydrogen bonds during the liquid-to-gas phase transition, which helps regulate temperature in living organisms through evaporative cooling (sweating)

Hydrogen Bonding and Water's Behavior

• Ice is less dense than liquid water due to the open lattice structure formed by hydrogen bonds, allowing ice to float on water and providing insulation for aquatic life during winter
• Hydrogen bonding enables water to dissolve a wide range of polar substances, making it an excellent solvent for biological processes
  • Polar molecules (sugars, amino acids) and ions (sodium, chloride) readily dissolve in water
• The slightly negative oxygen atom and slightly positive hydrogen atoms of water molecules interact with charged or polar regions of other molecules, facilitating dissolution
• Hydrogen bonding between water and other molecules plays a crucial role in the folding and stability of proteins, as well as the formation of lipid bilayers in cell membranes

Structure of Macromolecules

Proteins

• Hydrogen bonds play a crucial role in the secondary structure of proteins, such as alpha helices and beta sheets, by stabilizing the folding patterns of the polypeptide chain
• In the tertiary structure of proteins, hydrogen bonds contribute to the overall three-dimensional shape by forming between amino acid residues that are far apart in the primary sequence
• Hydrogen bonding between the amino and carbonyl groups of the peptide backbone helps maintain the structure and stability of proteins
• Example: Hydrogen bonds between the amino acid residues in the alpha helix of hemoglobin help maintain its structure for oxygen binding and transport

Nucleic Acids

• In DNA, hydrogen bonds form between the complementary base pairs (adenine-thymine and guanine-cytosine) on opposite strands, contributing to the stability and specificity of the double helix
• The strength of hydrogen bonding in DNA is sufficient to maintain the double helix structure while still allowing the strands to be separated during replication and transcription
• RNA molecules also rely on hydrogen bonding to maintain their secondary structures, such as hairpin loops and stem-loops, which are essential for their function
• Hydrogen bonds between sugar residues and between sugar and base molecules contribute to the overall structure and stability of nucleic acids
• Example: Hydrogen bonding between the complementary base pairs in tRNA helps maintain its cloverleaf structure, which is essential for its role in protein synthesis

Polysaccharides

• In polysaccharides like cellulose, hydrogen bonds between adjacent glucose molecules help maintain the rigid structure of plant cell walls
• Hydrogen bonding between the hydroxyl groups of neighboring sugar residues contributes to the structural integrity and strength of polysaccharides
• Example: In starch, hydrogen bonds between amylose and amylopectin molecules help maintain the granular structure for energy storage in plants