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Biochemistry I - The Importance of Weak Interactions

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    Study Guide Biochemistry I – The Importance of Weak Interactions 1. The Hydrophobic Effect Water is very good at forming hydrogen bonds with itself . Because of this, water is most comfortable around substances that do not interfere with this bonding network. This explains a familiar observation: oil floats on water . Oil molecules are nonpolar , meaning they have no charge and do not mix well with water. When a nonpolar molecule (like oil) enters water, it disrupts water’s hydrogen - bonding network . To deal with this disruption, water molecules rearrange themselves around the nonpolar substance, forming a kind of ordered “cage.” However, creating this ordered structure goes against the Second Law of Thermodynamics , which says that natural processes tend to increase entropy (disorder). Figure 1 1.1 Why Nonpolar Molecules Stick Together So how does the system reduce this problem?

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    Study Guide The solution is for nonpolar molecules to cluster together . When they do this: • Fewer water molecules are needed to surround them • Less order is imposed on the water • Overall disorder (entropy) increases This makes the system more stable. 1.2 A Simple Analogy: Blocks and Paint Imagine water forming a “coating” around nonpolar molecules, like paint on cubes. • If you paint four separate cubes , each with six sides, you must paint 24 sides total. • If you stack the four cubes together, some sides are hidden, and you only need to paint 16 sides . In the same way, when nonpolar molecules come together, less surface area touches water , so fewer water molecules need to form ordered cages. This tendency of nonpolar molecules to group together in water is called the hydrophobic effect . Although the name suggests that molecules “hate water,” the effect is really driven by water’s preference to hydrogen - bond with itself . 1.3 Amphipathic Molecules: Both Water - Loving and Water - Hating Many biological molecules are amphipathic , meaning they contain: • A hydrophobic (water - repelling) part • A hydrophilic (water - attracting) part Example: Fatty Acids and Soap Palmitic acid has: • A charged carboxyl group (hydrophilic) • A long hydrocarbon chain (hydrophobic) When its sodium salt (sodium palmitate) is placed in water:

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    Study Guide • The hydrophobic tails cluster together • The hydrophilic heads face the water This forms a micelle — a small spherical structure with: • Hydrophobic parts inside • Hydrophilic parts outside Soap works this way. Micelles trap grease and oils inside their hydrophobic core, allowing them to be washed away in water . Detergents are similar but often more powerful , because their hydrophilic ends are more strongly charged. A common example is sodium dodecyl sulfate (SDS) , found in shampoos and widely used in laboratories to disrupt membranes and proteins. 1.4 Membrane Associations Cell membranes are built using the same principles. Phospholipids Many membrane lipids are glycerol esters of fatty acids , but unlike fats: • They contain two fatty acid chains • They also have a hydrophilic head group This makes them amphipathic. A common example is phosphatidylcholine , which contains: • Two hydrophobic fatty acid tails • A charged phosphate – choline head

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    Study Guide 1.5 From Molecules to Membranes When phospholipids are placed in water: • The hydrophobic tails cluster together • The hydrophilic heads face the water Instead of forming micelles, phospholipids form a bilayer . This bilayer can close into a sphere called a liposome , which has: • An inside • An outside Liposomes closely resemble cell membranes . 1.6 Properties of Biological Membranes Biological membranes: • Are made of lipid bilayers • Contain many proteins • Have different lipids on the inner and outer surfaces • Are semipermeable This means they: • Block many hydrophilic molecules (like ions and sugars) • Allow small nonpolar molecules (like oxygen) and water to pass more easily Key Takeaway The hydrophobic effect — driven by water’s hydrogen bonding — is a fundamental force in biology. It explains why: • Oils clump together in water • Soap cleans grease • Membranes form naturally Without the hydrophobic effect, cell membranes, protein structure, and life itself would not exist .

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    Study Guide 2. Electrostatic and van der Waals Interactions Inside cells, molecules are held together and shaped by weak, noncovalent forces . Two of the most important are electrostatic interactions and van der Waals interactions . Individually these forces are small, but together they play a major role in determining the structure and behavior of biological molecules. 2.1 Electrostatic Interactions: Opposites Attract Electrostatic interactions occur between charged particles . • Opposite charges attract , and like charges repel . • For example, Mg² ⁺ ions (positively charged) are attracted to the negatively charged phosphate groups in nucleotides and nucleic acids. • Inside proteins, a salt bridge can form when: o A positively charged amino group is close to o A negatively charged carboxylate group These interactions are especially important in nucleic acids (DNA and RNA) because: • Each building block carries a full negative charge • Electrostatic forces help stabilize their folded structures and interactions with ions and proteins

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    Study Guide Figure 1 2.2 van der Waals Interactions: Weak but Widespread van der Waals interactions arise from the way atomic charges are distributed . • An atom has: o A positively charged nucleus o A surrounding cloud of negatively charged electrons • When two atoms get close: o The nucleus of one atom attracts the electron cloud of the other o This creates a weak attractive force Distance Matters • If atoms are too far apart , the attraction is negligible • If atoms are too close , their electron clouds overlap and repel each other • Therefore, each pair of atoms has an optimal distance where attraction is strongest o For identical atoms, this distance is about twice the atomic radius (d = 2r)

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