2027 · High School Specialised Subject · Biochemistry

2.1 Weak Interactions in Aqueous Solutions

Hydrogen bonds, the hydrophobic effect, colligative properties

Hydrogen bonding and electrostatics, entropy of dissolution, the hydrophobic effect, van der Waals contacts, and how weak interactions together stabilise macromolecules and set colligative properties.

10 SECTIONS19 FIGURESEnglish edition

01

Hydrogen bonding gives water its unusual properties

Two O–H bonds at about 104.5° give a bent, strongly dipolar molecule
Each water can donate two hydrogen bonds and accept two, forming a dynamic network
Consequences: high boiling point, high surface tension, high heat capacity, open ice lattice

Figure 2-1-1Hydrogen bonding gives water its unusual properties — reference figure

Table 2.1-1Properties of water and their biological meaning

Table 2.1-4Classes of noncovalent interaction

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01

Hydrogen bonding gives water its unusual properties

Two O–H bonds at about 104.5° give a bent, strongly dipolar molecule
Each water can donate two hydrogen bonds and accept two, forming a dynamic network
Consequences: high boiling point, high surface tension, high heat capacity, open ice lattice

Figure 2-1-1Hydrogen bonding gives water its unusual properties — reference figure

02

Water hydrogen-bonds with polar solutes

Hydration shells surround hydroxyl, amino and carbonyl groups
Sugars, amino acids and short peptides dissolve because hydration compensates for lost crystal contacts
Directionality of hydrogen bonds makes recognition specific rather than merely tight

Figure 2-1-2Hydrogen bonding gives water its unusual properties — reference figure

Table 2.1-2Hydrogen-bond donors and acceptors in biological groups

Table 2.1-6Concentration measures and tonicity

03

Water interacts electrostatically with charged solutes

Ion–dipole orientation: cations attract oxygen lone pairs, anions attract hydrogens
Solvation shells screen charge–charge attraction
Dielectric constant of water near 78 at 25 °C weakens salt bridges compared with vacuum

Figure 2-1-3Water hydrogen-bonds with polar solutes — reference figure

04

Dissolution of crystalline solids increases entropy

A crystal has few accessible arrangements; a solution has many
The entropy gain of dispersal offsets part of the lattice enthalpy
ΔG = ΔH − TΔS decides whether dissolution is favourable

Figure 2-1-4Water hydrogen-bonds with polar solutes — reference figure

Table 2.1-3Thermodynamic contributions to dissolution of a crystal

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04

Dissolution of crystalline solids increases entropy

A crystal has few accessible arrangements; a solution has many
The entropy gain of dispersal offsets part of the lattice enthalpy
ΔG = ΔH − TΔS decides whether dissolution is favourable

Figure 2-1-4Water hydrogen-bonds with polar solutes — reference figure

05

Nonpolar gases are poorly soluble in water

Cavity formation costs energy that a nonpolar solute cannot repay
Water around a nonpolar surface becomes more ordered: an entropic cost
Gas solubility falls with rising temperature, so warm water holds less oxygen

Figure 2-1-5Water interacts electrostatically with charged solutes — reference figure

06

Nonpolar compounds disturb the structure of water

Nonpolar groups cluster to minimise the ordered water surface around them
Driving force is the entropy gained by released water, not attraction between nonpolar groups
Underlies bilayer formation, protein cores, micelles and ligand binding

Figure 2-1-6Water interacts electrostatically with charged solutes — reference figure

07

Van der Waals interactions are weak attractions between atoms

Fluctuating dipoles induce dipoles in neighbouring atoms
Attractive as 1/r⁶ with strong short-range repulsion; optimum at contact distance
0.4–4 kJ/mol per contact, but hundreds act together across a complementary surface

Figure 2-1-7Dissolution of crystalline solids increases entropy — reference figure

08

Weak interactions appear together in many combinations

An interface typically combines hydrogen bonds, ion pairs, hydrophobic contacts and packing
Individually weak energies sum to tens of kJ/mol for a well-matched surface
Geometry requirements create specificity; weak bonds allow reversible assembly

Figure 2-1-8Dissolution of crystalline solids increases entropy — reference figure

09

Weak interactions are central to macromolecular structure and function

Protein folding: hydrophobic core, backbone hydrogen bonds, side-chain packing
Nucleic acids: base pairing plus base stacking
Membranes, enzyme recognition, hormone–receptor binding and allostery all use this toolkit

Figure 2-1-9Nonpolar gases are poorly soluble in water — reference figure

10

Solutes affect the colligative properties of aqueous solutions

Osmotic pressure, boiling-point elevation, freezing-point depression depend on particle number
π = iMRT for dilute ideal solutions
Cells regulate osmolarity to control water movement
ΔTf = i·Kf·m underlies biological antifreeze strategies

Figure 2-1-10Nonpolar gases are poorly soluble in water — reference figure

Table 2.1-5The four colligative properties

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10

Solutes affect the colligative properties of aqueous solutions

Osmotic pressure, boiling-point elevation, freezing-point depression depend on particle number
π = iMRT for dilute ideal solutions
Cells regulate osmolarity to control water movement
ΔTf = i·Kf·m underlies biological antifreeze strategies

Figure 2-1-10Nonpolar gases are poorly soluble in water — reference figure

References

**Table 2.1-1**Properties of water and their biological meaning
Property	Molecular explanation	Biological meaning
High heat capacity	Energy changes in the hydrogen-bond network	Damps temperature fluctuation
High heat of vaporisation	Much energy needed for the liquid-to-gas transition	Evaporative cooling
Cohesion and surface tension	Free-energy cost of creating a surface	Water movement and interfacial phenomena
Low density of ice	Formation of an open solid lattice	Ice floats; aquatic environments are protected
High dielectric constant	Weakened electrostatic interaction between ions	Hydration and dissolution of ionic substances

**Table 2.1-2**Hydrogen-bond donors and acceptors in biological groups
Group or molecule	Typical role	Biological example
Hydroxyl –OH	Both donor and acceptor	Sugars, serine residues
Carbonyl C=O	Oxygen acts as acceptor	Peptide bond
Amide –NH–	N–H acts as donor	Polypeptide backbone
Carboxylate –COO⁻	Oxygen accepts; charge interaction	Aspartate residue
Ammonium –NH₃⁺	N–H donates; positive charge	Lysine residue

**Table 2.1-3**Thermodynamic contributions to dissolution of a crystal
Factor	Typical contribution	Caution
Breaking the crystal lattice	Overcoming attractions costs energy	Depends on the lattice and the process
Hydration or solvation	New solute–water interactions, often stabilising	Water ordering and entropy also change
Dispersal of solute particles	Tends to increase the number of arrangements	Not identical to the total ΔS
Overall ΔG	Combined result of ΔH and TΔS	Depends on temperature, concentration and saturation

**Table 2.1-4**Classes of noncovalent interaction
Interaction	Main principle	Example	Important condition
Hydrogen bond	Directional X–H···Y interaction	Peptide backbone, base pairs	Geometry; competition with water
Electrostatic	Forces between charges or partial charges	Salt bridge, ion–dipole	Ionic strength, dielectric constant
Van der Waals	Dipole and dispersion forces with short-range repulsion	Substrate contact, base stacking	Distance and surface complementarity
Hydrophobic effect	Hydration free energy of nonpolar surfaces	Protein interior, lipid bilayer	Temperature, solute size, solvent

**Table 2.1-5**The four colligative properties
Property	Dilute ideal solution	Interpretation and unit
Vapour-pressure lowering	p(solvent) = x(solvent) × p°	Nonvolatile solute, ideal solution, mole fraction
Boiling-point elevation	ΔTb = i·Kb·m	Kb: ebullioscopic constant (°C·kg/mol)
Freezing-point depression	ΔTf = i·Kf·m	For water Kf ≈ 1.86 °C·kg/mol
Osmotic pressure	π = iMRT	M in mol/L; R in L·atm/(mol·K)

**Table 2.1-6**Concentration measures and tonicity
Concept	How it is defined	Caution
Molarity M	Amount of solute species / volume of solution	Volume changes with temperature
Molality m	Amount of solute species / mass of solvent	Based on solvent mass, not volume
Osmolarity	Amount of osmotic particles / volume of solution	Account for non-ideal ion behaviour
Tonicity	No sustained change in cell volume across a given membrane	Permeable solutes must be considered separately

Hydrogen bonding gives water its unusual properties — reference figure — Figure 2-1-1Hydrogen bonding gives water its unusual properties — reference figure

Hydrogen bonding gives water its unusual properties — reference figure — Figure 2-1-1Hydrogen bonding gives water its unusual properties — reference figure

Hydrogen bonding gives water its unusual properties — reference figure — Figure 2-1-2Hydrogen bonding gives water its unusual properties — reference figure

Water hydrogen-bonds with polar solutes — reference figure — Figure 2-1-3Water hydrogen-bonds with polar solutes — reference figure

Water hydrogen-bonds with polar solutes — reference figure — Figure 2-1-4Water hydrogen-bonds with polar solutes — reference figure

Water hydrogen-bonds with polar solutes — reference figure — Figure 2-1-4Water hydrogen-bonds with polar solutes — reference figure

Water interacts electrostatically with charged solutes — reference figure — Figure 2-1-5Water interacts electrostatically with charged solutes — reference figure

Water interacts electrostatically with charged solutes — reference figure — Figure 2-1-6Water interacts electrostatically with charged solutes — reference figure

Dissolution of crystalline solids increases entropy — reference figure — Figure 2-1-7Dissolution of crystalline solids increases entropy — reference figure

Dissolution of crystalline solids increases entropy — reference figure — Figure 2-1-8Dissolution of crystalline solids increases entropy — reference figure

Nonpolar gases are poorly soluble in water — reference figure — Figure 2-1-9Nonpolar gases are poorly soluble in water — reference figure

Nonpolar gases are poorly soluble in water — reference figure — Figure 2-1-10Nonpolar gases are poorly soluble in water — reference figure

Nonpolar gases are poorly soluble in water — reference figure — Figure 2-1-10Nonpolar gases are poorly soluble in water — reference figure