Easy presentation on which all points to be included in this topic.

2. • Weak interactions are so important, because they add up and thus
generate very strong molecule conformations.
• The secondary, tertiary and quaternary structure of proteins, the double
helix of the DNA, the membrane structures and complex intracellular
particles like the ribosomes are all maintained by weak interactions.
• The important thing here is the shape of the reactants.
• The more complementary the structures, the better their fit, the more
weak interactions can form and the more stable is the resulting
conformation or aggregate.

3. HYDROGEN BONDING BETWEEN WATER MOLECULES
• Tetrahedral arrangement of oxygen electron allow each water molecule to form
hydrogen bonds with 4 neighboring water molecules.
• In continuous motion of liquid state ,hydrogen bonds are constantly and swiftly
being broken and formed.
• In ice ,each water molecules is fixed in space and form hydrogen bonds to
produce a regular lattice surface.
• In lattice ,much thermal energy is needed to break the hydrogen bond – reason
for the relatively high melting point of water.
• Heat is taken by the system when ice melts, water evaporates.

5. Hydrogen Bonding in Other Molecules

6. HYDROGEN BONDING BETWEEN WATER AND SOLUTE MOLECULES
• Hydrogen bond is not unique to water.
• Readily form between an electronegative atom and a hydrogen atom
covalently bonded to another electronegative atom in same and another
molecule.
• Sugar(uncharged ) dissolve in water because of stabilizing effect of the many
hydrogen bond that form between the hydroxyl group or the carbonyl group
oxygen of sugar and polar water molecule.
• Hydrogen bonds are highly directional.

7. • Hydrogen bonds are highly directional in that strength depends on proper
alignment of interacting atoms.
• Best alignment occurs when the orbital containing the unshared electron pair of
the acceptor atom is in line with the covalent bond between the donor atom
and hydrogen.

9. INTERACTION BETWEEN WATER AND CHARGED SOLUTES
• Compounds dissolve in water are hydrophilic.
• Water dissolves salts such as Nacl by hydrating and stabilizing Na+ and cl-
ions.
• Nacl crystal lattice is disrupted ,ionic charges re partially neutralized and
electrostatic attractions are weakened.
• Increase in the entropy of system leads to the dissolving of salt in water.

11. INTERACTIONS BETWEEN WATER AND NON-POLAR GASES
• Biologically important gases CO2,O2,N2 are non polar.
• In O2 & N2 electrons shared equally by both atoms.
• In CO2,each C=O bond is polar ,but the two dipole are oppositely directed and
cancel each other.
• The movement of these molecules from disordered gas phase into aqueous
solution – decrease in entropy.
• So gas is poorly soluble in water.
• CO2 dissolve in aqueous solution and form carbonic acid (H2CO3).
• Gases like H2S and NH3 are polar and dissolve in water.

12. INTERACTION BETWEEN WATER AND NONPOLAR COMPOUNDS
• When water is mixed with a hydrocarbon such as benzene or hexane ,two
phase form neither liquid is soluble in other.
• Such compounds are hydrophobic as they are unable to undergo energetically
with hydrogen bonding among water molecules.
• They never offers any compensation as solutes .
• Dissolving hydrophobic solutes in water result in measurable decrease in
entropy and a small gain of enthalpy.
• To break the hydrogen bond ,energy has to be added to the system.

13. VAN DER WAALS INTERACTION
• Are the weak ,non specific ,interatomic attractions and come to play when any
two uncharged atoms are 3-4 Angstrom apart.
• Basis of this bond is that distribution of electronic charges around an atom
changes with time.
• All type of molecules exhibit van der Waals forces.
• At a certain distance defined as the van der Waals contact radius ,there is a
balance between the forces of attraction and those of repulsion.
• More symmetrical the molecule ,greater are the Vander Waals forces.

14.  Energy of a van der Waals interaction as a function of the distance between two
atoms
 Schematic of van der Waals balance between two atoms forming Van der Waals
radius

15. NON COVALENT INTERACTIONS
• These include charge - charge interactions ,hydrogen bond , Vander Waals
forces.
• It is the strongest non covalent interactions
• Example is stabilization of NACL crystals by interionic attractions.
• Nature of solvent determines the strength of charge - charge interactions and if
solvent is water ,it weakens these interactions.
• Salt bridge is term used for attractions between oppositely charged functional
groups.

16. HYDROPHOBIC INTERACTIONS
• A non polar molecule dissociates with another non polar molecule.
This is why oil and water don't mix: oil is nonpolar while water is polar.
• Hydrophobic interactions help in stabilization of micelles.
• Stability of a macromolecule largely determined by cumulative effect of many
hydrophobic interactions.
• During the spontaneous folding of polypeptide chain, hydrophobic interaction
are formed which determine three dimensional structure of many proteins.
• Hydrophobic interactions depend on the increased entropy of the surrounding
water molecules rather than one direct attraction between non polar groups.

17. stabilization of micelles

19. FITNESS OF AQUEOUS ENVIRONMENT OF LIVING ORGANISMS
• Life is possible on this planet due to the presence of water in abundance.
• Excess body heat is used by organism for evaporation of sweat by exploiting
high heat of vaporization of water.
• Water act as a heat buffer due to its high specific heat.
• Used by organisms to keep the body temperature constant even in the
surrounding fluctuations of temperature.
• Also beneficial to plants as transport of dissolved nutrients from roots to
leaves during transpiration because of high degree of internal cohesion of
liquid.

20. • Aqueous environment supports countless species.
• The evolution of life on earth was influenced by solvent and reactant
properties of water.
• Organisms are effectively adapted to their aqueous environment by
exploiting the unusual properties of water .
Coral reefs