Molecular Interactions Flashcards
What are molecular interaction ?
Mostly non-covalent
Primarily electrostatic (unlike chargesattract;like charges repel)
Biologically important molecular interaction are individually weak but specific -molecular recognition and nanoscale machines
Typical energies of bonds and interaction
Covalent bonds -highest
Ionic bond
Hydrogen bond
Dispersion forces
Dipole-dipole interaction - lowest
Polarised covalent bonds
Pure covalent bonding only when delta x =0
E.g) isolated H2,n2 o2
Most biologically important molecules are heteroatomic
-non identical atoms covalently bound together
-bonds have delta x ≠ 0
-bonding is partially ionic
The ketelaar triangle
Electrostatic interactions ;no overlap of atomic orbitals ;molecules are not formed
By
Polar molecules and dipoles interaction
Some molecules containing polar covalent bonds are polar overall :they have a permanent dipoles
Permanent dipole electrostatically attracts molecules and ions
Are all heteroatomic molecules polar ?
Not all heteroatomic
Depends on the shape of molecules
Highly symmetrical molecules
Dipoles may point in opposite directions and cancel out
What are dispersion forces ?
These polar interaction do not require a permanent dipole
Because ‘non polar species’ are only non-polar on average
Electron densities temporarily polarized :induced dipoles
Bigger atoms and compound experience bigger dispersion forces
Where are induced dipoles ?
Everywhere ??????
What are Van der walls interaction ?
Refers to overwall non-covalent electrostatic interaction
Sum of
Attraction. Repulsion
-dispersion. -electron-electron
-permanent dipole interactions steric effect ^
What is the energetic of vander waals interaction?
What is important interaction between polar molecules ?
Hydrogen bonds
-some of the most widespread interaction in biology
(DNA,proteins)
-between water molecules
What are the strongest kind of molecular interaction ?
The have an energy of approx 20kJ mol-1
Still much weaker than covalent or ionic bonds
Hydrogen bonds are directional
-require some overlap of atomic orbitals (slightly cov
-strongest H-bonds are linear
What are hydrogen bonds ?
Dipole interactions involving O-H,N-H
- O and N are highly electronegative
- H is very small (allow close approach)
Hydrogen bonds vs dipolar interaction ?
Many polar molecules dont form hydrogen bonds
They do interact electrostatically but much weaker than H-bonds
Eg)-hydrocarbons,SH2,chloroform
-non H-bonding compounds tend to be gases/volatile liquids at room temp ,pressure
-low solubility in water
What are the ionic forces between charged molecules ?
Some molecules are charged :IONS
-SO42-,PO43-,NH4+ etc
Many bigger biomolecules are charged
-eg DNA backbone contains negatively charged phosphate linkages
Some AA side chains charged at pH 7
Charges never exist in isolation
