[biochem] lecture 1-8

8 non-covalent interactions

• charge-charge

• charge-dipole

• dipole-dipole

• charge-induced dipole

• dipole-induced dipole

• van der Waals

• hydrogen bonds

K_w

ion product for water

• presents equilibrium const for the autoionization of water

pK_a value

dissociation const.

• measurement of acid strength

in acid-base titration, at mid point …

half of the acid is neutralized

pH = pK_a

buffer solution

resists pH change with addition of small amounts of acid or base 

buffer capacity 

total conc [HA] + [A^-]

ampholyte

molecule that contains group w both acidic and basic pK_a value 

tautomers

2 molecules w same chemical formula but different connectivity

resonance structure

different Lewis structures of the same molecule/ion that show how electrons can be delocalized

nucleophile

have electron-rich atom → able to donate a pair of electrons to form a new covalent bond 

electrophile

have electron-poor atom → an attractive target for electron-rich atom

nucleophilic substitute reaction

nucleophile replaces a leaving group attached to a C atom

• at saturated C:

  • S_N1 substitution: leaving group leaves first → nucleophile attacks 

  • S_N2 substitution: nucleophile pushes out the leaving group 

• at carbonyl group: involves a tetrahedral intermediate