lecture 5

Question 1

what is pH?

Answer 1

Measure of conc. Of hydrogen ions in aqeuous solutions

Question 2

what is pKa

Answer 2

○ Related to pH, predicts what a molecule will so at a specific pH
* pKa is the pH value at which a chemical species will accept or donate a proton
Lower the pKa, stronger the acid and the greater the ability to donate a proton in aqeuous solutions

Question 3

what is the Henderson-Hasselbalch equation

Answer 3

• Is used to estimate the pH of a buffer solution
  Consider a weak acid HA, the acid dissociation constant is given by … refer to onenote

Question 4

describe the acid-base properties of amino acids

Answer 4

• Amino acids are weak polyprotic acid
  
  • Degree of dissociation depends on the pH of the medium
  • All amino acids have at least 2 dissociable hydrogens
    For amino acids which do not have a dissociable side chain, the first dissociation is that of the alpha-carboxyl group

For amino acids which do not have a dissociable side chain, the second dissociation is that of the alpha-amino group

refer to onenote

Question 5

describe the acid-base properties of glycine

Answer 5

○ At LOW ph, both the amino + carboxyl group are protonated = net pos. Charge
○ If pH increase, the carboxyl group H+ dissociates first, yields neutral zwittrionic species
Further increase in Ph results in dissociation of the amino group + net neg. Charge

Where Gly+ = pK1
Gly0 = isoelectric point
Gly- = pK2

refer to onenote

Question 6

what is the pKa values of carbonyl and amino groups

Answer 6

• Alpha carboxyl group: pKa = ¬ 2, will be neg.
  Alpha amino group: pKa = ¬ 9, will be pos.

Question 7

describe the reactions of amino acids

Answer 7

• Alpha carboxyl groups + alpha amino groups show similar reactions
  
  • Carboxyl groups form amides + esters (refer to image)
  • Amino groups form Schiff bases + amides (refer to image)
    Cysteine forms disulphide bridges (refer to image)