Amino Acid Reactivity

Question 1

Definition of Amphoteric

Answer 1

amino acids can act as either acids or bases

Question 2

Which mathematical equation pH, pKa and the position of equilibrium in an acid-base reaction?

Answer 2

Henderson-Hasselbalch Equation

pH = pKa + log [A-] / [HA]

Question 3

What is the pKa of the carboxyl group?

Answer 3

2 (stronger acid)

Question 4

What is the pKa of the amino group?

Answer 4

9-10 (weaker acid)

Question 5

Which will be deprotonated first (the carboxyl or the amino group)?

Answer 5

carboxyl group
the stronger the acid, the more likely to donate protons
lower pKa = higher Ka = stronger acid = more likely to deprotonate

Question 6

When pH < pKa …

Answer 6

The site is mostly protonated (has H present)

NH3+ and COOH

Question 7

When pH > pKa …

Answer 7

The site is mostly deprotonated (no H present)

NH2 and COO-

Question 8

Definition of pI

Answer 8

The pH at which a molecule has an overall neutral charge (the zwitterion)

Question 9

How to calculate pI

Answer 9

pI = pKa1 + pKa2 / 2

Question 10

Reaction Coupling

Answer 10

Thermodynamically unfavorable reactions in the cell can be driven forward by reaction coupling (one very favorable reaction is used to drive an unfavorable one)