Amino acid charge, pH and Structure

Question 1

what is a zwitterion?

Answer 1

A neutral ion with (-) and (+) charges- overall neutral charge

Question 2

what is the charge of the amino and carboxyl group of an amino acid at pH 7?

Answer 2

amino-NH3+
carboxyl- COO-

Question 3

what is the charge of the carboxyl group of an amino acid when pKa is < 7 but >1?

Answer 3

COO-
pka ~2
-deprotonation of the carboxyl

Question 4

what is the charge of the carboxyl group of an amino acid when pH is >7 but < 14?

Answer 4

COO-
pka ~2
-deprotonation of the carboxyl

Question 5

what is the charge of the amino group of an amino acid when pH is 14 ?

Answer 5

NH2
pka ~9.5

Question 6

what is the charge of the amino group of an amino acid when pH is equal or less than 7?

Answer 6

NH3+

pka~9.5

Question 7

In a low pH solution will the ionizable groups be protonated or deprotonated?

Answer 7

protonated

Question 8

In a high pH solution will the ionizable groups be protonated or deprotonated?

Answer 8

deprotonated

Question 9

what is pKa? what does it tell us?

Answer 9

-a measure of protonation or deprotonation relative to pH
- tells us the strength of a weak acid; it’s tendency to ionize

low pKa = more dissociation
lower pKa = stronger acid

Question 10

what does it mean if pKa > pH?

Answer 10

the protonated fraction is greater than the deprotonated fraction

[HA] > [A-]

Question 11

what does it mean if pKa < pH?

Answer 11

the protonated fraction is less than the deprotonated fraction

[A-] > [HA]

Question 12

what amino acids are non-polar at pH 7?

Answer 12

alanine, valine, phenylalanine, tryptophan, leucine, isoleucine, methionine, proline

Question 13

what amino acids are polar uncharged at pH 7?

Answer 13

serine, threonine, tyrosine, cysteine, asparagine, glutamine, histidine, glycine*weakly polar

Question 14

what is the pKa of tyrosine ?

Answer 14

> 7

10.5

Question 15

what R groups have ionizable functional groups?

Answer 15

Neutral polar- tyrosine, cysteine and histidine

charged polar- asparagine, glutamine , lysine, and arginine

Question 16

what polar charged R groups are neutral at high pH?

Answer 16

Lysine and arginine

Question 17

what polar charged groups are neutral at low pH?

Answer 17

Asparagine and glutamine

Question 18

what neutral polar R group is (+) at low pH?

Answer 18

histidine
pka ~6.0

Question 19

what neutral polar R groups are (-) at high pH?

Answer 19

tyrosine and cysteine

pka~10.5 and 8.5 respectively

Question 20

what is the pKa of cysteine

Answer 20

> 7

8.5

Question 21

what is the pKa of Histidine?

Answer 21

<7

6.0
-acts as a donor and a base

Question 22

what are the polar charged amino acids at pH 7?

Answer 22

aspartate, glutamate, lysine, arginine

pka ~4.0, 4.0, 10.0 and 12.5 respectively

Question 23

what is the charge of aspartate at pH 7? what’s it’s pKa?

Answer 23

(-) at pH 7

pKa = 4.0

Question 24

what is the charge of glutamate at pH 7? what’s it’s pKa?

Answer 24

(-) at pH 7

pKa = 4.0

Question 25

what is the charge of lysine at pH 7? what’s it’s pKa?

Answer 25

(+) at pH 7

pKa = 10.0

Question 26

what is the charge of arginine at pH 7? what is its pKa?

Answer 26

(+) at pH 7
pKa = 12.5