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Protonation & Deprotonation
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when protonated have a formal charge of 0 & when deprotonated have a formal charge of -1
CDESTY side chains & carboxylate
(Cysteine has an S & all the other ones have an O.)
when protonated have a formal charge of +1 & when deprotonated have a formal charge of 0
HKR (Histidine, Lysine, Arginine) side chains & amino
(They all have N. Remember that NQ are NOT ionizable.)
Calculate the charge on a fully protonated non-ionizable free amino acid
NH3+
: +1
COOH
: 0
R
: N/A
Total charge = +1
(non-ionizable amino acids: all hydrophobic + NQ)
Calculate the charge on a fully deprotonated non-ionizable free amino acid
NH2
: 0
COO-
: -1
R
: N/A
Total charge = -1
(non-ionizable amino acids: all hydrophobic + NQ)
Calculate the charge on a fully protonated CDESTY free amino acid
NH3+
: +1
COOH
: 0
R
: 0
Total charge = +1
Calculate the charge on a fully deprotonated CDESTY free amino acid
NH2
: 0
COO-
: -1
R
: -1
Total charge = -2
Calculate the charge on a fully protonated HKR free amino acid
NH3+
: +1
COOH
: 0
R
: +1
Total charge
: +2
Calculate the charge on a fully deprotonated HKR free amino acid
NH2
: 0
COO-
: -1
R
: 0
Total charge = -1
Which amino acids do NOT exist as a zwitterion at a pH of 7.40?
DEKR
DE will have an overall charge of -1
KR will have an overall charge of +1
zwitterion: sum of formal charges in a molecule is 0 (the neutral state). All the other 16 amino acids will have a charge of 0 at a pH of 7.40
Determine the protonated:deprotonated ratios of each ionizable functional group for the free amino acid Lysine.
pH is set to 7.40.
If pH > pKa, expect more deprotonated.
If pH < pKa, expect more protonated.
Add exponent value |pH-pKa| to the 10.
alpha COO- pKa = 2.20. pH > pKa. 1:10^5.2 (prot:deprot)
alpha NH3+ pKa = 9.40. pH < pKa. 10^2:1 (prot:deprot)
Lys R group pKa = 10.50. pH < pKa. 10^3.1:1 (prot:deprot)
Author
sophathida
ID
350541
Card Set
Protonation & Deprotonation
Description
Protonated, deprotonated
Updated
2020-02-06T17:13:40Z
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