Talk:Isoelectric point

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Should add an example of pI calculation for lysine. I added a simple method, someone might put a blurb about Henderson-Hasselbach...

Should there be mention of ZPC? Lizz612 21:47, 29 January 2006 (UTC)[reply]

tried to introduce the topic more. Expanded on information and elaborated on some topics. It's not perfect but at least I tried.140.203.16.53 19:54, 11 October 2007 (UTC)[reply]

The 20:00, 12 March 2009 130.91.112.68 version is translated into Chinese Wikipedia.--Wing (talk) 13:44, 28 March 2009 (UTC)[reply]

The article pulls the average formula out of nowhere. Horrible, horrible way to introduce the concept. I don't have time to fix it right now, but someone should. John Riemann Soong (talk) 06:49, 13 April 2010 (UTC)[reply]

A full derivation can be found in the pKa article. 96.54.32.44 (talk) 20:05, 20 February 2011 (UTC)[reply]

From the article: "Glycine may exist as a zwitterion at the isoelectric point, but the equilibrium constant for the isomerization reaction in solution is not known"

   H2NCH2CO2H is in equilibrium with H3N+CH2CO2-"

The reason that one can't find a published equilibrium constant for this reaction is simply because it is a two step process. The equilibrium constants for each step are easily obtained from the published pKa1 = 2.34 and pKa2 = 9.58 for glycine (see table in proteinogenic amino acid):

step 1, protonation of the amino group:

H₂NCH₂CO₂H + H₃O⁺ ↔ H₃N⁺CH₂CO₂H + H₂O

K = 4.0 x 10⁹

step 2, deprotonation of the carboxylic acid:

H₃N⁺CH₂CO₂H + H₂O ↔ H₃N⁺CH₂CO₂^- + H₃O⁺

K = 4.6 x 10^-3

There is no particular order for the two steps, and deprotonation of the carboxylic acid can just as easily precede protonation of the amino group.

The overall equilibrium constant for the two step process is the product of the two K values shown.

H₂NCH₂CO₂H ↔ H₃N⁺CH₂CO₂^-

K = 1.9 10⁷

So there is one molecule of H₂NCH₂CO₂H for every 19 million zwitterion H₃N⁺CH₂CO₂^-

96.54.32.44 (talk) 18:31, 20 February 2011 (UTC)[reply]

There is a problem with the pKa for phosphate esters, in that the first deprotonation of the phosphoric acid group is almost a strong acid (pK_a < 1) and very difficult to measure. For this reason, phosphate esters are often quoted with a single pKa, but the ionization is for the second deprotonation, typical pK_a 6-6.5

HOPO₂^-OR + H₂O ↔ OPO₂^2-OR + H₃O⁺

The quoted pK_as for 5'-AMP are 0.9, 3.8, 6.1, and should be attributed as

pKa₁ = 0.9 - first phosphoric ester ionization

pKa₂ = 3.8 - deprotonation from adenine-H⁺

pKa₃ = 6.1 second deprotonation of hydrogen phosphate ester

That would make the charge states

AMP⁺ ↔ AMP ↔ AMP^- ↔ AMP^2-

so pI = ½(0.9 + 3.8) = 2.4

See Data for Biochemical Research by RMC Dawson et al. Oxford. 96.54.32.44 (talk) 05:07, 25 February 2011 (UTC)[reply]