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Specialty Chemistry Forums => Biochemistry and Chemical Biology Forum => Topic started by: Joules23 on September 25, 2008, 07:57:26 PM

Title: Chemical structures at diff pH's based on pKa's
Post by: Joules23 on September 25, 2008, 07:57:26 PM

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fi492.photobucket.com%2Falbums%2Frr283%2FJoules233%2FHis1.jpg&hash=86c213056130edbbd2b625b08bfea5371c8a014e)

Is this correct?

Title: Re: Chemical structures at diff pH's based on pKa's
Post by: macman104 on September 25, 2008, 11:15:06 PM

You are close.  You're first two are correct.  Think about it this way, you have 3 possible site for acid/base chemistry.  They are, with their pKa's:

2:  alpha-carboxyl group
6:  R-group
9:  alpha-amino group

Now, for each pH situation ask, which groups have are protonated, and which are deprotonated...

At a pH of 2, the acidity is sufficient to have all sites protonated, giving you the first structure correctly.

At a pH of 4, the acidity is insufficient  to protonate our first site with a pKa of 2, this gives you a structure with the carboxyl group deprotanted.

At a pH of 7.7, the acidity is insiffucient to keep the first TWO sites protonated, because they have pKa's of 2 and 6.  This gives us a structure with the carobxyl and R-Group deprotonated.

Finally at a pH of 11, the acidity is so insufficient that it cannot keep any of the THREE sites protonated, because they all of pKa's less than 11.  This gives us a structure where all 3 sites are deprotonated.

You were correct about the charges, but you had a little trouble identifying who the contributing agents were!

Title: Re: Chemical structures at diff pH's based on pKa's
Post by: Joules23 on September 25, 2008, 11:24:09 PM

Thanks for explaining.. on the last structure i simply forgot to change the alpha amino group, but i meant it to be the way you stated :) ... Your right though, on the 3rd structure i wasnt sure if the alpha-amino or R-group amino was to be deprotonated.

If for example aspartic acid or glutamic acid was used.. Which Carboxyl group would be deprotonated first? the R-group or alpha-carboxyl

Title: Re: Chemical structures at diff pH's based on pKa's
Post by: macman104 on September 25, 2008, 11:57:09 PM

The two carboxylic acid groups (or the structures with two amino groups) both have slightly different pKas, because the protons are not in indentical environments.  So for the first one to be deprotonated, it would be whichever one had a lower pKa.