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physio-biochem questions
- Thread starter saburi
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Question from Exam (released)
What Is the net charge of glutamic acid at a pH of 1 ?
1. +2
2. +1
3. 0
4. -1
5. -2
At a pH of 1 glutamic acid, will be well below its PkA, and since its an acid it will still have all the H+ ions attached thus it should be +1.
hey, you really have some sound foundation on your biochem. I'm hoping you can help me out on this. Ok what about in terms of basic side chain, like for instance argine, at a pH of 14 would the over all charge be 0? And how did you get the pKa value of a certain amino acid? Did you solve it mentally or is there a table that you used for reference?
If you maybe so kind to please show how it was done if you had done it mentally.
Thanks in advance.
And another thing, from wikipedia its says pKa value of glutamine is 4 yet the pK values of the different compents of the amino acid, are different, why is that?
What Is the net charge of glutamic acid at a pH of 1 ?
1. +2
2. +1
3. 0
4. -1
5. -2
At a pH of 1 glutamic acid, will be well below its PkA, and since its an acid it will still have all the H+ ions attached thus it should be +1.
hey, you really have some sound foundation on your biochem. I'm hoping you can help me out on this. Ok what about in terms of basic side chain, like for instance argine, at a pH of 14 would the over all charge be 0? And how did you get the pKa value of a certain amino acid? Did you solve it mentally or is there a table that you used for reference?
If you maybe so kind to please show how it was done if you had done it mentally.
Thanks in advance.
And another thing, from wikipedia its says pKa value of glutamine is 4 yet the pK values of the different compents of the amino acid, are different, why is that?
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At a pH of 1 there would actually be a charge of NEGATIVE 1, I had problems at first with this too. But you almost have to think in reverse and focus on the remaining molecule and where the H+'s have gone, for instance:
In this case we have 2 COO- groups left and one NH3+ group (because an acid LOSES its H+ when the pH is below its pK and a base GAINS an H+ when the pH is below its pK) this is a VERY important distinction.
An easier way to look at it overall:
Basic side chains (and amino end of the acid): they are +1 until their pK is reached (usually from 1-11ish) but from 12-14 they are NEUTRAL
Acidic side chains (and carboxyl end of the acid): they are -1 BELOW their pK (usually from 1-4ish) and above their pK they are NEUTRAL
Hope this helps
Oh yeah, the pKa is the sum of all the pK's of the molecule, its basically the point at which if left in a solution at that pH would have NO charge at all and would not move if a charge was applied (like in an electrophoresis box)
In this case we have 2 COO- groups left and one NH3+ group (because an acid LOSES its H+ when the pH is below its pK and a base GAINS an H+ when the pH is below its pK) this is a VERY important distinction.
An easier way to look at it overall:
Basic side chains (and amino end of the acid): they are +1 until their pK is reached (usually from 1-11ish) but from 12-14 they are NEUTRAL
Acidic side chains (and carboxyl end of the acid): they are -1 BELOW their pK (usually from 1-4ish) and above their pK they are NEUTRAL
Hope this helps
Oh yeah, the pKa is the sum of all the pK's of the molecule, its basically the point at which if left in a solution at that pH would have NO charge at all and would not move if a charge was applied (like in an electrophoresis box)
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discussion
the amino group pK is 9.7 (Im assuming this is the pH when it gains an electron). so at neutral pH it is +1, on NH3+
the carboxyl group's pk is 2.1 (now for sure I know this is when when H+ dissociates) so at neutral pH its is-1, being C00-
and the R side chain pK is 4.3. And from that, we can conclude that the charge of the amino acid at neutral pH would be negative 1
my thought on getting pKa on acidic acids such as glutamate and asparate, is just estimating when the R group would dissociate its H, which both would be roughly at 4, so in essence pKa value is the value of when the R group dissociates? Does that sound about right?
the amino group pK is 9.7 (Im assuming this is the pH when it gains an electron). so at neutral pH it is +1, on NH3+
the carboxyl group's pk is 2.1 (now for sure I know this is when when H+ dissociates) so at neutral pH its is-1, being C00-
and the R side chain pK is 4.3. And from that, we can conclude that the charge of the amino acid at neutral pH would be negative 1
my thought on getting pKa on acidic acids such as glutamate and asparate, is just estimating when the R group would dissociate its H, which both would be roughly at 4, so in essence pKa value is the value of when the R group dissociates? Does that sound about right?
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thanks 🙂
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I think for the most part zwitter ions are applicable only to non polar AA and uncharged polar AA at neutral pH.
wouldn't it be -1 since at physiological pH its already +1, hence the the term basic amino acid? the COO- and NH3+ group would already cancel out leaving the R group as the only charged group. increasing the pH from neutral pH (or isoelectric point of the AA) to reach the particular pKb of the two positvely charge group would cause both them to accept electrons.
wouldn't it be -1 since at physiological pH its already +1, hence the the term basic amino acid? the COO- and NH3+ group would already cancel out leaving the R group as the only charged group. increasing the pH from neutral pH (or isoelectric point of the AA) to reach the particular pKb of the two positvely charge group would cause both them to accept electrons.
