Biochem question

[dentheartthrob]

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hey dr. tacoelf, I hope you can help me out on this question, or if anyone knows it, then it would be great to.

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.
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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?

 

[PlaqueAttack]

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)