pI (zwitterion)

[piojita63]

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I know this is probably very basic, but I find that I am slow at it. Does anyone have a quick way to determine what will happen to an amino acid (when they give you the pI) if it is placed in an acidic/basic solution? Like something I can memorize... If pI is more than x then in basic it is protonated/deprotonated and in acidic it is protonated/deprotonated, etc... Help! Thanks in advance.

 

[pfaction]

THIS IS FOR GLYCINE

pH<pI: Lots of H+ floating around, NH2 and COO- will be protonated; so NH3+ COOH = +1 -> goes towards negative electrode.
pH = pI: zwitterion; COO- and NH3+ net charge 0, goes nowhere.
pH>pI: Less hydrogens. COO-, and depending on the pH, NH3 or NH2, if it's above the pKa of NH3 then it'll be NH2 and thus a net charge of -1. Goes towards positive electrode.

 

[folktale]

THIS IS FOR GLYCINE

pH<pI: Lots of H+ floating around, NH2 and COO- will be protonated; so NH3+ COOH = +1 -> goes towards negative electrode.
pH = pI: zwitterion; COO- and NH3+ net charge 0, goes nowhere.
pH>pI: Less hydrogens. COO-, and depending on the pH, NH3 or NH2, if it's above the pKa of NH3 then it'll be NH2 and thus a net charge of -1. Goes towards positive electrode.

Is the negative electrode going to be the anode, and the positive is the cathode? B/c I saw somewhere they tend to flip the charges when dealing with amino acids rather than galvanic/electrolytic cells.

 

[SaintJude]

Someone else may add some clever mnemoic but I like to rationalize it.

I think of pI as "pH comfort zone" of amino acid ("aa").

So if pH > pI, that means that there are less H+ than the aa is accustomed to. So it will donate some H+ to solution. So it will deprotonate and become more negatively charged.

If pH < pI , that means the solution is more acidic than aa is normally comfortable, so aa will "sequester" (as in take on some H+) to push the solution to become a little more basic. So aa will be protonated and more positively charged.

 

[pfaction]

It all depends on the type of cell, yeah. In galvanic, the anode is negative and is being oxidized so it depends on the pH, if it's negative it'll go to the anode and if it's positive it'll go to the cathode (where electrons are flowing into and reductions are occuring).

 

[SaintJude]

Is the negative electrode going to be the anode, and the positive is the cathode? B/c I saw somewhere they tend to flip the charges when dealing with amino acids rather than galvanic/electrolytic cells.

It's same convention as in electrolytic cell (technique is called isoelectric focusing)

So anode = positive and cathode = negative.

edit: didn't see previous post before I answered (redundantly) ...oh well.

 

[folktale]

It all depends on the type of cell, yeah. In galvanic, the anode is negative and is being oxidized so it depends on the pH, if it's negative it'll go to the anode and if it's positive it'll go to the cathode (where electrons are flowing into and reductions are occuring).

?

Wait so you said that the anode is negative, but if the amino acid is negative, it will go to the anode? The electrons are moving from anode to cathode, so you're saying that in the case of aa, it'd be the cathode that's going to be more negative since the electrons moves TOWARDS it?

 

[folktale]

It's same convention as in electrolytic cell (technique is called isoelectric focusing)

So anode = positive and cathode = negative.

edit: didn't see previous post before I answered (redundantly) ...oh well.

I'm looking at BR Gen Chem II, p. 269.

it says that physicists see cathodes as + and anodes as -

but biochemists see it as cathodes as - and anodes as +

????

 

[SaintJude]

I'm looking at BR Gen Chem II, p. 269.

it says that physicists see cathodes as + and anodes as -

but biochemists see it as cathodes as - and anodes as +

????

Forget BR designation. (Why do they try so hard to confuse ppl?)

Kaplan says:
Galvanic cells : anode - negative / cathode - positive
(The way I remember it: At the anode electrons are lost so they are leaving the anode. Like charges repel.)

Isolectric focusing/ Electrolytic cells: reverse (Both techniques are using a battery to drive the process)

 

[folktale]

Forget BR designation. (Why do they try so hard to confuse ppl?)

Kaplan says:
Galvanic cells : anode - negative / cathode - positive
(The way I remember it: At the anode electrons are lost so they are leaving the anode. Like charges repel.)

Isolectric focusing/ Electrolytic cells: reverse (Both techniques are using a battery to drive the process)

Ahh I see it now. So discharging/galvanic batteries have negative anodes and positive cathodes b/c an ox , red cat.

And gel electrophoresis is like an example of an electrolytic cell, so it's the opposite. This makes sense. Thank you.


EDIT: and btw, so the cathode and anodes for galvanic vs. electrolytic cells are also reversed?
Like let's say: Electrodes of Zn would be anode and Cu would be cathode on a galvanic cell, but Zn is cathode and Cu electrode is anode for an electrolytic cell?

 

[piojita63]

Very helpful! Thank you! I guess I got a 2 for 1 with this post... It turned into an electrochemisty lesson too! Great!