Peptide Charge

[anonymousername]

---

Members do not see this ad.

All of my usual resources are letting me down b/c they all seem to explain in the same way...this seems like a fairly easy concept but i can't figure out how to determine the charge of a peptide at a certain pH.

Lets start out with an easy one. At pH = 6 find the charge on these di/tripeptides

lys-phe
glu-glu
phe-glu
lys-phe-glu

I have the answers but if you could explain exactly what the thought processes is, i might get a better idea.

I'm usually pretty good at orgo, i don't know whats going on. Can someone please help?

 

[Slack3r]

Isn't this asking about isoelectric points?

 

[UCB05]

This seems a bit deep for a DAT question. It's biochemistry, especially if they expect you to have pka values memorized and for kicks toss in a non-physiological pH. In any case, the terminal carboxyl and amino charges cancel, so you just have to know the pkas of the side-chains. lys is basic for a +1, glutamate is acidic for a -1, and phe is neutral. Add up your charges to get your answers.

 

[CANgnome]

i don't even have to remember these things in my graduate biochemistry class O_O

 

[anonymousername]

This seems a bit deep for a DAT question. It's biochemistry, especially if they expect you to have pka values memorized and for kicks toss in a non-physiological pH. In any case, the terminal carboxyl and amino charges cancel, so you just have to know the pkas of the side-chains. lys is basic for a +1, glutamate is acidic for a -1, and phe is neutral. Add up your charges to get your answers.

I see what you're saying..i think I need to start of by drawing out all the side chains instead of visualizing it all in my head.

thanks!

 

[dentalWorks]

All of my usual resources are letting me down b/c they all seem to explain in the same way...this seems like a fairly easy concept but i can't figure out how to determine the charge of a peptide at a certain pH.

Lets start out with an easy one. At pH = 6 find the charge on these di/tripeptides

lys-phe
glu-glu
phe-glu
lys-phe-glu

I have the answers but if you could explain exactly what the thought processes is, i might get a better idea.

I'm usually pretty good at orgo, i don't know whats going on. Can someone please help?

I'll do lys-phe one for ya.

The general rule of thumb with determining charge of an amino acid is to pay attention to the COO(H) and NH2(H) groups (this includes the side chains as well). By that I mean if the current pH of the solution (6 in our problem) is
1) BELOW the Pka value of the group, then it is PROTONATED --> +1 charge
2) ABOVE the pka value of the group, then its deprotonated --> either neutral (if NH2) or -1 (if COO-)
3) SAME as the pka value, its neutral (cause its 50/50 from both species)

This website lists all the pka values of all amino acids and thier side chains (http://homepage.smc.edu/kline_peggy/Organic/Amino_Acid_pKa/Amino_Acid_pKa.htm)
For lys: COOH=2, NH2=9, and side chain=11
For phe: COOH=2, NH2=9, and there is no side chain

I have these circled in red:

However, our two amino acids are linked by a peptide bond, I think your supposed to ignore the H's associated with the peptide bond since that bond is showing partial double-bond properties (someone verify this plz)
Here is the structure for lys-phe and the charges of each side chain at pH6

 

[anonymousername]

I'll do lys-phe one for ya.

The general rule of thumb with determining charge of an amino acid is to pay attention to the COO(H) and NH2(H) groups (this includes the side chains as well). By that I mean if the current pH of the solution (6 in our problem) is
1) BELOW the Pka value of the group, then it is PROTONATED --> +1 charge
2) ABOVE the pka value of the group, then its deprotonated --> either neutral (if NH2) or -1 (if COO-)
3) SAME as the pka value, its neutral (cause its 50/50 from both species)

This website lists all the pka values of all amino acids and thier side chains (http://homepage.smc.edu/kline_peggy/Organic/Amino_Acid_pKa/Amino_Acid_pKa.htm)
For lys: COOH=2, NH2=9, and side chain=11
For phe: COOH=2, NH2=9, and there is no side chain

I have these circled in red:

However, our two amino acids are linked by a peptide bond, I think your supposed to ignore the H's associated with the peptide bond since that bond is showing partial double-bond properties (someone verify this plz)
Here is the structure for lys-phe and the charges of each side chain at pH6

👍

wow...that's exactly the kind of explanation I was looking for. Thanks!


(and now I get to use this smiley face)

dental works =

 

[Slack3r]

Are you sure you need to know this for the DAT?

 

[anonymousername]

Are you sure you need to know this for the DAT?

i don't think so.

 

[dentalWorks]

Are you sure you need to know this for the DAT?

I highly doubt they'll expect you to know how to figure out a charge of a peptide.... but Im pretty sure knowing the concept of isoelectric point is recommended. You don't have to memorize the pka of anything of that sort, but just be familiar with what isoelectric is