histidine charge at pH 7

[aegistitan]

wanted to know what you guys thought of what to think of histidine's charge at pH 7 for the MCAT. I realize histidine is 25% (+) charge at pH 7

NH3(+)-H(+)IPAGE(-)ATE(-)K(+)ALR(+)GD(-) - COO(-) --> this is how I initially calculated the charge which would be net 0. But the solution says the net charge is -1. Does this mean when I see a histidine at pH 7 to not include it into charge calculations?

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[theonlytycrane]

the pKa for His is 6.0. Go with this rule:

if pH > pKa: proton off
if pH = pKa: neutral (50% Ha 50% A-)
if pH < pKa: proton on

so at pH = 7, His will be neutral (deprotonated)

 

[Jouster]

Only the C-terminal, N-terminal, and R-groups will participate in ion exchange. Since the imidazole group on histidine has a pKa of 6.0, a solution with a pH of 7 will favor the negative ion on the functional group of histidine. While a low pH (less than 6) solution adds an H+ to the group, a high pH (greater than 6) reacts aqueous OH- with the hydrogen on the functional group. A negative charge will result. Remember that pH=pKa when [A-]=[HA]. If pH=7, pKa=6.0 and pH=pKa+log[A-/HA] then 7=6+log[A-/HA] and then log [A-/HA]=1. [A-/HA]=10, more [A-] at pH=7!

Hopefully you can follow my thought process

 

[Integrated MCAT Course]

Only the C-terminal, N-terminal, and R-groups will participate in ion exchange. Since the imidazole group on histidine has a pKa of 6.0, a solution with a pH of 7 will favor the negative ion on the functional group of histidine. While a low pH (less than 6) solution adds an H+ to the group, a high pH (greater than 6) reacts aqueous OH- with the hydrogen on the functional group. A negative charge will result. Remember that pH=pKa when [A-]=[HA]. If pH=7, pKa=6.0 and pH=pKa+log[A-/HA] then 7=6+log[A-/HA] and then log [A-/HA]=1. [A-/HA]=10, more [A-] at pH=7!

Hopefully you can follow my thought process

This is all fine except but I want to make the comment that the conjugate base of the imidazole group has no charge, so no negative charge results. I think you might have lost the track here a little bit because the traditional expression of the Henderson-Hasselbach equation represents interchange between a neutral (acid) form and negative (base) so there is a bit of mental work sometimes in accustoming yourself to amine-type moeities where the exchange is between a positive (acid) form and a neutral (base). The equation works the same. You just see [A-] as the symbol for the base form and [HA] as the symbol for the acid form realizing that sometimes they are standing in for something negative and neutral, sometimes neutral and positive (in this case) or maybe doubly negative and singly negative as with carbonate and bicarbonate. In the equation they are just symbols for base and acid.

Anyway, at pH 7 histidine has a positive charge about 10% of the time (proton exchange occurs about every 10^-5 s). I don't think this extra bit is meant by the question-writer to figure into the whole number approximations implied in the answers to the problem. I guess a piece of advice that over-thinking can be its own hazard. During MCAT review a person can get to be like a dog that's been hit too much. Sure there are questions where the deeper level is going to get you, but it's also good to remember that most of the time if a question looks too easy it's just because it seems easy to you because you have studied the material. Most of the time if it looks too easy that's just because it's easy.

 

[Sloppy Toppy]

This is all fine except but I want to make the comment that the conjugate base of the imidazole group has no charge, so no negative charge results. I think you might have lost the track here a little bit because the traditional expression of the Henderson-Hasselbach equation represents interchange between a neutral (acid) form and negative (base) so there is a bit of mental work sometimes in accustoming yourself to amine-type moeities where the exchange is between a positive (acid) form and a neutral (base). The equation works the same. You just see [A-] as the symbol for the base form and [HA] as the symbol for the acid form realizing that sometimes they are standing in for something negative and neutral, sometimes neutral and positive (in this case) or maybe doubly negative and singly negative as with carbonate and bicarbonate. In the equation they are just symbols for base and acid.

Anyway, at pH 7 histidine has a positive charge about 10% of the time (proton exchange occurs about every 10^-5 s). I don't think this extra bit is meant by the question-writer to figure into the whole number approximations implied in the answers to the problem. I guess a piece of advice that over-thinking can be its own hazard. During MCAT review a person can get to be like a dog that's been hit too much. Sure there are questions where the deeper level is going to get you, but it's also good to remember that most of the time if a question looks too easy it's just because it seems easy to you because you have studied the material. Most of the time if it looks too easy that's just because it's easy.

Could you or anyone explain. Why histudune is either neutral or +1? With a pKa of 6 wouldn't it get deprotonated?

 

[weanprednisone]

Could you or anyone explain. Why histudune is either neutral or +1? With a pKa of 6 wouldn't it get deprotonated?

You might not be understanding the difference between pKa and pH.
You can't ask at a pKa of 6, would it be protonated/deprotonated...
pKa is the measurement of how strong the acid is, pH is the measurement of how acidic or basic a sovlent is.
pKa is like what you were born with, and pH is like environmental factors.

The questions is that the "environment" is pH=7, would His be protonated (HA) or deprotonated (A-)
Just like everyone has been replying. Given that His pKa=6 @ pH=7; When pH>pKa (7>6)=we know that its in the deprotonated form=A- (no H+ attached)

this is the deprotonated form pKa(6)<pH=neutral; we can assume that this probably exists in a solution pH=7+

this is the protonated form=exists only when pKa(6)>pH=+1 charge; we can assume that this probably exists in a solution pH=0-6

Another way to remember pH><pKa is to connect the dots of pH vs. [H+]
when pH is low=[H+] is high meaning a lot of H+ ions are floating around.
So when pH<pKa (pH is low), we can assume that H+ ions are high concentration wanting to protonate something. so that molecule must be protonated.
therefore, when pH<pKa, [H+] High, molecule in protonated state.

Hope this helps. You can look up all the amino acid chart pKa's and practice at different pHs drawing whether the structure will be deprotonated or protonated...

 

[Sloppy Toppy]

You might not be understanding the difference between pKa and pH.
You can't ask at a pKa of 6, would it be protonated/deprotonated...
pKa is the measurement of how strong the acid is, pH is the measurement of how acidic or basic a sovlent is.
pKa is like what you were born with, and pH is like environmental factors.

The questions is that the "environment" is pH=7, would His be protonated (HA) or deprotonated (A-)
Just like everyone has been replying. Given that His pKa=6 @ pH=7; When pH>pKa (7>6)=we know that its in the deprotonated form=A- (no H+ attached)

this is the deprotonated form pKa(6)<pH=neutral; we can assume that this probably exists in a solution pH=7+

this is the protonated form=exists only when pKa(6)>pH=+1 charge; we can assume that this probably exists in a solution pH=0-6

Another way to remember pH><pKa is to connect the dots of pH vs. [H+]
when pH is low=[H+] is high meaning a lot of H+ ions are floating around.
So when pH<pKa (pH is low), we can assume that H+ ions are high concentration wanting to protonate something. so that molecule must be protonated.
therefore, when pH<pKa, [H+] High, molecule in protonated state.

Hope this helps. You can look up all the amino acid chart pKa's and practice at different pHs drawing whether the structure will be deprotonated or protonated...

Thanks. I get all that, but I guess what is confusing me a little is since Histidine is deprotonated due to the pH 7 environment why isn't it negative. Instead it's neutral?

 

[weanprednisone]

Thanks. I get all that, but I guess what is confusing me a little is since Histidine is deprotonated due to the pH 7 environment why isn't it negative. Instead it's neutral?

Ah I see! I had the same problem when I was studying the AAs.
I was confused because His falls under the category of basic AAs, which usually have neg. charges because they can receive a H+.
But remember how general basis can also act as good nucleophiles, and good nucleophiles can have a negative charge or a lone pair, or both!
In this case, the ring just happens to be a nucleophile when it is neutral because of the lone pair, and the positive protonated state as the acid.
I believe His is just unique that way due to the imidazole ring...
If anyone else could give a better explanation, that works too!

 

[aldol16]

Thanks. I get all that, but I guess what is confusing me a little is since Histidine is deprotonated due to the pH 7 environment why isn't it negative. Instead it's neutral?

Ah I see! I had the same problem when I was studying the AAs.
I was confused because His falls under the category of basic AAs, which usually have neg. charges because they can receive a H+.
But remember how general basis can also act as good nucleophiles, and good nucleophiles can have a negative charge or a lone pair, or both!
In this case, the ring just happens to be a nucleophile when it is neutral because of the lone pair, and the positive protonated state as the acid.
I believe His is just unique that way due to the imidazole ring...
If anyone else could give a better explanation, that works too!

Acids do not always have to become neutral upon formation of the conjugate base. The only imperative conferred by conservation of charge is that the acid must go down one unit in charge upon deprotonation. So if it was positively charged to begin with, it'll go neutral. This is not an effect isolated to imidazole rings. The ammonium-ammonia system works this way as well. So does lysine and arginine, actually.

 

[Sloppy Toppy]

Acids do not always have to become neutral upon formation of the conjugate base. The only imperative conferred by conservation of charge is that the acid must go down one unit in charge upon deprotonation. So if it was positively charged to begin with, it'll go neutral. This is not an effect isolated to imidazole rings. The ammonium-ammonia system works this way as well. So does lysine and arginine, actually.

Got that, but how do you know the Histidine is a positive charge to begin with?

 

[aldol16]

Got that, but how do you know the Histidine is a positive charge to begin with?

Well, you first have to know the structure of His or at least what an imidazole looks like. There's no working around that for the MCAT. You're assumed to have a certain level of basic knowledge and the amino acid structures, pKas, general properties, three and one letter codes are expected. Rarely will you have to just simply identify an amino acid - more often, it will be reasoning based on this knowledge. So you have to know that.

But if you know what an imidazole looks like, it's a heterocycle with two nitrogens - each has three bonds to it, one is singly-bonded to one carbon and doubly-bonded to another while the other nitrogen is singly-bonded to two carbons and singly-bonded to a hydrogen. From there, there's only two possibilities - lose the proton to leave an anionic NR2 group or gain a proton on the other nitrogen to give a positively-charged HNR2 group. The former will not happen because anionic NR2 is very basic and unfavorable - the same reason why peptide bond hydrolysis is not spontaneous under normal conditions. So the only thing that makes chemical sense is the lone pair on the other nitrogen serving as a Lewis base.

 

[akb5633]

pKa is the measurement of how strong the acid is, pH is the measurement of how acidic or basic a sovlent is.
pKa is like what you were born with, and pH is like environmental factors.

This thread is from like 4 months ago, but wow, that was a beautiful way to describe the difference between pka and pH.

 

[Asma Ben Saeed]

Please Please clarify for histidine as it is written

 

[lpp06]

Please Please clarify for histidine as it is written

Lysine/Arginine are AA's with basic side chains, with high pKa's. This means that the side chain will be protonated (eg -NH3+) until the pH gets quite high (>10). Since physiological pH is near 7, these side chains will be unable to participate in any proton exchange. This makes them a poor buffer in the human body. On the other hand, since Histidine has a side chain with a pKa of ~6 (closer to physiological pH). It's more likely to gain/lose a proton when pH suddenly changes. This makes histidine a buffer.

 

[medium yus]

When I look at this question, I don't look at charge, I look at overall property (which obviously dictates charge) i.e. is it aliphatic/hydroxylic, basic or acidic. If it's aliphatic/hydroxylic, I cross it out. Based on your photo, it looks like it is looking at these peptides at a pH at or above physiological pH and above the peptides isoelectric point (pI) since we are using anion-exchange chromatography.

C. HIPAGEATEKALRGD

Crossed out amino acids: Isoleucine (I), Proline (P), Alanine (A), Glycine (G), Threonine (T), and Leucine (L).

Histidine (H) - Amphoteric (+/0)
Glutamic Acid (E) - Acidic (-)
Glutamic Acid (E) - Acidic (-)
Lysine (K) - Basic (+)
Arginine (R) - Basic (+)
Aspartic Acid (D) - Acidic (-)

Total Charge = between -1 and 0 (I'm not happy so I digress)

Digression....

Henderson-Hasselbalch Equation derivation...

Dissociation (α) = 1 / 10^(pKa−pH) + 1
α = 1/10^(6-7.4) + 1 = .96 or 96% dissociation at physiological pH (pH = 7.4). This means that 4% of the histidine is positive and 96% is deprotonated and neutral.

Assuming all other amino acids are either full protonated or not except for histidine the total charge should be:

(+1)(.04) + 2 - 3 = -0.96 net charge or about -1