Acid/Base Chemistry with Amino Acids EK Question

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balopathic45

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I understand why the answer choice A is correct, but why isn't D correct also. If Arginine's side chain, when protonated, is more stable that means it's less likely to give off a proton and therefore has a higher pKa.

For instance, OH- has a high pKa because its conjugate acid (H2O) is stable.

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That's a reasonable explanation. D sound correct as well.

Maybe, it was saying in body's ph. Those are in protonated forms. So answering with conjugated base ( unprotonated) is more sounded. I"m guessing here
 
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That's a reasonable explanation. D sound correct as well.

Maybe, it was saying in body's ph. Those are in protonated forms. So answering with conjugated base ( unprotonated) is more sounded. I"m guessing here

Yeah, I think you're right about the body pH
 
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lysine is more acidic (lower pKa than arginine) so we can reason that it's conjugate base is more stable. I was looking at the structure of the conjugate base of both amino acids and couldn't come up with a good reason as to why lysine's conjugate base would be more stable than arginine's?

Arginine's will actually have more resonance structures. :confused:
 
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lysine is more acidic (lower pKa than arginine) so we can reason that it's conjugate base is more stable. I was looking at the structure of the conjugate base of both amino acids and couldn't come up with a good reason as to why lysine's conjugate base would be more stable than arginine's?

Arginine's will actually have more resonance structures. :confused:

I think in this scenario it's easier to think about it in terms of which structure is the most stable while protonated - the more stable, the less acidic. Since arginine's positive charge is delocalized between the three nitrogens, it's not as likely to give off a proton than lysine is. Thus, lysine is more acidic.
 
lysine is more acidic (lower pKa than arginine) so we can reason that it's conjugate base is more stable. I was looking at the structure of the conjugate base of both amino acids and couldn't come up with a good reason as to why lysine's conjugate base would be more stable than arginine's?

Arginine's will actually have more resonance structures. :confused:

are you thinking of the conjugate base for arginine's or lysine's side chain as having a negative charge? the conjugate base for the side chain in each case is neutral. The resonance structures in arginine help stabilize the positive charge in the conjugate acid.
 
I think in this scenario it's easier to think about it in terms of which structure is the most stable while protonated - the more stable, the less acidic. Since arginine's positive charge is delocalized between the three nitrogens, it's not as likely to give off a proton than lysine is. Thus, lysine is more acidic.
but then isnt histidine more stable than lysine and arginine because of delocalization of the positive charge into the ring?
 
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