TBR CBT 5 Q 127 BS section

[Sailor Senshi Dermystify]

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I am not understanding why if the amino acid is negative then that means it is in a negative environment and it's acidic.

I thought by using the Henderson-Hasselbach equation
If ph=pka the molecule is neutral
If ph>pka the molecule is an anion, which means it has a negative charge, so I assume it is basic
If ph<pka the molecule is a cation, which means it has a positive charge, so I assume it is acidic.

Their explanations say otherwise especially when it comes to the environment and if the molecule is acidic or basic.

Attach is the problem and thank you so much 🙂

 

[Cawolf]

If the acid has a pka that is below the pH (as in the correct answer choice) then the acid will be deprotonated. An acid group that is deprotonated will have a formal negative charge.

 

[Quinoline]

You want proof that DNA is made of an acid. From HH eq., when [A-]=[HA], pH=pka. So at pH=pka, half of the A- are free and half are bound to H+. At a pH greater than the pka, there are less free H+, so even more A- are free, giving a net negative charge if the molecule is an acid.

pH=pka (2 free, 2 bound) :
- - - -
+ +

pH>pka (2 free, 1 bound) (net negative charge):
- - - -
+

 

[Sailor Senshi Dermystify]

I thought net negative charge is basic and protonated or having a cation is acidic. Why having a net negative charge is acidic, that doesn't make sense to me.

 

[Cawolf]

If the acid has a pka that is below the pH (as in the correct answer choice) then the acid will be deprotonated. An acid group that is deprotonated will have a formal negative charge.

 

[Quinoline]

For an acid like HCl for example, when it is deprotonated, you are left with Cl- which is negative. When H2SO4 is deprotonated it gives HSO4-, which is negative.

Choices C and D: For a base, like OH-, when it is protonated it has a neutral charge in H20. For a weaker base like NH3, when it is protonated it has a positive charge in NH4+.