TBR Bio I Chap I #22

[shffl]

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What I gathered from the passage is that the drug (lidocaine) will be in its basic form under a more basic condition because if it is protonated in an acidic condition then it is inactivated (correct me if I'm wrong here). The answer states that if the pKa of the local anesthetic (the lidocaine) is high, then the concentration of the base in the tissue will be low. That contradicts what I thought because if the pKa is high (more basic?) then why will the concentration of its basic form be low? Where did I go wrong?

 

[hdub]

There will be lower concentration of base in the tissue based on this principle:

A) When pH solution (ie physiological pH in this scenario) > pKa anesthetic , more of the anesthetic will exist it its protonated form CATION IS ACID FORM
VS
B) When pH sltn < pKa anesthetic, more anesthetic will exist in its deprotonated form

Now: Which situation can be attributed to the lower concentration of base in the tissue?? Situation A because the anesthetic exists primarily in its protonated/acid form at high pKa in a physiological pH

 

[sps27]

Bumping this thread. I am still confused about this. Psg4 in TBR from Nerve and Muscle and question number 22 in particular. Correct ans is A). My doubt is the same as the OP.

Q: Based on the passage, which of the following statements is most likely true?

A) The higher the pKa of the local anesthetic, lower the concentration of base in the tissue
B) The lower the pKa of the local anesthetic, higher the concentration of the acid in the tissue
C) The higher the Ka of the local anesthetic, lower the concentration of base in the tissue
D) The lower the Ka of the local anesthetic, lower the concentration of acid in the tissue.

 

[Schenker]

A higher pKa implies that the compound is more basic, which seems to be clear to everyone. The confusion around this problem seems to be a linguistic red herring(?) of sorts: if a compound is more basic, then shouldn't it stick around more in its basic form? No, because it'll do what good bases do and grab a proton, converting it to its conjugate acid. Thus, raising the pKa of a compound will raise the concentration of its conjugate acid and lower the concentration of base.

 

[sps27]

I appreciate your explanation and thank you; I can live with that.