Re: is the chemical acetylcholine acidic?

The short answer is not really. It doesn't have a proton (ie, an H+) that will come off in solution under normal conditions. Techically, a couple of the protons could come off at a very, very high pH or possibly the positively charged nitrogen could cause it to act as a "Lewis acid" (although I'm not sure about this). So, I think that you could technically say that acetylcholine is a very, very weak acid, but for pratical purposes, it is non-acidic.

By the way, I was a little unsure of my answer for this question, so I checked with some friends. I've copied the correspondence below, which might give you some leads if you want to learn more. Good luck!

The author is answering on his own behalf, and his answer should not be considered a statement from his employer.

Things can be acidic in ways other than the Brønsted–Lowry acid-base theory ("acid is something that loses H+"). If you put this N+ thing in water, how might it interact (remember water exists partially as a mixture of "H+ and OH-")? If you change the balance of H+ and OH-, you're changing the pH of the solution. DMacks (talk) 17:52, 10 November 2008 (UTC)

I suppose it will either remove some free OH- from solution, consequently increasing the ratio of H+ in solution and lowering pH, or it will have no effect at all. How would you describe acidity/basicity of this molecule, short of being quantitative about it? Would you simply call it a weak acid, or it there something more precise you can say? ike9898 (talk) 18:06, 10 November 2008 (UTC)

Asuming that the -NH3+ bit is the most acidic bit, it will have some measurable pKa value. Any solution that has a pH above this number will result in deprotonation of the molecule. According to this page, the Ka of acetylcholine is 1.8e-5, which means the pKa is 4.74. So any solution of a pH greater than 4.74 will deprotonate the -NH3+. In other words, in a pure water solution (pH = 7.00) acetylcholine will be acidic. Cheers. -- Jayron32.talk.contribs 17:59, 10 November 2008 (UTC)

The N doesn't have a proton to lose. It has 4 substituets. ike9898 (talk) 18:06, 10 November 2008 (UTC)

You're right. I misread the article. But the pKa of the molecule is measured, so it really is a weak acid. It doesn't matter WHERE the proton comes from. Looking at the structure, the base form of Acetylcholine is likely to be a zwitterion of the form CH3COOCH-CH2N+(CH3)3 or maybe on the next carbon to the right... --Jayron32.talk.contribs 18:13, 10 November 2008 (UTC)

What about the effect cited by DMacks above? Is the + charge going to pull OH- ions out of solution? ike9898 (talk) 18:19, 10 November 2008 (UTC)

I don't see why that would work. After all, Na+ ions aren't acidic in any way; they don't "pull" OH- ions out of water at all... -- Jayron32.talk.contribs 18:25, 10 November 2008 (UTC)

Wrong on both counts. That paper is talking about the dissociation constant of a complex with a receptor, not a pKa. Na+ is indeed acidic, although very weak. Acetylcholine is also a very weak acid; you can deprotonate a carbon next to the nitrogen to form a nitrogen ylide, or the carbon next to the ester carbonyl to form an enolate. But both pKa's are greater than 14, which means that the deprotonation will be largely negligible in water. Acetylcholine could also act as a Lewis acid and react with water as RNMe3+ + H2O -> RNMe2 + MeOH + H+, but that's probably not what the OP had in mind because it "destroys" the molecule rather than simply deprotonating it. --Itub (talk) 20:22, 10 November 2008 (UTC)

Good call, Itub. Yes, I was lazy in not fully reading the article. After looking over the structure, there's no reason to assume that ANY of the C- H bonds is particulatly acidic; they certainly don't look much more acidic than, say, the C-H bonds in acetylacetone, which, while being more acidic than alkyl C-H bonds, really doesn't compare to classically defined "acids". --Jayron32.talk.contribs 21:13, 10 November 2008 (UTC)