[matzkdav]

I'm having trouble with this one.
My answer would be that the overall charge on serotonin would be negative since acidity increases at low pH levels?
Any advice?

Thanks!

[Perito]

Serotonin has one -OH group on a phenyl ring. The hydrogen on that is acidic. It also has an -NH2 group that can accept a proton (hydrogen atom) and a -NH group where the N is in a 5-member ring. You can look up the structure to see what I'm talking about.

As acidity increases, the -OH will be less and less ionized so it will be neutral. The -NH2 group will become -NH3+ and the -NH group will become NH2+. The latter group will disperse the electrical charge around the conjugated ring structure so it will actually accept a proton more readily than the -NH2 group.

Overall, the structure will become more positive. There will, of course, be other anions hanging around.