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I was looking up the chemical structure of glycine, and I found the following two images: glycineH2N

and

glycineH3N.

My question is, what is the "base state", or the "natural state" of glycine? Under what scenarios does glycine become oxidized/reduced?

Also, I know that amino acids contain a carboxyl group, an amino group, a lone hydrogen atom, and a side chain. What is the formula for the amino group? Is it $\ce{-NH_2}$ or $\ce{-NH_3^+}$?

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    $\begingroup$ Both are correct, both are the natural state of glycine and both represent the base state. Amino acids are zwitterions, which switch freely between the two states. Try googling zwitterions. $\endgroup$ – Jeppe Nielsen Jul 9 '17 at 22:38
  • $\begingroup$ So, they are just different ways of representing the same thing? Is there a "more correct" diagram? $\endgroup$ – DrPepper Jul 9 '17 at 22:39
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    $\begingroup$ Correct, both are entirely correct. However at physiological pH (6-8), typically the zwitterform with charges are used. $\endgroup$ – Jeppe Nielsen Jul 9 '17 at 22:50
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    $\begingroup$ For the sake of completeness: the second molecule should have a negative charge at the -COO group $\endgroup$ – Arsak Jul 10 '17 at 5:42
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    $\begingroup$ Please note that these are acid/base reactions and not oxidation/reduction reactions. If glycine is oxided or reduced its not glycine anymore. $\endgroup$ – Nicolai Jul 10 '17 at 20:45
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Short Answer: You need to define base state.

Background: The point to keep in mind here is that the base state (as you call it) is not constant. As @jeppenielsen points out in the comments, amino acids are zwitterions. According to Wikipedia:

In chemistry, a zwitterion, formerly called a dipolar ion, is a neutral molecule with both positive and negative electrical charges. (In some cases multiple positive and negative charges may be present.) Zwitterions are sometimes called inner salts. Unlike simple amphoteric compounds that may only form either a cationic or anionic species, a zwitterion simultaneously has both ionic states.

In glycine, the center of positive charge is the -NH2 group while the center of negative charge is the -COOH group. At different pH (acidic, neutral or basic), the structure of the molecule keeps on changing as shown in the image below:

glycine structure at different pHsource

Structure of other amino acids also changes similarly. To know why this change occurs, just keep in mind that (in very simple terms) pH is inversely proportional to the concentration of H+ ions in a solution (higher the pH, lower the concentration of H+). The exact formula is $\ce{pH = - log[H_3O^+]}$ (yes, it means that a pH of 4 is 10 times more acidic than pH 5 and so on). Also, as pH changes, so does the formula for the amino group (see the diagram again).

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  • $\begingroup$ Good answer, as always, but this is incorrect: "keep in mind that pH is inversely proportional to the concentration of H+ ions". The bigger the pH the lower the H+ ions concentration, that's correct... However, inversely proportional has another meaning in math: it means that the multiplication of the two variables is a constant. And that is clearly not the case here, do the math and you'll see. Actually, you can have a variable growing while the other one decreases and still they can be directly proportional! $\endgroup$ – user24284 Jul 10 '17 at 5:25
  • $\begingroup$ That was a simplification, thats why I also put the formula with it. But I'll change it if it seems incorrect :) seems better now? $\endgroup$ – another 'Homo sapien' Jul 10 '17 at 5:27
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    $\begingroup$ Kind of. Just say that "the higher the pH the lower the H+ concentration". $\endgroup$ – user24284 Jul 10 '17 at 5:49
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    $\begingroup$ or say pH=-log(H+) and do it correctly. ah I see it was added alredy. $\endgroup$ – Jeppe Nielsen Jul 13 '17 at 0:44
  • $\begingroup$ @jeppenielsen thanks! There is a post on chemistry.SE (whose link I am unable to find right now) which discusses about why it is better to write $\ce{H_3O^+}$ instead of $\ce{H^+}$ :) $\endgroup$ – another 'Homo sapien' Jul 13 '17 at 4:00

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