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I have a universal tube with 10 mg mL-1 ampicillin. When I got it, it was supposed to be sterile. It was opened for approximately 20 minutes for an experiment and has since been standing around sealed for a good month now.

Within the last couple weeks, it has gradually turned yellow. Right now the colour is faint, with a green-ish tint.

Why would it turn yellow? I know NADH is yellow, so that was my first guess. But I couldn't exaplain why ampicillin would cause NADH to accumulate, so I discarded that.

Side info: Ampicillin acts on bacterial cell walls, maybe that might help.

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I assume you know that, but ampicillin solution should be stored at -20°C in aliquots, it's not the most stable molecule. –  Mad Scientist Jun 26 '12 at 13:24
    
Ah, I never thought it could just be an unstable molecule. I'm going to read up a bit more about the ampicillin itself then; I thought it had something to do with contamination. –  Armatus Jun 26 '12 at 13:26
    
Just take a look at the structure, does that four-membered ring look strained to you? ;-) –  Mad Scientist Jun 26 '12 at 13:30
    
Oh wow I didn't even know that was possible. So could the colour come from the Phe-like member when it breaks off? –  Armatus Jun 26 '12 at 13:39
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up vote 6 down vote accepted

Why is NADH the first thing to come to mind? Time for some physical chemistry.

Beta-lactams has a shorter ring structure which give it a different absorbance (around 322 nm apparently (source needed)) which gives it a blueish hue. Typically Penicillin and Ampicillin start off as an off-white. As hinted by @Mad Scientist, the beta-lactam rings are unstable and will undergo acid-catalyzed hydrolysis which breaks the 4-membered ring.

The Penicillin core also near a ketone which can attack the ketone in the lactam (I'm pretty sure this qualifies as enol chemistry but it has been awhile). The cyclization will form a 5-membered oxazole ring. Based on my experience, unsaturated 5-membered rings will absorb at a higher wavelength tending to result in a yellowish color (570-590 nM) so I am going to suggest that is the source of your color.

Penicillin core

Ampicillin degradation

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Cheers, very informative. NADH was just because that's the only yellow compound our classes have dealt with until now. –  Armatus Jun 26 '12 at 19:07
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