I would also consider the possibility that the effectiveness of the ninhydrin test is pH-dependent, and the optimal pH is different for different amino acids, as shown in Lemothe and McCormick, Analytical Chemistry, 44(4), 1972, 821

https://pubs.acs.org/doi/pdf/10.1021/ac60312a003

There is no universal method, best approach typically depends on the identity of the active substance.

I’m trying to degrade aspartame at pH levels 2, 3, 5, 6, and 8.  Initially, my research says that when degrading, aspartame will break into the amino acids aspartic acid and phenylalanine. Thus, I used the ninhydrin test to detect the degradation of aspartame, since ninhydrin reacts with amino acids to form the compound Ruhemann’s Purple. However, after 6 days, though the purple colored compound formed for pH 3, 5, and 6, there was no results on pH 2 and 8.  The results are understandable for pH 2, since aspartame degrades faster at higher pH levels and much slower at lower ones.  However, theoretically, for pH 8, it should degrade faster than pH 6, and hence the ninhydrin test should have been tested positive at pH 8.  Does anybody know why is this the case?  I hypothesized that it was because aspartame degrades differently at basic pH levels than at acidic pH levels.  However, when I did further research, I got conflicting responses. 

https://www.tandfonline.com/doi/full/10.1080/10942912.2011.565903
This article said that at low pH levels, aspartame will degrade into aspartyl-phenylalanine and methanol. At pH 5 and above, aspartame will cyclize to form its diketo-piperazine, which will then be hydrolyzed to form the amino acids.

http://lib3.dss.go.th/fulltext/Journal/Journal%20of%20food%20science/2001%20v.66/no.6/jfsv66n6p0808-0809ms20000528%5B1%5D.pdf
However, this article says that it will degrade into the diketo-piperazine at pH levels 7-10, and aspartyl-phenylalanine at pH 12.

https://www.researchgate.net/figure/Decomposition-of-aspartame-Adapted-from-Bell-and-Labuza-1991-and-Prodolliet-and_fig3_601130
And yet this photo shows that it will degrade into phenylalanine and aspartic acid at acidic pH levels, while it will degrade into aspartyl-phenylalanine at high pH levels.


Can someone please clarify what compounds does aspartame degrade into at different pH levels?  If it degrades into different compounds at basic and acidic pH levels, how can I detect the degradation of aspartame at high pH levels then?  Thank you so much!

I have a mass spectrum for 4-methoxybenzaldehyde and have 2 fragment ion peaks at 107 and 92. It is an EI+ GCMS and I wanted to know what the fragment ion at 107 and 92 was and the mechanism to form those 2 fragments. Thank you

I agree this is dubious, two competing factors changing at the same time. The only way to say for sure is to compare pKa data (no, I don't have a source at hand).