Ninhydrin Reaction with Alanine in Latent Fingerprints

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This is an ab initio computational chemistry project and all energies of the compounds in this multi-step reaction were calculated via WebMo, using the Hartree-Fock theory and 6-31G basis set to calculate molecular energy.

Background

Fingerprinting is an important tool used in forensic science and solving crimes. Latent fingerprints is one of the easiest ways to connect an individual with the crime. These types of fingerprints are created when any surface is touched and there is a transfer of a thin layer of chemical residue from the tips of your fingers. Multiple methods of lifting fingerprints are used, but chemical methods is the most effective for nonporous surfaces.[1] In this project, I am focusing on the use of a ninhydrin spray to lift a fingerprint. Ninhydrin reacts with the proteins in the latent fingerprint and create an amino acid residue that becomes a purple color after a multi-step reaction. The ninhydrin condenses the primary amine of an amino acid, alanine in this case, to form an imine. The resulting imine decarboxylates and hydrolyzes in water to form another primary amine that condenses with a second ninhydrin molecule to form Ruhemann's purple.[2]

Alanine
Ninhydrin
Figure 1. McCaldin Mechanism. The numbers of the compounds correspond with the numbers in the chart
Figure 2. Lamothe mechanism. The numbers of the compounds correspond with the numbers in the chart.
Figure 3. Friedman mechanism. The numbers of the compounds correspond with the numbers in the chart

Calculations

The molecular energy calculations were ran with WebMo/Gaussian using the Hartree-Fock theory and 6-31G basis set. The numbers in the chart can be found by looking at the above figures, the numbers correspond with specific compounds. The ΔE of reaction a was also calculated by using the Products - Reactants formula.

Structure Energy (Hartree) Run Time
Ninhydrin -643.27 7.3 sec
Alanine -321.77 2.1 sec
1 -889.50 27.6 sec
2 -626.84 21.3 sec
3 -548.97 10.6 sec
Acetaldehyde -152.91 0.6 sec
4 -625.77 16.7 sec
5 -1041.85 43.0 sec
5' -1117.79 50.4 sec
6 -568.77 9.7 sec
7 -568.84 9.9 sec
8 -1128.16 7 min 11 sec
9 547.69 9.7 sec
10 -567.65 4.9 sec
11 -813.39 24.3 sec
12 -625.82 14.1 sec
Ruhemann's purple, isomer 1 -1040.48 1 min 15 sec
Ruhemann's purple, isomer 2 -1040.57 1 min 1 sec
Ruhemann's purple, isomer 3 -1041.07 2 min 43 sec
Water -75.98 0.6 sec
Carbon Dioxide -185.58 0.6 sec
Ammonia -56.10 0.6 sec

ΔE Calculations

The multiple reactions are denoted by letters a through w. The ΔE calculation was done by converting the energy values from the chart to kcal/mol. The conversion factor is 627.51 kcal/mol = 1 Hartree. I took Products - Reactants to get my ΔE.

The reaction: Ninhydrin+alanine --> 1 +H2O

(-889.50 Hartree + -75.98 Hartree) - (-643.27 Hartree + -321.77 Hartree) = -0.44 Hartree

-.044 Hartree * 627.51 kcal/mol = 276.1 kcal/mol

  • My ΔE value was different than the original paper's value was. They got the value of 7.08 kcal/mol.

Conclusion

It can be concluded that under high temperatures, where extra energy is available, this reaction is energetically feasible. All three different mechanisms work best under high temperature conditions. The multiple reaction paths lead to higher yields of Ruhemann's purple. Higher yields of Ruhemann's purple creates more of a chromophore when it reacts with the amino acid.

Citations