Towards elucidation of the reaction mechanism between lawsone and amino acids

Author: Simone Madaras

Madaras, Simone, 2021 Towards elucidation of the reaction mechanism between lawsone and amino acids, Flinders University, College of Science and Engineering

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Research is constantly ongoing to find new fingerprinting reagents that can be applied to a variety of latent fingermarks and substrates. Particular substrates, such as paper and blood, prove to be challenging due to the high degree of background signals and possibility for reagent interaction with other compounds in the complex chemical matrix. Additionally, commonly used fingermark enhancing reagents such as ninhydrin, 1,2-indandione, and DFO and their formulations have proven toxicity. In 2008, Jelly et al. were prompted to investigate lawsone, a primary constituent in henna, as an alternative and safer reagent for the detection of latent fingermarks on porous surfaces. They found that lawsone reacts with amino acids in fingerprint deposits on paper, returning a purple-brown coloured stain with red fluorescent properties. Two contrasting structures have been proposed to be responsible for this red fluorescence, but limited research has been conducted to verify either of these structures. There has also been no investigation into how the reaction of lawsone with amino acids proceeds. Further investigations into lawsone’s use have resulted in the inconsistent replication of the observed red fluorescence. This thesis details the identification of the structure responsible for the observed red fluorescence from the reaction between lawsone and glycine, as well as a proposed reaction pathway towards its formation.

The microwave assisted reaction between lawsone and glycine in ethanol was performed in this work, where a red compound exhibiting fluorescence and NMR spectra consistent to that reported by Jelly et al. was isolated. Unlike the two previously reported structures, the structure determined in this work consisted of two lawsone molecules connected by an amine and alkyl linker. This structure was consistent with existing literature and therefore the identity of the red fluorescent product was verified. Two additional compounds were isolated from the reaction, where one was identified as an amine intermediate toward red compound formation, and the other was deemed a reaction by-product.

Further investigation into the reaction mixture using different amino acids, deuterated ethanol, and lawsone analogues revealed that the amino acid is incorporated into the red fluorescent structure at the -OH site of both lawsone molecules to form the amine linker. Surprisingly, the alkyl linker originated from the ethanol solvent and was incorporated at the vinyl position of both lawsone molecules. This alkylation chemistry was unexpected as the traditional conditions for alkylation are seemingly not present in the reaction mixture.

Possible intermediates along the reaction pathway towards the red fluorescent product formation were postulated and synthesised. These proposed intermediates imitated varying potential stages of linker formation. The ability of the reactions between these intermediates with each other, lawsone, glycine, and ethanol to form the fluorescent red product was subsequently used to propose a complete reaction pathway towards dimer formation. It was postulated that the previously isolated amine intermediate is formed first via Strecker degradation between lawsone and the amino acid, followed by condensation with a second lawsone molecule to form a NH-linked dimer intermediate. The alkyl linker is then formed via nucleophilic substitution at one vinyl position of the existing NH-linked dimer intermediate, followed by intramolecular cyclisation to completely form the fluorescent red product. The knowledge obtained from the understanding of this pathway may help to facilitate further optimisation of reagent conditions for use in the reliable and reproducible detection of latent fingermarks using lawsone.

Keywords: lawsone, amino acids, fingermark reagent, reaction mechanism

Subject: Forensic & Analytical Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2021
School: College of Science and Engineering
Supervisor: Claire Lenehan