Assessment of the PCR inhibitory effect of the hair dye constituents and its role in forensic DNA analysis

Hair is among the most frequent types of biological evidence recovered at crime scenes, playing a crucial role in identifying culprits. Often, the recovered hair samples are dyed with various routinely used hair dyes. A survey of 182 individuals showed that the majority (82 %) of individuals dye their hair frequently, at least once a month. Cetearyl alcohol, propylene glycol, and disodium EDTA are the most common ingredients of the commercially available hair dyes. In-silico analysis predicted that the citric acid component of hair dye has the strongest affinity with Taq Polymerase (−6.1 Kcal/mol), followed by ascorbic acid (−5.5 Kcal/mol), resorcinol (−5.0 Kcal/mol), trisodium EDTA (−4.8 Kcal/mol), phosphoric acid (−4.0 Kcal/mol), glycerin and cetyl alcohol (−3.7 Kcal/mol), propylene glycol (−3.5 Kcal/mol), ethanolamine (−3.0 Kcal/mol) and hydrogen peroxide (−2.9 Kcal/mol). Molecular docking studies further revealed that the residues of arginine, threonine, glutamine, lysine, threonine, asparagine, serine, aspartic acid, phenylalanine, leucine, methionine, and tryptophan are the responsible motifs of Taq Polymerase which bind with different hair dye chemical constituents. In a singleplex PCR, the CYCLO gene was amplified only in the presence of cetyl alcohol, glycerin, and ethanolamine. All 23 STR markers were amplified using the Fusion 6C kit in the presence of hair dyes. However, the dye constituents adversely affected the Locus Balance of the STR profiles. Thus, most of the hair dye components act as potential PCR inhibitors by interacting with Taq Polymerase and suitable mitigation strategies should be employed for such forensic biological samples.