Rheological profiles of DNA extracts from forensic bloodstain samples

Abstract

Rheology can be used to probe the differences in physical properties and in-solution behaviours of synthetic and biological polymers, like deoxyribonucleic acid (DNA). Large fragments of highly concentrated genomic and phage DNA have been characterized using rheology; however, this amount and size of DNA are atypical in DNA extracted from forensic evidence. To determine the applicability of rheology for the analysis of dilute concentrations of short DNA fragments expected from forensic specimens (low molecular weight DNA at concentrations in the ng/µL range), we conducted an optimization experiment in which we varied the size, concentration, and conformation of synthetic DNA in various solvents and polymer matrices. We found that incorporating DNA into an alginate-based, ionically crosslinked hydrogel produced the greatest differences in rheological profiles from synthetic DNA comprised of different physical properties. The conformation and size of encapsulated DNA provided significantly different responses during dynamic oscillatory measurements (p<0.05). Additionally, a time since deposition (TSD) study was performed using rotational and oscillatory tests to understand the changes in DNA extracts from bloodstains left to degrade for up to 19 months. DNA extracted from all timepoints could be detected and quantified for the tested temperature conditions (-20°C, 4°C, 22°C). Statistical analyses revealed moderate correlations between the rheological responses of DNA-containing materials and TSD (r = -0.57 to r = 0.62). Our results highlight the viability of rheology as a technique for the analysis of dilute DNA oligos and DNA extracts, and as a complementary technique for the determination of bloodstain TSD.