Use of aspartic acid racemization in fingerprints as a molecular clock

Abstract

Aspartic acid is the chemical, amongst amino acids, whose racemization process is considered to be the fastest and the most reliable way to conduct an ageing study in some living-being tissues. Racemization is a natural-occurring event in which L-stereoisomer of aspartic acid is converted into D-stereoisomer in an equilibrium dependent on factors namely pH, environmental conditions and, most critically, on temperature.

The main objective of the current study is to assess the influence of temperature on kinetics of aspartic acid racemization and to implement this methodology in fingermarks. The hypothesis is that different starting points of deposition of a biological specimen will give rise to significant differences in L-/D- racemization after the sample has been heated, allowing to determine time since deposition of a fingerprint. In this way, it would be possible to find the time since build a molecular clock for fingerprints, consisting of time since deposition of fingermarks based on racemization degree of aspartic acid.

Firstly, a calibration curve was built within the concentrations range expected to be quantified in actual samples both with L-aspartic acid (L-Asp) and D-aspartic acid (D-Asp) separately, Gas Chromatography - Flame Ionization Detector (GC-FID) being the chosen technique and detection mode to set up the calibration experiments. Secondly, L-aspartic acid standards were treated at different temperatures during different time periods and were analyzed by GC-FID with a chiral column, aimed for detecting D- isomer, providing with information about racemization rate of aspartic acid.

Eventually a downward trend was observed for L-Asp but not for D-Asp and a pseudo-rate constant could be calculated for the former, demonstrating that temperature degrades this chemical. Fingerprints study was not successful to determine aspartic acid racemization but a simple handling of them and an adequate, sensitive response of the chromatographic system could quantify racemization degree.