Two different incorporation sites of manganese in single-crystalline monohydratedL-asparagine studied by electron paramagnetic resonance

Single crystals of monohydrated $L$-asparagine have been grown from aqueous solutions using $\mathrm{Mn}{\mathrm{Cl}}_{2}$ as doping material. Electron paramagnetic resonance (EPR) was used to determine the incorporation sites of ${\mathrm{Mn}}^{2+}$ ions in the crystal structure. Depending on small $p\mathrm{H}$ changes and crystal growth kinetics in the aqueous solutions, ${\mathrm{Mn}}^{2+}$ ions are incorporated in two chemically distinct sites in asparagine crystals. The first shows isotropic six-line hyperfine-split EPR spectra, whereas the second shows anisotropic multiple line splitting due to ${\mathrm{Mn}}^{2+}$ fine structure $(S=5∕2)$ and hyperfine interaction $(I=5∕2)$. Angular dependencies of the ${\mathrm{Mn}}^{2+}$ EPR spectra in three mutually perpendicular crystal planes were measured and analyzed. The results are discussed in terms of the metal incorporation site symmetry in the crystal structure of monohydrated $L$-asparagine.