Molecular conformations of crystalline L-cysteine determined with vibrational cross angle measurements.

The molecular conformations of crystalline L-cysteine prepared in its orthorhombic form were determined by the vibrational cross angle measurements. Its major dihedral angles of chemical bonds determined by this method are consistent with the results from diffraction experiments. In addition, the relative orientations of the chemical bonds associated with the hydrogen atoms of the NH3(+) group and the thiol group are also determined. The results demonstrate that the vibrational cross angle method based on the multiple-mode approach can potentially become a structural tool for determining molecular conformations. The major challenges for the method to become a general molecular structural tool are discussed, and some approaches to address them are proposed.