methanethiosulfonate and pyrroline

Site-directed spin labeling is a versatile tool to study structure as well as dynamics of proteins using EPR spectroscopy. Methanethiosulfonate (MTS) spin labels tethered through a disulfide linkage to an engineered cysteine residue were used in a large number of studies to extract structural as well as dynamic information on the protein from the rotational dynamics of the nitroxide moiety. The ring itself was always considered to be a rigid body. In this contribution, we present a combination of high-resolution X-ray crystallography and EPR spectroscopy of spin-labeled protein single crystals demonstrating that the nitroxide ring inverts fast at ambient temperature while exhibiting nonplanar conformations at low temperature. We have used quantum chemical calculations to explore the potential energy that determines the ring dynamics as well as the impact of the geometry on the magnetic parameters probed by EPR spectroscopy.

Topics: Crystallography, X-Ray; Electron Spin Resonance Spectroscopy; Mesylates; Models, Molecular; Oxides; Proteins; Pyrroles

The title compound, C(10)H(18)NO(3)S(2), which finds application as a spin label, has triclinic (P\\overline{1}) symmetry at 100 (2) K with two independent molecules in the asymmetric unit. Both molecules are very similar with respect to bond lengths and angles, but molecule 2 shows disordering of its side chain. The pyrroline rings differ slightly with respect to the position of the NO group, which in both cases are sterically shielded by the surrounding methyl groups. The crystal structure of the title compound represents the first example of a 2,2,5,5-tetramethyl-1-oxyl-Delta(3)-pyrroline derivative with a side chain at the double bond which is linked to it through an sp(3)-hybridized C atom. In the solid state, the side chain adopts a conformation with the methyl group above/below the pyrroline ring and a H atom directed towards a C atom of the double bond. The disordered side chain of molecule 2 represents a second conformation with low potential energy. Both molecules exhibit planar chirality, but in the solid state both pairs of stereoisomers are present. These four stereoisomers are stacked one behind the other in four different columns, denoted A, A', B and B', the angle between the vectors of the N-O bonds in columns A and B being 80.38 (8) degrees .

Topics: Crystallization; Crystallography; Mesylates; Models, Molecular; Molecular Conformation; Pyrroles; Spin Labels