Crystal structure of seleno-l-cystine di-hydro-chloride

Abstract

Numerous crystal structures are available for the dimeric amino acid cystine. In proteins it is formed by oxidation of the -SH thiol groups of two closely spaced cysteine residues, resulting in the formation of a familiar di-sulfide bridge. The title compound [systematic name: (R,R)-1,1'-dicarb-oxy-2,2'-(diselanedi-yl)diethanaminium dichloride], C6H14N2O4Se2 (2+)·2Cl(-), is the first example of a small mol-ecule structure of the biologically important analogue with a -CH2-Se-Se-CH2- bridging unit. Bond lengths and angles of seleno-l-cystine di-hydro-chloride and its isotypic sulfur analogue l-cystine di-hydro-chloride are compared.

Keywords: Se—Se bridge; cancer therapy; crystal structure; isotypism; l-cystine analogue.

Figure 1

(a) The mol­ecular structure of seleno-l-cystine di­hydro­chloride. The right-hand part, coloured in a light tone, is generated by application of twofold rotation symmetry in space group C2; Se1*, C3* etc are generated by the symmetry code −x + 1, y, −z. Displacement ellipsoids are shown at the 50% probability level. (b) Best overlap between the structures of (I) (dark grey O, N and C atoms) and (II) (light grey; Leela & Ramamurthi, 2007 ▸) with a root-mean-square deviation of 0.133 Å. The view is along the twofold rotation axis (lens-shaped symbol), the dashed line gives the direction of the z axis.

Figure 2

The crystal packing of seleno-l-cystine di­hydro­chloride viewed approximately along the b axis.

Figure 3

(a) Stereodrawing showing the coordination of hydrogen-bond donors around a Cl⁻ anion (see Table 2 ▸ for symmetry operators). (b) Tape motif along the b axis generated from C^α—H⋯O hydrogen bonds. O1* is at (x, y + 1, z), O1′ at (−x + , y + , −z + 1). Side chains have been truncated beyond C^β.