Crystal structure of seleno-l-cystine di­hydro­chloride

Görbitz, C.H.; Levchenko, V.; Semjonovs, J.; Sharif, M.Y.

doi:10.1107/S205698901501021X

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ISSN: 2056-9890

Volume 71| Part 6| June 2015| Pages 726-729

doi:10.1107/S205698901501021X

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Crystal structure of seleno-L-cystine dihydrochloride

Carl Henrik Görbitz,^a ^* Vladimir Levchenko,^a Jevgenijs Semjonovs ^a and Mohamed Yusuf Sharif ^a

^aDepartment of Chemistry, University of Oslo, PO Box 1033 Blindern, N-0315 Oslo, Norway
^*Correspondence e-mail: c.h.gorbitz@kjemi.uio.no

Edited by M. Weil, Vienna University of Technology, Austria (Received 28 April 2015; accepted 27 May 2015; online 30 May 2015)

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 disulfide bridge. The title compound [systematic name: (R,R)-1,1′-dicarboxy-2,2′-(diselanediyl)diethanaminium dichloride], C₆H₁₄N₂O₄Se₂²⁺·2Cl⁻, is the first example of a small molecule structure of the biologically important analogue with a —CH₂—Se—Se—CH₂— bridging unit. Bond lengths and angles of seleno-L-cystine dihydrochloride and its isotypic sulfur analogue L-cystine dihydrochloride are compared.

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

CCDC reference: 1403356

1. Chemical context

In addition to the 20 amino acids directly encoded by the genetic code, three more are incorporated into proteins during translation. These three, selenocystine, pyrrolysine and N-formylmethionine, are considered to belong to a group of 23 proteinogenic amino acids. The UGA codon, normally a stop codon, is made to encode selenocysteine by the presence of a selenocysteine insertion sequence (SECIS) in the mRNA (Kryukov et al., 2003 ).

Analogous to the common sulfur analogue cysteine, selenocysteine dimerizes through the formation of an Se—Se bridge to selenocystin, a substance that has received considerable attention recently for its anticancer efficacy (Yu et al., 2015 ) as well as its potential in the prevention of cardiovascular and neurodegenerative diseases (Weekley & Harris, 2013 ).

In the Cambridge Structural Database (CSD, version 5.36; Groom & Allen, 2014 ) there are about 80 distinct structures of cystine deposited, either as an amino acid, a modified amino acid or as an integrate part of a peptide or another large organic molecule. In contrast, there are no entries for selenocystine (and also none for sulfur–selenium hybrids with a —CH₂—S—Se—CH₂— bridge). To provide detailed structural information for this biologically important link, an investigation of its structure, in the dihydrochloride salt C₆H₁₄N₂O₄Se₂²⁺·2Cl⁻, (I), has been undertaken.

2. Structural commentary

The molecular structure of (I) is shown in Fig. 1a. A twofold rotation axis relates the two parts of the molecule. The crystal packing is depicted in Fig. 2, with molecules stacked on top of each other along the 5.2529 (4) Å monoclinic axis. Compound (I) is isotypic with the structure of L-cystine dihydrochloride, (II) (Gupta et al., 1974 ; Jones et al., 1974 ; Leela & Ramamurthi, 2007 ), but not with the structure of L-cystine dihydrobromide (Anbuchezhiyan et al., 2010 ), which forms a related packing arrangement but crystallizes in the orthorhombic space group P2₁2₁2. The disulfide/diselenide bridges adopt helical conformations in all three structures, characterized by having gauche —C—C—X—X—, —C—X—X—C— and —X—X—C—C— torsion angles (X = S or Se) of the same sign, in this case between −81 and −89° [Table 1; —C—C—X—X— = —X—X—C—C— by symmetry]. Geometric parameters for (I) and (II) are furthermore compared in Table 1 with average values from 16 acyclic —CH₂—Se—Se—CH₂— links in non-amino acid structures retrieved from the CSD (Groom & Allen, 2014). The bond lengths and bond angles of (I) are similar to those in the previous seleno structures. The most important differences with respect to (II) [X-ray data at 173 K: a = 18.4405 (15), b = 5.2116 (6), c = 7.2191 (6) Å, β = 103.856 (6)°; Leela & Ramamurthi, 2007] are (obviously) the two Se—Se and S—S bond lengths, with modest changes for bond angles and torsion angles. Concerning the dimensions of the unit cell, there is above all an increase in the length of the cell edge a (+ 0.364 Å, 2%) due to longer C—Se than C—S bonds. An equivalent, anticipated effect on c as a result of the increased length of the Se—Se bond, which runs parallel to the z axis, is effectively counteracted by a 2.52° decrease for the two C—Se—Se angles along the bridge compared to the C—S—S angles, see: Fig. 1b and Table 1. The length of the short monoclinic axis b is determined by direct stacking of amino acid molecules, for which the S-to-Se substitution has less impact since neither is involved in any close intermolecular contacts.

Table 1
Geometric parameters (Å, °) of diselenide and disulfide bridges

Compound	C—Se/C—S	Se—Se/S—S	C—C—Se/S	C—Se/S—Se/S	C—C—Se/S—Se/S	C—Se/S—Se/S—C
(I)	1.9671 (18)	2.3213 (4)	113.96 (12)	100.88 (5)	−88.72 (12)	−83.05 (10)
Average^a	1.967	2.310	114.17	101.29
(II)^b	1.817	2.040	114.48	103.40	−89.04	−81.04
Notes: (a) average of 16 –CH₂—Se—Se—CH₂– bridges in acyclic non-amino acid structures; (b) Leela & Ramamurthi (2007).

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.

3. Supramolecular features

The four strong hydrogen bonds with N—H and O—H donors all have Cl⁻ as the acceptor atom (Fig. 3a). The geometric parameters of the hydrogen bonds listed in Table 2 are almost identical to those of (II). There is also a three-centre interaction with a C^α—H donor and two carbonyl oxygen atoms as acceptors, Fig. 3b.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1⋯Cl1ⁱ	0.91	2.25	3.1425 (16)	167
N1—H2⋯Cl1ⁱⁱ	0.91	2.40	3.2110 (18)	149
N1—H3⋯Cl1ⁱⁱⁱ	0.91	2.32	3.1794 (15)	157
O2—H4⋯Cl1	0.79 (6)	2.22 (6)	3.0080 (19)	172 (4)
C2—H21⋯O1^iv	1.00	2.39	3.292 (2)	150
C2—H21⋯O1ⁱⁱⁱ	1.00	2.55	3.216 (2)	124
Symmetry codes: (i) x, y+1, z-1; (ii) x, y, z-1; (iii) $[-x+{\script{3\over 2}}, y+{\script{1\over 2}}, -z+1]$ ; (iv) x, y+1, z.

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 + $[{3\over 2}]$ , y + $[{1\over 2}]$ , −z + 1). Side chains have been truncated beyond C^β.

4. Synthesis and crystallization

Selenocystine has very low solubility in water as well as in organic solvents, including trifluoroethanol and 1,1,1,3,3,3-hexafluoropropan-2-ol, so a saturated solution was prepared in 0.1 M NaOH solution. 100 µl of this solution was pipetted into a small test tube (5 × 50 mm) to which a small amount of BTB pH indicator was added. The tube was sealed with parafilm punctured with a needle (one small hole) and placed inside a larger tube with concentrated hydrochloric acid. After 15 h the colour had shifted from blue to green, and small crystals of the hydrochloride could be harvested.

5. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. The position of the carboxyl H atom was restrained to the plane defined by O1, O2, C1 and C2; other H atoms were positioned with idealized geometry with fixed C/N—H distances for NH₃, CH₂ (methylene) and CH (methine) groups of 0.91, 0.99 and 1.00 Å, respectively. Free rotation was permitted for the ammonium group. U_iso(H) values were set to 1.2U_eq of the carrier atom, or 1.5U_eq for the ammonium group.

Table 3
Experimental details

Crystal data
Chemical formula	C₆H₁₄N₂O₄Se₂²⁺·2(Cl⁻)
M_r	407.02
Crystal system, space group	Monoclinic, C2
Temperature (K)	100
a, b, c (Å)	18.8045 (16), 5.2529 (4), 7.2719 (6)
β (°)	102.219 (1)
V (Å³)	702.03 (10)
Z	2
Radiation type	Mo Kα
μ (mm⁻¹)	5.65
Crystal size (mm)	0.85 × 0.08 × 0.07

Data collection
Diffractometer	Bruker D8 Advance single-crystal CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2014 )
T_min, T_max	0.514, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	11089, 4209, 4080
R_int	0.024
(sin θ/λ)_max (Å⁻¹)	0.908

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.023, 0.062, 1.10
No. of reflections	4209
No. of parameters	78
No. of restraints	3
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	4.55, −0.87
Absolute structure	Flack x determined using 1708 quotients (Parsons et al., 2013 )
Absolute structure parameter	0.044 (4)
Computer programs: APEX2 and SAINT (Bruker, 2014), SHELXT (Sheldrick, 2015a ), SHELXL2014 (Sheldrick, 2015b ) and Mercury (Macrae et al., 2008 ).

A rather large residual peak in the electron density map, with Δρ_max = 4.55 e Å⁻³, remained after completion of the refinement. This peak is located on the twofold rotation axis at the center of the Se—Se bond, and evidently reflects bonding electrons. As a test, an extra isotropic C atom was introduced close to the axis. Its occupancy was subsequently refined to 0.17 (equivalent to one electron), and the R-factor fell from 0.0233 to 0.0180.

Supporting information

CCDC reference: 1403356

Crystal structure: contains datablocks I, global. DOI: 10.1107/S205698901501021X/wm5154sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S205698901501021X/wm5154Isup2.hkl

Supporting information file. DOI: 10.1107/S205698901501021X/wm5154Isup3.cml

checkCIF report

Chemical context top

Analogous to the common sulfur analogue cysteine, selenocysteine dimerizes through the formation of an Se—Se bridge to selenocystin, a substance that has received considerable attention recently for its anticancer efficacy (Yu et al., 2015) as well as its potential in the prevention of cardiovascular and neurodegenerative diseases (Weekley & Harris, 2013).

In the Cambridge Structural Database (CSD, version 5.36; Groom & Allen, 2014) there are about 80 distinct structures of cystine deposited, either as an amino acid, a modified amino acid or as an integrate part of a peptide or another large organic molecule. In contrast, there are no entries for selenocystine (and also none for sulfur–selenium hybrids with a —CH₂—S—Se—CH₂— bridge). To provide detailed structural information for this biologically important link, an investigation of its structure, in the dihydrochloride salt C₆H₁₄N₂O₄Se₂²⁺·2Cl^-, (I), has been undertaken.

Structural commentary top

The molecular structure of (I) is shown in Fig. 1a. A twofold rotation axis relates the two parts of the molecule. The crystal packing is depicted in Fig. 2, with molecules stacked on top of each other along the 5.2529 (4) Å monoclinic axis. Compound (I) is isotypic with the structure of L-cystine dihydrochloride, (II) (Gupta et al., 1974; Jones et al., 1974; Leela & Ramamurthi, 2007), but not with the structure of L-cystine dihydrobromide (Anbuchezhiyan et al., 2010), which forms a related packing arrangement but crystallizes in the orthorhombic space group P2₁2₁2. The disulfide/diselenide bridges adopt helical conformations in all three structures, characterized by having gauche —C—C—X—X—, —C—X—X—C— and —X—X—C—C— torsion angles (X = S or Se) of the same sign, in this case between -81 and -89° [Table 1; (—C—C—X—X— = —X—X—C—C— by symmetry]. Geometric parameters for (I) and (II) are furthermore compared in Table 1 with average values from 16 acyclic —CH₂—Se—Se—CH₂— links in non-amino acid structures retrieved from the CSD (Groom & Allen, 2014). The bond lengths and bond angles of (I) are similar to those in the previous seleno structures. The most important differences with respect to (II) [X-ray data at 173 K: a = 18.4405 (15), b = 5.2116 (6), c = 7.2191 (6) Å, β = 103.856 (6)°; Leela & Ramamurthi, 2007] are (obviously) the two Se—Se and S—S bond lengths, with modest changes for bond angles and torsion angles. Concerning the dimensions of the unit cell, there is above all an increase in the length of the cell edge a (+ 0.364 Å, 2%) due to longer C—Se than C—S bonds. An equivalent, anticipated effect on c as a result of the increased length of the Se—Se bond, which runs parallel to the z axis, is effectively counteracted by a 2.52° decrease for the two C—Se—Se angles along the bridge compared to the C—S—S angles, see: Fig. 1b and Table 1. The length of the short monoclinic axis b is determined by direct stacking of amino acid molecules, for which the S-to-Se substitution has less impact since neither is involved in any close intermolecular contacts.

Supramolecular features top

The four strong hydrogen bonds with N—H and O—H donors all have Cl^- as the acceptor atom (Fig. 3a). The geometric parameters of the hydrogen bonds listed in Table 2 are almost identical to those of (II). There is also a three-centre interaction with a C^α—H donor and two carbonyl oxygen atoms as acceptors, Fig. 3b.

Synthesis and crystallization top

Refinement top

Crystal data, data collection and structure refinement details are summarized in Table 3. The position of the carboxyl H atom was restrained to the plane defined by O1, O2, C1 and C2; other H atoms were positioned with idealized geometry with fixed C/N—H distances for NH₃, CH₂ (methylene) and CH (methine) groups of 0.91, 0.99 and 1.00 Å, respectively. Free rotation was permitted for the ammonium group of the zwitterionic molecule. U_iso(H) values were set to 1.2U_eq of the carrier atom, or 1.5U_eq for the ammonium group.

A rather large residual peak in the electron density map, with Δρ_max = 4.55 e Å^-3, remained after completion of the refinement. This peak is located on the twofold rotation axis at the center of the Se—Se bond, and evidently reflects bonding electrons. As a test, an extra isotropic C atom was introduced close to the axis. Its occupancy was subsequently refined to 0.17 (equivalent to one electron), and the R-factor fell from 0.0233 to 0.0180.

Related literature top

For related literature, see: Allen (2002); Anbuchezhiyan et al. (2010); Gupta et al. (1974); Jones et al. (1974); Kryukov et al. (2003); Leela & Ramamurthi (2007); Weekley & Harris (2013); Yu et al. (2015).

Computing details top

Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b).

Figures top

	Fig. 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 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.
	Fig. 2. The crystal packing of seleno-L-cystine dihydrochloride viewed approximately along the b axis.
	Fig. 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 + 3/2, y + 1/2, -z + 1). Side chains have been truncated beyond C^β.

(R,R)-1,1'-Dicarboxy-2,2'-(diselanediyl)diethanaminium dichloride top

Crystal data top

C₆H₁₄N₂O₄Se₂²⁺·2(Cl⁻)	F(000) = 396
M_r = 407.02	D_x = 1.925 Mg m⁻³
Monoclinic, C2	Mo Kα radiation, λ = 0.71073 Å
a = 18.8045 (16) Å	Cell parameters from 9938 reflections
b = 5.2529 (4) Å	θ = 2.2–40.2°
c = 7.2719 (6) Å	µ = 5.65 mm⁻¹
β = 102.219 (1)°	T = 100 K
V = 702.03 (10) Å³	Needle, colourless
Z = 2	0.85 × 0.08 × 0.07 mm

Data collection top

Bruker D8 Advance single-crystal CCD diffractometer	4209 independent reflections
Radiation source: fine-focus sealed tube	4080 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.024
Detector resolution: 8.3 pixels mm^-1	θ_max = 40.2°, θ_min = 2.2°
Sets of exposures each taken over 0.5° ω rotation scans	h = −34→34
Absorption correction: multi-scan (SADABS; Bruker, 2014)	k = −9→9
T_min = 0.514, T_max = 1.000	l = −13→13
11089 measured reflections

Refinement top

Refinement on F²	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	H atoms treated by a mixture of independent and constrained refinement
R[F² > 2σ(F²)] = 0.023	w = 1/[σ²(F_o²) + (0.0169P)² + 0.004P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.062	(Δ/σ)_max < 0.001
S = 1.10	Δρ_max = 4.55 e Å⁻³
4209 reflections	Δρ_min = −0.87 e Å⁻³
78 parameters	Absolute structure: Flack x determined using 1708 quotients (Parsons et al., 2013)
3 restraints	Absolute structure parameter: 0.044 (4)

Crystal data top

C₆H₁₄N₂O₄Se₂²⁺·2(Cl⁻)	V = 702.03 (10) Å³
M_r = 407.02	Z = 2
Monoclinic, C2	Mo Kα radiation
a = 18.8045 (16) Å	µ = 5.65 mm⁻¹
b = 5.2529 (4) Å	T = 100 K
c = 7.2719 (6) Å	0.85 × 0.08 × 0.07 mm
β = 102.219 (1)°

Data collection top

Bruker D8 Advance single-crystal CCD diffractometer	4209 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2014)	4080 reflections with I > 2σ(I)
T_min = 0.514, T_max = 1.000	R_int = 0.024
11089 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.023	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.062	Δρ_max = 4.55 e Å⁻³
S = 1.10	Δρ_min = −0.87 e Å⁻³
4209 reflections	Absolute structure: Flack x determined using 1708 quotients (Parsons et al., 2013)
78 parameters	Absolute structure parameter: 0.044 (4)
3 restraints

Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Se1	0.50147 (2)	0.56610 (5)	0.16037 (2)	0.01306 (4)
Cl1	0.65288 (2)	0.16979 (8)	0.87546 (6)	0.01345 (7)
O1	0.69775 (8)	0.3451 (3)	0.4104 (2)	0.0158 (2)
O2	0.62968 (9)	0.5697 (5)	0.57259 (19)	0.0225 (3)
H4	0.6356 (18)	0.454 (12)	0.644 (6)	0.058 (15)*
N1	0.67823 (8)	0.6777 (3)	0.1266 (2)	0.0125 (2)
H1	0.6681	0.8028	0.0384	0.019*
H2	0.6563	0.5305	0.0787	0.019*
H3	0.7272	0.6536	0.1591	0.019*
C1	0.66257 (8)	0.5306 (3)	0.4320 (2)	0.0125 (3)
C2	0.65058 (9)	0.7531 (3)	0.2961 (2)	0.0114 (2)
H21	0.6808	0.8988	0.3574	0.014*
C3	0.57160 (9)	0.8414 (3)	0.2457 (3)	0.0123 (2)
H31	0.5667	0.9713	0.1451	0.015*
H32	0.5592	0.9238	0.3573	0.015*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Se1	0.00843 (6)	0.01357 (7)	0.01686 (7)	−0.00112 (6)	0.00199 (4)	0.00422 (7)
Cl1	0.01334 (15)	0.01271 (15)	0.01538 (15)	0.00216 (12)	0.00550 (12)	0.00565 (12)
O1	0.0175 (6)	0.0106 (5)	0.0193 (6)	0.0028 (4)	0.0037 (5)	0.0040 (4)
O2	0.0349 (7)	0.0201 (6)	0.0146 (5)	0.0104 (8)	0.0100 (5)	0.0067 (7)
N1	0.0104 (5)	0.0118 (5)	0.0159 (5)	0.0005 (4)	0.0045 (4)	0.0045 (5)
C1	0.0125 (5)	0.0113 (7)	0.0126 (6)	0.0005 (4)	0.0000 (4)	0.0034 (4)
C2	0.0114 (6)	0.0083 (5)	0.0138 (6)	0.0003 (5)	0.0011 (5)	0.0021 (5)
C3	0.0125 (6)	0.0096 (5)	0.0147 (6)	0.0022 (5)	0.0029 (5)	0.0019 (5)

Geometric parameters (Å, º) top

Se1—C3	1.9671 (18)	N1—H2	0.9100
Se1—Se1ⁱ	2.3213 (4)	N1—H3	0.9100
O1—C1	1.206 (2)	C1—C2	1.516 (2)
O2—C1	1.319 (2)	C2—C3	1.525 (2)
O2—H4	0.79 (6)	C2—H21	1.0000
N1—C2	1.490 (2)	C3—H31	0.9900
N1—H1	0.9100	C3—H32	0.9900

C3—Se1—Se1ⁱ	100.88 (5)	N1—C2—C3	112.00 (14)
C1—O2—H4	112 (4)	C1—C2—C3	113.22 (14)
C2—N1—H1	109.5	N1—C2—H21	107.9
C2—N1—H2	109.5	C1—C2—H21	107.9
H1—N1—H2	109.5	C3—C2—H21	107.9
C2—N1—H3	109.5	C2—C3—Se1	113.96 (12)
H1—N1—H3	109.5	C2—C3—H31	108.8
H2—N1—H3	109.5	Se1—C3—H31	108.8
O1—C1—O2	125.92 (18)	C2—C3—H32	108.8
O1—C1—C2	123.24 (17)	Se1—C3—H32	108.8
O2—C1—C2	110.84 (16)	H31—C3—H32	107.7
N1—C2—C1	107.72 (14)

O1—C1—C2—N1	9.3 (2)	N1—C2—C3—Se1	70.66 (16)
O2—C1—C2—N1	−170.64 (15)	C1—C2—C3—Se1	−51.39 (17)
O1—C1—C2—C3	133.72 (17)	C2—C3—Se1—Se1ⁱ	−88.72 (12)
O2—C1—C2—C3	−46.24 (19)	C3—Se1—Se1ⁱ—C3ⁱ	−83.05 (10)

Symmetry code: (i) −x+1, y, −z.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···Cl1ⁱⁱ	0.91	2.25	3.1425 (16)	167
N1—H2···Cl1ⁱⁱⁱ	0.91	2.40	3.2110 (18)	149
N1—H3···Cl1^iv	0.91	2.32	3.1794 (15)	157
O2—H4···Cl1	0.79 (6)	2.22 (6)	3.0080 (19)	172 (4)
C2—H21···O1^v	1.00	2.39	3.292 (2)	150
C2—H21···O1^iv	1.00	2.55	3.216 (2)	124

Symmetry codes: (ii) x, y+1, z−1; (iii) x, y, z−1; (iv) −x+3/2, y+1/2, −z+1; (v) x, y+1, z.

Geometric parameters (Å, °) of diselenide and disulfide bridges top

Compound	C—Se/C—S	Se—Se/S—S	C—C—Se/S	C—Se/S—Se/S	C—C—Se/S—Se/S	C—Se/S—Se/S—C
(I)	1.9671 (18)	2.3213 (4)	113.96 (12)	100.88 (5)	-88.72 (12)	-83.05 (10)
Average^a	1.967	2.310	114.17	101.29
(II)^b	1.817	2.040	114.48	103.40	-89.04	-81.04

Notes: (a) average of 16 –CH₂—Se—Se—CH₂– bridges in acyclic non-amino acid structures; (b) Leela & Ramamurthi (2007).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···Cl1ⁱ	0.91	2.25	3.1425 (16)	167.1
N1—H2···Cl1ⁱⁱ	0.91	2.40	3.2110 (18)	149.2
N1—H3···Cl1ⁱⁱⁱ	0.91	2.32	3.1794 (15)	156.8
O2—H4···Cl1	0.79 (6)	2.22 (6)	3.0080 (19)	172 (4)
C2—H21···O1^iv	1.00	2.39	3.292 (2)	150.2
C2—H21···O1ⁱⁱⁱ	1.00	2.55	3.216 (2)	123.7

Symmetry codes: (i) x, y+1, z−1; (ii) x, y, z−1; (iii) −x+3/2, y+1/2, −z+1; (iv) x, y+1, z.

Experimental details

Crystal data
Chemical formula	C₆H₁₄N₂O₄Se₂²⁺·2(Cl⁻)
M_r	407.02
Crystal system, space group	Monoclinic, C2
Temperature (K)	100
a, b, c (Å)	18.8045 (16), 5.2529 (4), 7.2719 (6)
β (°)	102.219 (1)
V (Å³)	702.03 (10)
Z	2
Radiation type	Mo Kα
µ (mm⁻¹)	5.65
Crystal size (mm)	0.85 × 0.08 × 0.07

Data collection
Diffractometer	Bruker D8 Advance single-crystal CCD diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2014)
T_min, T_max	0.514, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	11089, 4209, 4080
R_int	0.024
(sin θ/λ)_max (Å⁻¹)	0.908

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.023, 0.062, 1.10
No. of reflections	4209
No. of parameters	78
No. of restraints	3
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	4.55, −0.87
Absolute structure	Flack x determined using 1708 quotients (Parsons et al., 2013)
Absolute structure parameter	0.044 (4)

Computer programs: APEX2 (Bruker, 2014), SAINT (Bruker, 2014), SHELXT (Sheldrick, 2015a), SHELXL2014 (Sheldrick, 2015b), Mercury (Macrae et al., 2008).

References

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Volume 71| Part 6| June 2015| Pages 726-729

doi:10.1107/S205698901501021X

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