Crystallographic rationalization of the reactivity and spectroscopic properties of (2R)-S-(2,5-dihydroxyphenyl)cysteine

At 150 K, the title compound, C₉H₁₁NO₄S, crystallizes in the ortho­rhom­bic form as a zwitterion and has a low gauche conformation [χ = −46.23 (16)°] for an acyclic cysteine derivative. A difference in bond length is observed for the alkyl C—S bond [1.8299 (15) Å] and the aryl C—S bond [1.7760 (15) Å]. The –NH₃⁺ group is involved in four hydrogen bonds, two of which are inter­molecular and two intra­molecular. The compound forms an infinite three-dimensional network constructed from four inter­molecular hydrogen bonds. Characterization data (¹³C NMR, IR and optical rotation) are reported to supplement the incomplete data disclosed previously in the literature.

S1. Comment

Cysteinylhydroquinone, (I), is the adduct formed by nucleophilic attack of cysteine thiol on p-benzoquinone. It was investigated as a reducing agent (Hatanaka et al., 1972) and has been reported in the context of a study on inhibitors of betacyanin synthesis (Hayashi & Koshimizu, 1979). It has also been the subject of a mass spectrometry study (d'Ischia et al., 1996). It is a very useful biochemical tool: it has been used in the investigation of the metabolism of benzene, phenol and hydroquinone (Bratton et al., 1997; Lunte & Kissinger, 1983) and of the metabolism of acetaminophen/paracetamol (Pascoe et al., 1988; Axworthy et al., 1988). It has been shown to be cytotoxic to melanoma cells (Yamada et al., 1988) and it is believed that the mode of action is tyrosinase inhibition. Indeed, the tyrosinase inhibitory activity of (I) has been disclosed in a recent patent concerning the use of the compound as a skin-brightening agent (Wempe & Clauson, 2008). Applications of (I) in hair dyeing (Wenke & Prota, 1995) and permanent waving (Kubo & Schultz, 1994) have also been patented.

Concerning the synthesis of (I), an early reported procedure (Kuhn & Beinert, 1944) was subsequently reinvestigated (Crescenzi et al., 1988) and it was found that, under certain conditions, a mixture of 1,4-benzothiazine oligomers may be formed in preference to the desired product. We undertook synthesis of (I) by addition of p-benzoquinone in ethanol to L-cysteine in H₂O (Hayashi & Koshimizu, 1979). Spectroscopic characterization of (I) to date has been incomplete. Thus, we report here the high-field ¹H and ¹³C NMR spectra, IR spectrum and optical rotation of (I).

The asymmetric unit of (I) (Fig. 1) contains one molecule of L-cysteinylhydroquinone, which forms hydrogen bonds with every heteroatom except sulfur. The protonated amino group on its own forms two intramolecular hydrogen bonds, N—H1B···O2 and N—H1C···O3, and two intermolecular hydrogen bonds, N—H1A···O1ⁱⁱⁱ and N—H1B···O4^iv (symmetry codes as in Table 2). In combination with the two hydrogen-bond-forming OH groups of the hydroquinone ligand, this gives rise to an infinite three-dimensional network of N—H···O and O—H···O bonds. Details of these hydrogen bonds are given in Table 2 and the hydrogen-bond network is illustrated in Fig. 2. In the crystallographic literature only two other cysteine derivatives with a cyclic S-substituent have been reported, 5-S-cysteinyluracil monohydrate (Williams et al., 1977) and S-benzyl-L-cysteine (Troup et al., 2001). Bond lengths in these cysteinyl units are very similar to the corresponding bond lengths found in (I).

The structure we report here serves to explain the reported reactivity trends of (I) and aspects of the characterization data obtained for (I) by other techniques, as well as to suggest an application for (I) in peptide engineering. Firstly, phenolic alkylthioethers such as (I) have been reported (Costantini et al., 1994; d'Ischia et al., 1995) to show a marked proclivity for undergoing UV-induced desulfurization via regiospecific alkyl C—S bond cleavage and to fragment in a similar fashion under conditions of mass spectrometric analysis (d'Ischia et al., 1996). A rationale for the specific cleavage of the C3—S bond in this process may be derived from inspection of the crystal structure. Specifically, the observed C3—S bond length in (I) of 1.8302 (16) Å is appreciably longer than the C4—S bond length of 1.7765 (16) Å. Bond length and strength have previously been correlated for C—S bonds (Woodard et al., 1976; Polenov et al., 2006; Seidel et al., 2007). Thus, it may be deduced that the C3—S bond in (I) is the weaker of the two C—S bonds, so rationalizing the observation that the C3—S bond is particularly susceptible to regiospecific scission.

Secondly, discrepancies between the ¹³C NMR spectra for (I) and L-cysteine may be rationalized on the basis of the present crystal structure. The C3 methylene resonance is observed at δ = 34.5 p.p.m. for (I), but has been observed by us at δ = 25.5 p.p.m. for L-cysteine (both in DMSO-d₆). This significant downfield shift (Δδ = 9.0 p.p.m.) upon introduction of the S-substituent implies that the electron-rich aryl ring exerts a deshielding effect. We propose this to be a consequence of the arene ring current and of conjugation of the S atom with the arene π-system; electron donation into the ring lowers the electron density on the S atom, deshielding the nucleus of the adjacent atom C3. The difference in observed bond lengths for C4—S [1.7765 (16) Å] and C3—S [1.8302 (16) Å] provides compelling evidence for higher bond order for the C4—S bond and hence for the deshielding effect detailed above (the inequivalence of the C4—S and C3—S bond lengths will persist in solution).

Thirdly, as can be seen in Table 1, the cysteine N—C2—C3—S moiety adopts a very low gauche torsion angle of -46.23 (16)°, due to the intramolecular N—H1C···O3 hydrogen bond. No lower N—C—C—S torsion angle has thus far been reported for an acyclic cysteine derivative. In this conformation the small angle allows direct alignment of O3 along the N—H1C bond for optimal hydrogen bonding. A similarly low N—C—C—S torsion angle has previously only been described in four structures of cysteine derivatives: -46.5 (2) and -47.8 (3)° for cysteine mandelic acid diastereomers (Fujii et al., 2005), -48.2 (3)° for S-benzyl-L-cysteine (Troup et al., 2001), 48.8° for S-carboxymethyl-L-cysteine (Mighell et al., 1979) and 48.9 (3)° in L-cysteine L-tartrate monohydrate (Shan & Huang, 1999). It is noteworthy that the N—C2—C3—S torsion angle of -46.23 (16)° that we have observed is similar to the values of -45.6 (3) and 48.8 (3)° reported (Ranganathan et al., 2002) in a dimeric structure of a closely analogous L-cysteinylaryl derivative in which the N—C—C—S motif is explicitly constrained by incorporation into a seven-membered ring. This supports the conclusion that in the case of (I) it is the intramolecular hydrogen bond which is responsible for the anomalously low N—C—C—S torsion angle.

To conclude, it should be noted that this accurate structure constitutes an additional tool for rational peptide design. The OH functionality in the synthetic sulfur substituent of L-cysteinylhydroquinone results in a significant change in the N—C—C—S torsion angle in the amino acid moiety to -46.23 (16)°. Use of this synthetic amino acid with such an unusual torsion angle will allow the rational design of new peptides of defined secondary structure. Indeed, several oligopeptides containing L-cysteinylhydroquinone have been reported (Hansen et al., 2001; Ahlfors et al., 2003; Holmdahl et al., 2008), although none has been characterized by crystallography so far. We anticipate that the low torsion angle will orient the polarized aromatic ring such that it will exhibit π–π interactions with appropriate proximate aryl residues (Waters, 2004), which will confer defined secondary structure on the peptide.

S2. Experimental

Cysteinylhydroquinone was prepared as described previously (Hayashi & Koshimizu, 1979). To L-cysteine (6.00 g, 49.5 mmol, 1.34 equivalent) in H₂O (560 ml) at room temperature in air was added p-benzoquinone (4.00 g, 37.0 mmol, 1.00 equivalent) in ethanol (240 ml). The reaction mixture was stirred for 1 h, then the solvents were removed under reduced pressure. The crude product was dissolved in refluxing ethanol–H₂O (100 ml; Ratio?) and filtered whilst hot. The filtrate was allowed to cool to room temperature, then stored at 277 K for 3 d to give cysteinylhydroquinone, (I) (6.54 g, 77%), as a yellow crystalline solid of sufficient quality for X-ray analysis.

Spectroscopic analysis: [α]_D²⁵ +165° [c 1.0, 1M HCl_(aq)]; ¹H NMR (500 MHz, DMSO-d₆, 298 K, δ, p.p.m.): 8.50 (5H, br s, -OH and -NH), 6.84 (1H, d, J = 2.5 Hz, Ar-H), 6.71 (1H, d, J = 8.5 Hz, Ar-H), 6.54 (1H, dd, J = 8.5 and 2.5 Hz, Ar-H), 3.35 (1H, dd, J = 13.5 and 4.0 Hz, –S—CHH–), 3.31 (1H, dd, J = 8.5 and 4.5 Hz, –S—CH₂—CH–), 2.96 (1H, dd, J = 13.5 and 9.0 Hz, –S—CHH–); ¹³C NMR (75.4 MHz, DMSO-d₆, 298 K, δ, p.p.m.): 169.6, 150.5, 149.4, 120.1, 118.0, 116.4, 115.4, 53.4, 34.5; IR (film, ν_max, cm^-1): 3093, 3000, 2692, 2581, 1630, 1578, 1440, 1388, 1339, 1251, 1206, 1132, 1051, 939, 904, 855, 820, 779, 659.

S3. Refinement

All H atoms attached to N and O atoms were located in a difference Fourier map and refined freely. All other H atoms were placed in calculated positions and refined using a riding model, with C—H = 0.95–1.00 Å and U_iso(H) = 1.2U_eq(C). [Please check added text]

Computing details

Data collection: COLLECT (Nonius, 2000); cell refinement: HKL SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO–SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999) and DIAMOND (Brandenburg, 2005).

Figures

	Fig. 1. The molecular structure and atom-labelling scheme of (I). Displacement ellipsoids are depicted at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. The hydrogen-bond network (thin lines) of (I).

(2R)-S-(2,5-dihydroxyphenyl)cysteine

Crystal data

C9H11NO4S	F(000) = 480
Mr = 229.25	Dx = 1.549 Mg m^−3
Orthorhombic, P212121	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2ab	Cell parameters from 14443 reflections
a = 5.1661 (1) Å	θ = 2.9–30.0°
b = 10.3981 (2) Å	µ = 0.32 mm^−1
c = 18.2979 (3) Å	T = 150 K
V = 982.92 (3) Å^3	Block, pale yellow
Z = 4	0.40 × 0.38 × 0.38 mm

Data collection

Nonius Kappa CCD area-detector diffractometer	2875 independent reflections
Radiation source: fine-focus sealed tube	2507 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.062
300 1.8 degree images with \v scans	θmax = 30.0°, θmin = 3.9°
Absorption correction: multi-scan (SORTAV; Blessing, 1995)	h = −6→7
Tmin = 0.882, Tmax = 0.887	k = −14→14
22623 measured reflections	l = −25→25

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.034	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.077	w = 1/[σ^2(Fo^2) + (0.0411P)^2 + 0.1259P] where P = (Fo^2 + 2Fc^2)/3
S = 1.05	(Δ/σ)max = 0.001
2875 reflections	Δρmax = 0.22 e Å^−3
156 parameters	Δρmin = −0.28 e Å^−3
0 restraints	Absolute structure: Flack (1983), with 1188 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.06 (6)

Special details

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

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
S1	0.67164 (7)	0.64289 (4)	0.59127 (2)	0.02455 (10)
N	0.4824 (3)	0.69241 (15)	0.74707 (7)	0.0292 (3)
H1A	0.643 (6)	0.680 (2)	0.7605 (13)	0.056 (7)*
H1B	0.387 (6)	0.714 (3)	0.7904 (14)	0.067 (8)*
H1C	0.416 (5)	0.620 (3)	0.7223 (13)	0.057 (7)*
O1	0.2119 (2)	0.99595 (10)	0.69053 (6)	0.0290 (3)
O2	0.0738 (2)	0.83872 (12)	0.76495 (6)	0.0328 (3)
O3	0.2634 (2)	0.48601 (12)	0.66833 (6)	0.0290 (3)
H3	0.142 (6)	0.432 (3)	0.6909 (14)	0.075 (8)*
O4	0.2972 (3)	0.40051 (11)	0.36984 (6)	0.0303 (3)
H4	0.424 (4)	0.4330 (18)	0.3505 (10)	0.027 (5)*
C1	0.2295 (3)	0.88817 (14)	0.72070 (8)	0.0227 (3)
C2	0.4670 (3)	0.80796 (14)	0.69902 (8)	0.0224 (3)
H2	0.6268	0.8610	0.7056	0.027*
C3	0.4437 (3)	0.76864 (14)	0.61872 (8)	0.0227 (3)
H3A	0.4725	0.8454	0.5878	0.027*
H3B	0.2652	0.7379	0.6096	0.027*
C4	0.4517 (3)	0.53109 (14)	0.55217 (8)	0.0208 (3)
C5	0.2673 (3)	0.46593 (13)	0.59391 (8)	0.0221 (3)
C6	0.0907 (3)	0.38461 (14)	0.55989 (8)	0.0247 (3)
H6	−0.0380	0.3424	0.5882	0.030*
C7	0.1010 (3)	0.36459 (15)	0.48490 (8)	0.0254 (3)
H7	−0.0212	0.3095	0.4620	0.030*
C8	0.2899 (3)	0.42511 (14)	0.44346 (8)	0.0229 (3)
C9	0.4618 (3)	0.50919 (14)	0.47650 (8)	0.0224 (3)
H9	0.5877	0.5524	0.4477	0.027*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
S1	0.02095 (16)	0.02901 (18)	0.02369 (17)	−0.00207 (15)	0.00260 (14)	−0.00624 (15)
N1	0.0369 (8)	0.0313 (7)	0.0194 (6)	0.0126 (6)	0.0002 (6)	0.0019 (5)
O1	0.0372 (6)	0.0219 (5)	0.0278 (5)	0.0015 (5)	−0.0066 (5)	−0.0009 (4)
O2	0.0349 (6)	0.0347 (6)	0.0288 (6)	0.0118 (5)	0.0110 (5)	0.0035 (5)
O3	0.0325 (6)	0.0363 (6)	0.0183 (5)	−0.0076 (5)	0.0047 (5)	−0.0001 (4)
O4	0.0359 (7)	0.0364 (6)	0.0184 (5)	−0.0070 (5)	0.0023 (5)	−0.0048 (4)
C1	0.0268 (8)	0.0244 (7)	0.0169 (6)	0.0006 (6)	−0.0039 (5)	−0.0053 (5)
C2	0.0250 (7)	0.0239 (7)	0.0184 (6)	−0.0004 (6)	−0.0008 (6)	−0.0006 (5)
C3	0.0254 (8)	0.0227 (7)	0.0199 (6)	0.0006 (6)	0.0014 (6)	−0.0022 (5)
C4	0.0199 (7)	0.0217 (6)	0.0208 (7)	0.0004 (6)	0.0015 (5)	−0.0020 (5)
C5	0.0250 (7)	0.0226 (6)	0.0187 (6)	0.0030 (5)	0.0033 (6)	0.0003 (5)
C6	0.0264 (7)	0.0224 (7)	0.0254 (7)	−0.0021 (6)	0.0052 (6)	0.0016 (6)
C7	0.0251 (7)	0.0226 (7)	0.0285 (8)	−0.0029 (6)	0.0005 (6)	−0.0030 (6)
C8	0.0273 (8)	0.0228 (6)	0.0186 (7)	0.0027 (6)	−0.0003 (6)	−0.0014 (5)
C9	0.0241 (7)	0.0228 (7)	0.0203 (7)	−0.0001 (6)	0.0032 (5)	−0.0001 (5)

Geometric parameters (Å, º)

S1—C4	1.7760 (15)	C2—C3	1.530 (2)
S1—C3	1.8299 (15)	C2—H2	1.0000
N1—C2	1.491 (2)	C3—H3A	0.9900
N1—H1A	0.88 (3)	C3—H3B	0.9900
N1—H1B	0.96 (3)	C4—C5	1.397 (2)
N1—H1C	0.95 (3)	C4—C9	1.404 (2)
O1—C1	1.2527 (19)	C5—C6	1.391 (2)
O2—C1	1.2517 (19)	C6—C7	1.389 (2)
O3—C5	1.3779 (18)	C6—H6	0.9500
O3—H3	0.94 (3)	C7—C8	1.387 (2)
O4—C8	1.3718 (16)	C7—H7	0.9500
O4—H4	0.82 (2)	C8—C9	1.385 (2)
C1—C2	1.536 (2)	C9—H9	0.9500
C4—S1—C3	99.59 (7)	C2—C3—H3B	108.8
C2—N1—H1A	109.6 (17)	S1—C3—H3B	108.8
C2—N1—H1B	105.6 (16)	H3A—C3—H3B	107.7
H1A—N1—H1B	107 (2)	C5—C4—C9	119.08 (14)
C2—N1—H1C	110.0 (15)	C5—C4—S1	122.25 (11)
H1A—N1—H1C	111 (2)	C9—C4—S1	118.67 (11)
H1B—N1—H1C	113 (2)	O3—C5—C6	121.67 (13)
C5—O3—H3	110.7 (17)	O3—C5—C4	118.48 (13)
C8—O4—H4	111.7 (13)	C6—C5—C4	119.84 (14)
O2—C1—O1	127.29 (14)	C7—C6—C5	120.55 (14)
O2—C1—C2	117.22 (13)	C7—C6—H6	119.7
O1—C1—C2	115.47 (14)	C5—C6—H6	119.7
N1—C2—C3	110.81 (12)	C8—C7—C6	119.94 (14)
N1—C2—C1	109.13 (12)	C8—C7—H7	120.0
C3—C2—C1	109.28 (12)	C6—C7—H7	120.0
N1—C2—H2	109.2	O4—C8—C9	121.93 (14)
C3—C2—H2	109.2	O4—C8—C7	118.14 (14)
C1—C2—H2	109.2	C9—C8—C7	119.93 (13)
C2—C3—S1	113.81 (10)	C8—C9—C4	120.59 (14)
C2—C3—H3A	108.8	C8—C9—H9	119.7
S1—C3—H3A	108.8	C4—C9—H9	119.7
O2—C1—C2—N1	−9.67 (18)	C9—C4—C5—C6	−2.5 (2)
O1—C1—C2—N1	171.61 (13)	S1—C4—C5—C6	176.55 (11)
O2—C1—C2—C3	111.65 (14)	O3—C5—C6—C7	−179.09 (14)
O1—C1—C2—C3	−67.07 (17)	C4—C5—C6—C7	1.9 (2)
N1—C2—C3—S1	−46.23 (16)	C5—C6—C7—C8	0.6 (2)
C1—C2—C3—S1	−166.52 (10)	C6—C7—C8—O4	178.53 (14)
C4—S1—C3—C2	127.17 (11)	C6—C7—C8—C9	−2.4 (2)
C3—S1—C4—C5	−66.58 (13)	O4—C8—C9—C4	−179.15 (14)
C3—S1—C4—C9	112.45 (13)	C7—C8—C9—C4	1.9 (2)
C9—C4—C5—O3	178.50 (13)	C5—C4—C9—C8	0.6 (2)
S1—C4—C5—O3	−2.47 (19)	S1—C4—C9—C8	−178.46 (11)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3···O2^i	0.94 (3)	1.68 (3)	2.6214 (16)	177 (3)
O4—H4···O1^ii	0.82 (2)	1.82 (2)	2.6402 (18)	179 (2)
N1—H1A···O1^iii	0.88 (3)	2.24 (3)	2.8231 (17)	124 (2)
N1—H1B···O4^iv	0.96 (3)	2.11 (3)	2.8399 (18)	132 (2)
N1—H1B···O2	0.96 (3)	2.12 (3)	2.6223 (19)	111 (2)
N1—H1C···O3	0.95 (3)	1.88 (3)	2.822 (2)	175 (2)

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