N-Butanoyl-N-(3-chloro-1,4-dioxonaph­thalen-2-yl)butanamide

Butcher, R.J.; Berhe, S.; Anderson, A.J.; Bakare, O.

doi:10.1107/S1600536813016401

organic compounds

CRYSTALLOGRAPHIC
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ISSN: 2056-9890

Volume 69| Part 8| August 2013| Page o1230

doi:10.1107/S1600536813016401

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N-Butanoyl-N-(3-chloro-1,4-dioxonaphthalen-2-yl)butanamide

Ray J. Butcher,^a ^* Solomon Berhe,^a Alan J. Anderson ^b and Oladapo Bakare ^a

^aDepartment of Chemistry, Howard University, 525 College Street NW, Washington, DC 20059, USA, and ^bDepartment of Natural Sciences, Bowie State University, Bowie, MD 20715, USA
^*Correspondence e-mail: rbutcher99@yahoo.com

(Received 6 June 2013; accepted 12 June 2013; online 10 July 2013)

In the title compound, C₁₈H₁₈ClNO₄, the imide group with its two alkyl substituents is approximately perpendicular to the plane of the naphthoquinone ring system [dihedral angle = 78.5 (1)°]. Further, the imide carbonyl groups are oriented in an anti sense. In the crystal, the substituted naphthoquinone rings form π–π stacks in the a-axis direction [perpendicular centroid–centroid distance = 3.209 (2) Å and slippage = 4.401 Å].

Related literature

For the synthesis and biological evaluation of some imido-substituted 1,4-naphthoquinone derivatives, see; Bakare et al. (2003 ); Berhe et al. (2008 ); Brandy et al. (2013 ). For the anti-cancer and anti-trypanosomal activity of the title compound, see; Bakare et al. (2003); Berhe et al. (2008); Khraiwesh et al. (2012 ).

Experimental

Crystal data

C₁₈H₁₈ClNO₄
M_r = 347.78
Triclinic, $[P \overline 1]$
a = 8.1717 (10) Å
b = 8.3117 (10) Å
c = 14.6841 (15) Å
α = 93.119 (9)°
β = 98.369 (10)°
γ = 118.043 (12)°
V = 862.23 (17) Å³
Z = 2
Cu Kα radiation
μ = 2.15 mm⁻¹
T = 295 K
0.36 × 0.28 × 0.08 mm

Data collection

Agilent Xcalibur (Ruby, Gemini) diffractometer
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2012 ) T_min = 0.530, T_max = 1.000
5454 measured reflections
3398 independent reflections
2122 reflections with I > 2σ(I)
R_int = 0.043

Refinement

R[F² > 2σ(F²)] = 0.077
wR(F²) = 0.227
S = 1.12
3398 reflections
219 parameters
H-atom parameters constrained
Δρ_max = 0.39 e Å⁻³
Δρ_min = −0.24 e Å⁻³

Data collection: CrysAlis PRO (Agilent, 2012); cell refinement: CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL.

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536813016401/hg5322sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536813016401/hg5322Isup2.hkl

Supporting information file. DOI: 10.1107/S1600536813016401/hg5322Isup3.cml

checkCIF report

Comment top

We have been involved in the synthesis and biological evaluation of some imido-substituted 1,4-naphthoquinone derivatives [Bakare et al. (2003); Berhe et al. (2008); Brandy et al. (2013)]; and previously reported 2-chloro-3-dibutyrylamino-1,4-naphthoquinone (1) to possess inhibitory activities against certain protein kinases (Bakare et al. 2003). Compound 1 has subsequently been shown to possess a some desirable biological activities including anti-cancer [Bakare et al. (2003; Berhe et al. (2008)] and anti-trypanosomal activities [(Khraiwesh, et al., (2012)]. We present here the crystal structure of this anticancer and antiparasitic agent.

The title compound, C₁₈H₁₈ClNO₄, was synthesized as previously reported (Bakare et al. (2003)). The crystal structure of the title compound 1 shows that the imide group with its two alkyl substituents is almost perpendicular to the plane of the naphthoquinone ring (dihedral angle between planes of 78.5 (1)°. Further the two imide carbonyls are oriented anti to each other. The naphthoquinone rings form π–π stacks in the a direction (perpendicular Cg···Cg distance of 3.209 Å with slippage of 4.401 Å).

Related literature top

For the synthesis and biological evaluation of some imido-substituted 1,4-naphthoquinone derivatives, see; Bakare et al. (2003); Berhe et al. (2008); Brandy et al. (2013). For the anti-cancer and anti-trypanosomal activity of the title compound, see; Bakare et al. (2003); Berhe et al. (2008); Khraiwesh et al. (2012).

Experimental top

The title compound 1 was synthesized by refluxing 2-amino-3-chloro-1,4-naphthoquinone in butyryl chloride as previously reported (Bakare et al. (2003)). The compound was crystallized from the crude below 0°C with diethyl ether to obtain yellow crystals.

Computing details top

Data collection: CrysAlis PRO (Agilent, 2012); cell refinement: CrysAlis PRO (Agilent, 2012); data reduction: CrysAlis PRO (Agilent, 2012); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top

	Fig. 1. Diagram of C₁₈H₁₈ClNO₄ showing atom labeling.
	Fig. 2. The molecular packing for C₁₈H₁₈ClNO₄ viewed along the b axis and showing the π–π stacking in the a direction.

N-Butanoyl-N-(3-chloro-1,4-dioxonaphthalen-2-yl)butanamide top

Crystal data top

C₁₈H₁₈ClNO₄	Z = 2
M_r = 347.78	F(000) = 364
Triclinic, P1	D_x = 1.340 Mg m⁻³
Hall symbol: -P 1	Cu Kα radiation, λ = 1.54178 Å
a = 8.1717 (10) Å	Cell parameters from 1350 reflections
b = 8.3117 (10) Å	θ = 3.1–75.5°
c = 14.6841 (15) Å	µ = 2.15 mm⁻¹
α = 93.119 (9)°	T = 295 K
β = 98.369 (10)°	Plate, pale yellow
γ = 118.043 (12)°	0.36 × 0.28 × 0.08 mm
V = 862.23 (17) Å³

Data collection top

Agilent Xcalibur (Ruby, Gemini) diffractometer	3398 independent reflections
Radiation source: Enhance (Cu) X-ray Source	2122 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.043
Detector resolution: 10.5081 pixels mm^-1	θ_max = 75.7°, θ_min = 3.1°
ω scans	h = −8→10
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2012)	k = −10→8
T_min = 0.530, T_max = 1.000	l = −18→18
5454 measured reflections

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.077	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.227	H-atom parameters constrained
S = 1.12	w = 1/[σ²(F_o²) + (0.0739P)² + 0.4803P] where P = (F_o² + 2F_c²)/3
3398 reflections	(Δ/σ)_max < 0.001
219 parameters	Δρ_max = 0.39 e Å⁻³
0 restraints	Δρ_min = −0.24 e Å⁻³

Crystal data top

C₁₈H₁₈ClNO₄	γ = 118.043 (12)°
M_r = 347.78	V = 862.23 (17) Å³
Triclinic, P1	Z = 2
a = 8.1717 (10) Å	Cu Kα radiation
b = 8.3117 (10) Å	µ = 2.15 mm⁻¹
c = 14.6841 (15) Å	T = 295 K
α = 93.119 (9)°	0.36 × 0.28 × 0.08 mm
β = 98.369 (10)°

Data collection top

Agilent Xcalibur (Ruby, Gemini) diffractometer	3398 independent reflections
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2012)	2122 reflections with I > 2σ(I)
T_min = 0.530, T_max = 1.000	R_int = 0.043
5454 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.077	0 restraints
wR(F²) = 0.227	H-atom parameters constrained
S = 1.12	Δρ_max = 0.39 e Å⁻³
3398 reflections	Δρ_min = −0.24 e Å⁻³
219 parameters

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.

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 (Å²) top

	x	y	z	U_iso*/U_eq
Cl1	0.4121 (2)	0.34538 (16)	0.17915 (8)	0.0891 (4)
O1	0.2762 (5)	0.3535 (4)	−0.0134 (2)	0.0844 (9)
O2	0.4177 (5)	0.9507 (4)	0.2097 (2)	0.0938 (11)
O3	0.7195 (7)	0.7372 (8)	0.4060 (3)	0.1463 (19)
O4	0.1712 (5)	0.5863 (5)	0.3023 (2)	0.0968 (11)
N1	0.4736 (5)	0.6864 (5)	0.2928 (2)	0.0706 (9)
C1	0.3740 (6)	0.5151 (5)	0.1381 (3)	0.0639 (10)
C2	0.2990 (6)	0.4877 (5)	0.0361 (3)	0.0662 (10)
C3	0.2519 (6)	0.6265 (5)	0.0005 (2)	0.0607 (9)
C4	0.1688 (6)	0.6021 (6)	−0.0927 (3)	0.0722 (11)
H4A	0.1453	0.4994	−0.1325	0.087*
C5	0.1217 (7)	0.7286 (7)	−0.1261 (3)	0.0855 (13)
H5A	0.0661	0.7112	−0.1883	0.103*
C6	0.1562 (8)	0.8814 (7)	−0.0680 (3)	0.0886 (14)
H6A	0.1230	0.9664	−0.0909	0.106*
C7	0.2399 (7)	0.9083 (6)	0.0240 (3)	0.0785 (12)
H7A	0.2649	1.0126	0.0628	0.094*
C8	0.2869 (6)	0.7820 (5)	0.0590 (2)	0.0622 (9)
C9	0.3728 (6)	0.8109 (5)	0.1584 (3)	0.0680 (10)
C10	0.4067 (6)	0.6633 (5)	0.1942 (2)	0.0622 (10)
C11	0.6690 (8)	0.7503 (8)	0.3266 (3)	0.0961 (16)
C12	0.7987 (8)	0.8295 (9)	0.2605 (4)	0.1059 (18)
H12A	0.7738	0.9213	0.2331	0.127*
H12B	0.7675	0.7323	0.2108	0.127*
C13	1.0076 (11)	0.9175 (12)	0.2999 (6)	0.142 (3)
H13A	1.0310	0.8316	0.3343	0.171*
H13B	1.0747	0.9385	0.2487	0.171*
C14	1.0836 (12)	1.0872 (13)	0.3596 (6)	0.174 (4)
H14A	1.2123	1.1256	0.3876	0.261*
H14B	1.0106	1.0712	0.4073	0.261*
H14C	1.0791	1.1792	0.3241	0.261*
C15	0.3333 (8)	0.6480 (7)	0.3445 (3)	0.0796 (12)
C16	0.3897 (9)	0.6902 (10)	0.4493 (3)	0.1102 (19)
H16A	0.4304	0.6045	0.4716	0.132*
H16B	0.4976	0.8129	0.4656	0.132*
C17	0.2496 (10)	0.6815 (14)	0.4964 (4)	0.149 (3)
H17A	0.1405	0.5600	0.4784	0.179*
H17B	0.2116	0.7696	0.4751	0.179*
C18	0.2999 (10)	0.7179 (11)	0.6009 (4)	0.131 (2)
H18A	0.1924	0.7052	0.6254	0.197*
H18B	0.4029	0.8406	0.6204	0.197*
H18C	0.3366	0.6311	0.6236	0.197*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cl1	0.1260 (11)	0.0735 (7)	0.0734 (7)	0.0587 (7)	0.0014 (6)	0.0018 (5)
O1	0.118 (3)	0.0700 (17)	0.0620 (16)	0.0477 (18)	0.0066 (16)	−0.0132 (13)
O2	0.142 (3)	0.0725 (18)	0.0629 (17)	0.061 (2)	−0.0114 (18)	−0.0161 (14)
O3	0.135 (4)	0.212 (5)	0.079 (3)	0.083 (4)	−0.015 (2)	0.023 (3)
O4	0.100 (3)	0.121 (3)	0.0619 (18)	0.048 (2)	0.0175 (18)	−0.0028 (18)
N1	0.087 (2)	0.074 (2)	0.0479 (16)	0.0428 (19)	−0.0014 (16)	−0.0036 (14)
C1	0.075 (3)	0.058 (2)	0.057 (2)	0.0337 (19)	0.0084 (18)	0.0011 (16)
C2	0.076 (3)	0.062 (2)	0.052 (2)	0.028 (2)	0.0118 (18)	−0.0076 (16)
C3	0.065 (2)	0.062 (2)	0.0489 (18)	0.0283 (18)	0.0065 (16)	−0.0022 (15)
C4	0.083 (3)	0.077 (3)	0.048 (2)	0.035 (2)	0.0066 (19)	−0.0033 (18)
C5	0.098 (3)	0.096 (3)	0.055 (2)	0.046 (3)	0.000 (2)	0.008 (2)
C6	0.111 (4)	0.087 (3)	0.070 (3)	0.053 (3)	0.003 (3)	0.015 (2)
C7	0.101 (3)	0.069 (2)	0.063 (2)	0.045 (2)	0.001 (2)	−0.0012 (19)
C8	0.071 (2)	0.061 (2)	0.0501 (19)	0.0311 (19)	0.0041 (17)	0.0001 (15)
C9	0.084 (3)	0.061 (2)	0.052 (2)	0.034 (2)	0.0034 (18)	−0.0068 (16)
C10	0.073 (3)	0.062 (2)	0.0454 (18)	0.0316 (19)	0.0012 (16)	−0.0035 (15)
C11	0.109 (4)	0.107 (4)	0.064 (3)	0.057 (3)	−0.016 (3)	−0.006 (3)
C12	0.087 (4)	0.125 (5)	0.084 (3)	0.039 (3)	0.005 (3)	−0.009 (3)
C13	0.128 (6)	0.169 (7)	0.131 (6)	0.084 (6)	−0.009 (5)	0.007 (5)
C14	0.145 (7)	0.175 (8)	0.126 (6)	0.035 (6)	−0.032 (5)	0.002 (6)
C15	0.101 (4)	0.087 (3)	0.054 (2)	0.052 (3)	0.003 (2)	−0.003 (2)
C16	0.131 (5)	0.149 (5)	0.056 (3)	0.075 (4)	0.011 (3)	0.001 (3)
C17	0.131 (5)	0.253 (9)	0.055 (3)	0.090 (6)	0.014 (3)	−0.003 (4)
C18	0.140 (6)	0.185 (7)	0.057 (3)	0.072 (5)	0.015 (3)	−0.002 (4)

Geometric parameters (Å, º) top

Cl1—C1	1.703 (4)	C9—C10	1.484 (5)
O1—C2	1.218 (4)	C11—C12	1.487 (8)
O2—C9	1.215 (4)	C12—C13	1.508 (8)
O3—C11	1.207 (6)	C12—H12A	0.9700
O4—C15	1.221 (6)	C12—H12B	0.9700
N1—C15	1.389 (6)	C13—C14	1.424 (10)
N1—C11	1.422 (6)	C13—H13A	0.9700
N1—C10	1.440 (4)	C13—H13B	0.9700
C1—C10	1.335 (5)	C14—H14A	0.9600
C1—C2	1.496 (5)	C14—H14B	0.9600
C2—C3	1.475 (6)	C14—H14C	0.9600
C3—C4	1.395 (5)	C15—C16	1.512 (6)
C3—C8	1.397 (5)	C16—C17	1.397 (8)
C4—C5	1.369 (6)	C16—H16A	0.9700
C4—H4A	0.9300	C16—H16B	0.9700
C5—C6	1.377 (7)	C17—C18	1.505 (7)
C5—H5A	0.9300	C17—H17A	0.9700
C6—C7	1.377 (6)	C17—H17B	0.9700
C6—H6A	0.9300	C18—H18A	0.9600
C7—C8	1.376 (6)	C18—H18B	0.9600
C7—H7A	0.9300	C18—H18C	0.9600
C8—C9	1.479 (5)

C15—N1—C11	127.5 (4)	C13—C12—H12A	108.1
C15—N1—C10	113.5 (4)	C11—C12—H12B	108.1
C11—N1—C10	119.0 (4)	C13—C12—H12B	108.1
C10—C1—C2	121.8 (4)	H12A—C12—H12B	107.3
C10—C1—Cl1	121.9 (3)	C14—C13—C12	114.9 (7)
C2—C1—Cl1	116.3 (3)	C14—C13—H13A	108.5
O1—C2—C3	122.9 (4)	C12—C13—H13A	108.5
O1—C2—C1	120.0 (4)	C14—C13—H13B	108.5
C3—C2—C1	117.0 (3)	C12—C13—H13B	108.5
C4—C3—C8	119.0 (4)	H13A—C13—H13B	107.5
C4—C3—C2	119.9 (3)	C13—C14—H14A	109.5
C8—C3—C2	121.1 (3)	C13—C14—H14B	109.5
C5—C4—C3	120.4 (4)	H14A—C14—H14B	109.5
C5—C4—H4A	119.8	C13—C14—H14C	109.5
C3—C4—H4A	119.8	H14A—C14—H14C	109.5
C4—C5—C6	120.4 (4)	H14B—C14—H14C	109.5
C4—C5—H5A	119.8	O4—C15—N1	117.7 (4)
C6—C5—H5A	119.8	O4—C15—C16	123.7 (5)
C5—C6—C7	119.9 (4)	N1—C15—C16	118.6 (5)
C5—C6—H6A	120.0	C17—C16—C15	115.8 (5)
C7—C6—H6A	120.0	C17—C16—H16A	108.3
C8—C7—C6	120.6 (4)	C15—C16—H16A	108.3
C8—C7—H7A	119.7	C17—C16—H16B	108.3
C6—C7—H7A	119.7	C15—C16—H16B	108.3
C7—C8—C3	119.8 (4)	H16A—C16—H16B	107.4
C7—C8—C9	119.9 (3)	C16—C17—C18	117.0 (6)
C3—C8—C9	120.3 (4)	C16—C17—H17A	108.1
O2—C9—C8	122.0 (4)	C18—C17—H17A	108.1
O2—C9—C10	120.3 (4)	C16—C17—H17B	108.1
C8—C9—C10	117.6 (3)	C18—C17—H17B	108.1
C1—C10—N1	121.5 (4)	H17A—C17—H17B	107.3
C1—C10—C9	121.9 (3)	C17—C18—H18A	109.5
N1—C10—C9	116.6 (3)	C17—C18—H18B	109.5
O3—C11—N1	118.8 (6)	H18A—C18—H18B	109.5
O3—C11—C12	124.2 (6)	C17—C18—H18C	109.5
N1—C11—C12	117.0 (4)	H18A—C18—H18C	109.5
C11—C12—C13	116.7 (5)	H18B—C18—H18C	109.5
C11—C12—H12A	108.1

C10—C1—C2—O1	177.8 (4)	C2—C1—C10—C9	−0.7 (6)
Cl1—C1—C2—O1	−3.1 (6)	Cl1—C1—C10—C9	−179.8 (3)
C10—C1—C2—C3	−3.5 (6)	C15—N1—C10—C1	−102.5 (5)
Cl1—C1—C2—C3	175.6 (3)	C11—N1—C10—C1	79.8 (5)
O1—C2—C3—C4	3.2 (6)	C15—N1—C10—C9	76.4 (5)
C1—C2—C3—C4	−175.4 (4)	C11—N1—C10—C9	−101.2 (5)
O1—C2—C3—C8	−177.6 (4)	O2—C9—C10—C1	−174.2 (4)
C1—C2—C3—C8	3.7 (6)	C8—C9—C10—C1	4.7 (6)
C8—C3—C4—C5	−0.3 (7)	O2—C9—C10—N1	6.8 (6)
C2—C3—C4—C5	178.9 (4)	C8—C9—C10—N1	−174.3 (4)
C3—C4—C5—C6	0.1 (8)	C15—N1—C11—O3	16.4 (8)
C4—C5—C6—C7	0.5 (8)	C10—N1—C11—O3	−166.3 (5)
C5—C6—C7—C8	−1.1 (8)	C15—N1—C11—C12	−163.7 (5)
C6—C7—C8—C3	0.9 (7)	C10—N1—C11—C12	13.6 (7)
C6—C7—C8—C9	−178.6 (5)	O3—C11—C12—C13	−7.2 (10)
C4—C3—C8—C7	−0.3 (6)	N1—C11—C12—C13	172.9 (5)
C2—C3—C8—C7	−179.4 (4)	C11—C12—C13—C14	−70.9 (9)
C4—C3—C8—C9	179.3 (4)	C11—N1—C15—O4	−176.9 (4)
C2—C3—C8—C9	0.1 (6)	C10—N1—C15—O4	5.7 (6)
C7—C8—C9—O2	−5.9 (7)	C11—N1—C15—C16	5.1 (7)
C3—C8—C9—O2	174.5 (4)	C10—N1—C15—C16	−172.3 (4)
C7—C8—C9—C10	175.3 (4)	O4—C15—C16—C17	−9.5 (10)
C3—C8—C9—C10	−4.3 (6)	N1—C15—C16—C17	168.4 (6)
C2—C1—C10—N1	178.2 (4)	C15—C16—C17—C18	178.4 (7)
Cl1—C1—C10—N1	−0.9 (6)

Experimental details

Crystal data
Chemical formula	C₁₈H₁₈ClNO₄
M_r	347.78
Crystal system, space group	Triclinic, P1
Temperature (K)	295
a, b, c (Å)	8.1717 (10), 8.3117 (10), 14.6841 (15)
α, β, γ (°)	93.119 (9), 98.369 (10), 118.043 (12)
V (Å³)	862.23 (17)
Z	2
Radiation type	Cu Kα
µ (mm⁻¹)	2.15
Crystal size (mm)	0.36 × 0.28 × 0.08

Data collection
Diffractometer	Agilent Xcalibur (Ruby, Gemini) diffractometer
Absorption correction	Multi-scan (CrysAlis PRO; Agilent, 2012)
T_min, T_max	0.530, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	5454, 3398, 2122
R_int	0.043
(sin θ/λ)_max (Å⁻¹)	0.628

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.077, 0.227, 1.12
No. of reflections	3398
No. of parameters	219
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.39, −0.24

Computer programs: CrysAlis PRO (Agilent, 2012), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Acknowledgements

This work was supported in part by grant No. 5-U54—CA914–31 (Howard University/Johns Hopkins Cancer Center Partnership); in part by grant G12MD007597 from the National Institute On Minority Health and Health Disparities of the National Institutes of Health; and in part by MRI grant No. CHE-1126533 from the NSF. RJB wishes to acknowledge the NSF–MRI program (grant CHE-0619278) for funds to purchase the diffractometer.

References

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