Charge Density and Electrostatic Potential Study of 16Î±,17Î²-Estriol

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Charge Density and Electrostatic Potential Study of 16#,17#-Estriol and the Binding of Estrogen Molecules to the Estrogen Receptors ER and ER #

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Elizabeth A Zhurova, Vladimir V Zhurov, Poomani Kumaradhas, Simone Cenedese, and A. Alan Pinkerton J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b05961 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 10, 2016

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The Journal of Physical Chemistry

Charge Density and Electrostatic Potential Study of 16α,17βEstriol and the Binding of Estrogen Molecules to the Estrogen Receptors ERα and ERβ Elizabeth A. Zhurova1, Vladimir V. Zhurov1, Poomani Kumaradhas1,2, Simone Cenedese1and A. Alan Pinkerton1*

1

2

Department of Chemistry, the University of Toledo, Toledo, OH 43606, USA; Department of Physics, Laboratory of Biocrystallography and Computational Molecular Biology, Periyar University, Salem-636 011, India E-mail: [email protected] Tel: (419) 530-4580

Abstract An accurate X-ray diffraction study at 20K combined with DFT theoretical calculations has been performed for the estriol crystal with two conformationally different molecules in the asymmetric unit. The electron density has been modeled via a multipole expansion, using both experimental and theoretical structure factors, and a topological analysis has been performed. The experimental molecular geometry, hydrogen bonding, atomic charges, dipole moments and other topological characteristics are compared with those calculated theoretically. In particular, the molecular electrostatic potential has been extracted and compared with those reported for other estrogen molecules exhibiting different binding affinities to the estrogen receptors (ERα and ERβ).

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Introduction Although estrogens are commonly recognized as female sex hormones influencing growth, development and behavior, they also affect many other parts of the human body. Indeed, there has been a vast amount of research over the past quarter century to elucidate their role in initializing the formation of cancer of the breast. Despite significant achievements in the treatment of hormone dependent breast cancer, there remain many unanswered questions and intense research activity continues to this day.1,2,3 Subtle changes in the structure of estrogen molecules affect their chemical/biological behavior and can even result in the development of growth inhibitors for tumors; for instance, moving the hydroxyl group from C(3) (estradiol)4 to one of the adjacent Catoms can change the carcinostatic potentials from agonistic (estradiol) to inhibitory (2- and 4-hydroxy estratrien-17-ol).5,6 More than 60% of all breast cancers are known to be hormone dependent, i.e. initiation and progress can be influenced by estrogens and related compounds. These molecules bind as ligands to the estrogen receptor and hereby initiate a series of events resulting in the activation or repression of selective genes. A comparative QSAR analysis of estrogen receptor ligands generalizes the effect of different substituents in estrogen molecules.7 The substituents which increase the electron density in the A-ring (Fig. 1) appear to increase the binding affinity, whereas the steric character of the substituent and its polarity may reduce the ligand receptor binding affinity.7 An extensive crystallographic structural study reports the conformational features of these molecules and proposes mechanisms of ligand binding.8,9 A knowledge of the receptor or receptor complex structural model is essential to understand the binding mechanism. In this context, the structural analysis of the ligand binding domain of two estrogen receptors ( and ) complexed with various ligands has been reported.10,11,12 Since the first structures reported in the late 1990’s, there has been intense activity with over 100 entries currently in the PDB.13 These studies demonstrate the agonist/antagonist effects of certain hormones and drugs on the receptor. Furthermore, molecular modeling of a series of estrogen molecules has been conducted suggesting that conformational flexibility is one of the important properties of ligands binding to the estrogen receptor.14 Although the mechanism by which the ligands regulate the gene expression is currently unknown, it has been suggested15 that the differences in the electrostatic potential (ESP) are


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responsible for the variation in the regulation of hormone dependent genes. This information can be obtained from the studies of the electronic properties of estrogen derivatives. We have initiated a systematic study of estrogen compounds by investigating crystals of the principal estrogens (α- and β-estradiol, estrone), the phytoestrogen genistein, as well as the synthetic estrogens diethylstilbestrol (DES) and dienestrol (DNS).16,17,18,19,20 The next study in this series is on estra-1,3,5(10)-triene-3,16,17-triol, (Estriol, Fig. 1) and is reported here. Estriol is one of the active estrogens found in the body, however, there is controversy about its role in breast cancer and other tissues.21 It has been suggested that estriol is not associated with initiating cancer activity in the female body, but that it protects against this disease.22,23,24 Estriol has a much less stimulating effect on the breast and uterine lining than estradiol and estrone.25 Other studies suggest that estriol has a significant carcinogenic potential26 and is capable of stimulating macromolecular synthesis in breast cancer.21 Receptor binding studies have indicated that estriol has low binding affinity to estrogen receptors27,28,29 and that the receptor/ligand complex has a short nuclear retention time.30 It can thus be suggested that estrogen molecules must differ significantly in their electronic properties. To obtain a better understanding of the binding mechanism of these molecules, the structures need to be studied at the electronic level. In the present study, the electronic properties of estriol have been determined from analysis of the experimental and theoretical charge density. A preliminary study in our group was performed with a Bruker SMART 2K CCD diffractometer at 100K. We have now repeated this analysis with more advanced equipment at a lower temperature (20 K) and using our in-house developed integration software to provide significant improvement of the X-ray intensity data. Theoretical calculations have been performed to compare with the experimental results.

Fig. 1. Representation of estriol molecule.



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Experimental Details Crystals of 16α,17β-estriol were grown by slow evaporation from 1:1 mixture of ethanol and acetone at room temperature. A clear, colorless crystal (0.34 x 0.22 x 0.07 mm) was oil-mounted (paratone and mineral oil mix) in a 0.3 mm loop made from 0.04 mm nylon fiber. A Rigaku diffractometer equipped with an ULTRAX18 generator operated at 50 kV and 300 mA power, Mo rotating anode, R-Axis Rapid curved image plate detector, flat graphite monochromator and 0.5 mm collimator were used for the data collection. The temperature of the crystal was maintained at 20.0(1) K using an open flow helium cryostat.31,32,33 In order to obtain quality data with sufficiently high resolution and redundancy in a reasonable amount of time, 8 different runs34 (divided into 4 pairs) ranging from 0-180 in ω were collected at different χ and φ settings. A 4 ω-scan range was used for the entire measurement to avoid significant overlap of reflections. For each pair of runs, a 2 shift in start angle provided a half oscillation range overlap for precise scaling and avoided the use of partial reflections. A frame time of 300 seconds was chosen to maximize the intensity of the Bragg reflections while avoiding saturation of the strongest reflections. The entire experiment was completed in approximately 40 hours. The reflections were indexed using HKL200035 and the collected intensity data integrated using the VIIPP data integration program36,37 based on the reflection positions predicted from HKL2000. The background and experimental reflection profiles were averaged over the entire set of images. The reflection profiles averaged over 200×200 pixel areas were used for integration of peaks with I/σ(I)1 suggest incipient (incomplete) covalent bonding or partial covalent character of ACS Paragon Plus Environment

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these two hydrogen bonds.59 The estimated dissociation energies are also the highest for these two interactions. All other hydrogen bonds listed in Table 2 are weaker and belong to the closed-shell interaction type. For all closed-shell interactions, the electron density calculated from theory is higher, having several other weak bonding interactions not found in the experimental electron density. For two of the C···H interactions, bond paths curve to a different atom in the theoretical electron density than in the experimental density (Table 2). Due to the ubiquity of C-H bonds in bio-molecules, the characterization of H···H interactions has solicited significant interest over the last several years.60 Surprisingly, the H(1)···H(11B) intra-molecular bonding

interaction present in other estrogen molecules16,17,18 was not observed in the experimental electron density of estriol. Although we were able to locate the CPs in the theoretical electron density in both estriol molecules, no virial paths58 could be found, indirectly confirming the absence of this 'signature' estrogen interaction in the estriol molecules. However, several other intermolecular H ··· H bonding interactions are listed in the Table 2. Table 2. Hydrogen bonds and other non-covalent bonding interactions in the estriol crystal.a Bond

, eÅ-3

Strongest hydrogen bonds: O(2A) ··· H(1OA){-x+1/2, y-1, -z+1} 0.319 0.334 O(2B) ··· H(1OB){-x+3/2, y, -z} 0.317 0.343 O(3B) ··· H(2OA){x+1, y+1, z} 0.217 0.246 O(1B) ··· H(3OB){-x+5/2, y-1, -z} 0.198 0.235 O(1A) ··· H(2OB){-x+3/2, y-1, -z+1} 0.181 0.211 O(1A) ··· H(3OA){-x+3/2, y, -z+1} 0.167 0.182 Other bonding interactions: O(1A) ··· H(17B){-x+3/2, y-1, -z+1} 0.053 0.061 O(2B) ··· H(12AB){x-1, y, z} 0.045 0.068 O(3A) ··· H(16B){x, y-1, z+1} 0.039 0.047 O(3A) ··· H(18BB){x, y-1, z+1} 0.035 0.051 C(4B) ··· H(18BA){-x+3/2, y, -z+1} 0.064 0.074 C(3A) ··· H(18AB){-x+3/2, y-1, -z} 0.049 C(4A) ··· H(18AB){-x+3/2, y-1, -z} 0.051 C(4B) ··· H(12BA){-x+3/2, y, -z} 0.042

2, eÅ-5

Rij, Å

d1, Å

d2, Å

g, a.u.

v, a.u.

h e, a.u.

De, kJ/mol

4.14 3.02 3.97 2.77 3.09 2.08 2.66 1.73 1.93 1.54 2.19 1.78

1.658 1.129 1.128 1.653 1.139 1.127 1.826 1.213 1.213 1.859 1.224 1.218 1.921 1.261 1.257 1.908 1.259 1.265

0.534 0.531 0.521 0.529 0.618 0.614 0.636 0.641 0.661 0.664 0.651 0.645

0.0464 0.0400 0.0451 0.0392 0.0307 0.0259 0.0264 0.0226 0.0203 0.0196 0.0212 0.0193

-0.0498 -0.0487 -0.0489 -0.0497 -0.0294 -0.0302 -0.0253 -0.0273 -0.0205 -0.0232 -0.0197 -0.0201

-0.0034 -0.0087 -0.0038 -0.0104 0.0013 -0.0043 0.0012 -0.0047 -0.0002 -0.0036 0.0015 -0.0008

65.4 63.9 64.2 65.2 38.6 39.6 33.2 35.8 26.9 30.5 25.9 26.4

0.64 0.74 0.74 0.52 0.48 0.42 0.56 0.53 0.75 0.69 0.54 0.51 0.60

2.589 1.635 1.507 2.552 1.551 1.527 2.713 1.610 1.625 2.622 1.597 1.484 2.663 1.580 1.577 2.919 1.716 2.807 1.682 2.730 1.712

0.998 1.084 1.009 1.032 1.107 1.110 1.040 1.146 1.090 1.088 1.207 1.129 1.022

0.0053 0.0063 0.0058 0.0050 0.0039 0.0036 0.0043 0.0045 0.0064 0.0063 0.0046 0.0044 0.0047

-0.0040 -0.0048 -0.0039 -0.0045 -0.0027 -0.0029 -0.0028 -0.0035 -0.0050 -0.0055 -0.0035 -0.0035 -0.0033

0.0013 0.0015 0.0019 0.0005 0.0011 0.0007 0.0015 0.0010 0.0014 0.0008 0.0011 0.0009 0.0015

5.3 6.3 5.1 5.9 3.5 3.8 3.7 4.6 6.6 7.2 4.6 4.6 4.3



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0.053 0.48 1.636 1.118 0.0042 -0.0034 0.0008 4.5 C(5B) ··· H(9A){-x+3/2, y, -z} 0.043 0.36 2.757 1.672 1.102 0.0031 -0.0026 0.0006 3.4 0.056 0.38 1.625 1.148 0.0036 -0.0032 0.0004 4.2 C(4A) ··· H(14B){-x+3/2, y-1, -z+1} 0.038 0.27 2.967 1.821 1.167 0.0023 -0.0019 0.0004 2.5 0.041 0.31 1.770 1.201 0.0027 -0.0022 0.0005 2.9 C(15A) ··· H(11BA){x-1, y, z} 0.038 0.56 2.926 1.867 1.079 0.0044 -0.0030 0.0014 3.9 H(15BA) ··· H(11BA){x-1, y, z} 0.039 0.52 2.417 1.254 1.169 0.0041 -0.0029 0.0013 3.8 H(1A) ··· H(11BA)* 2.072 0.084 1.25 1.035 1.126 0.0106 -0.0082 0.0024 10.8 H(1B) ··· H(11BB)* 2.079 0.076 1.13 1.047 1.086 0.0095 -0.0072 0.0023 9.5 H(15BB) ··· H(11AB){x-1, y, z} 0.040 0.50 2.352 1.055 1.362 0.0040 -0.0029 0.0011 3.8 0.050 0.51 1.136 1.251 0.0043 -0.0033 0.0010 4.3 H(7AA) ··· H(6AB){-x+1/2, y-1, -z} 0.036 0.51 2.285 1.122 1.163 0.0040 -0.0027 0.0013 3.5 0.050 0.46 1.157 1.131 0.0040 -0.0032 0.0008 4.2 H(7AA) ··· H(11BA){x-1, y, z} 0.036 0.26 2.366 1.231 1.140 0.0023 -0.0018 0.0004 2.4 0.045 0.32 1.209 1.161 0.0029 -0.0025 0.0004 3.3 a   is the electron density;   is the Laplacian; d1, d2 are the distances from the critical point to atoms 1 and 2; R is the interatomic distance; g, v and he are the kinetic, potential and total electronic energy densities; De is the dissociation energy estimated as De= - v/2.61 First line – experimental data, second line –theoretical calculation (multipole refinement). Possible interactions with no virial paths found are marked with *.

Table 3. Atomic charges and volumes in the estriol crystal.a

Atom O(1) O(2) O(3) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10)

Molecule A q(), e V(), Å3 -1.159 17.33 -1.139 16.84 -1.053 16.51 -1.049 15.58 -1.057 20.16 -1.131 19.53 0.032 13.04 -0.164 12.91 0.076 12.88 0.060 12.45 0.207 9.35 0.318 8.51 0.025 11.39 0.005 11.24 -0.032 10.75 -0.061 10.25 0.032 8.80 0.073 8.12 0.194 8.01 0.023 7.96 -0.034 7.00 0.086 6.49 0.096 7.09 -0.009 7.01 -0.193 11.31

Molecule B q(), e V(), Å3 -1.174 19.89 -1.218 18.86 -1.020 17.04 -1.077 16.65 -1.069 19.38 -1.131 18.73 -0.003 13.26 0.051 12.21 0.025 13.43 -0.061 13.23 0.281 9.79 0.359 8.68 -0.080 10.90 -0.015 10.65 0.033 9.28 -0.048 9.04 0.089 8.58 0.076 7.88 0.165 8.53 -0.038 8.23 0.067 6.87 0.025 6.80 0.039 7.05 0.068 6.80 -0.031 10.73

Molecule A q(), e V(), Å3 0.614 1.79 0.621 1.71 H(1) 0.021 6.64 0.013 6.88 H(2) 0.018 7.13 0.042 7.74 H(4) 0.017 7.78 0.074 7.28 H(6A) -0.043 6.84 0.001 6.94 H(6B) -0.057 8.44 0.032 8.69 H(7A) -0.020 6.39 -0.046 6.98 H(7B) -0.017 7.47 0.000 7.87 H(8) 0.013 6.34 -0.021 6.97 H(9) 0.030 5.77 0.023 6.38 H(11A) 0.000 6.60 -0.027 7.09 H(11B) 0.013 5.93 -0.048 6.42 H(12A) -0.040 8.30 Atom H(3O)


Molecule B q(), e V(), Å3 0.612 1.65 0.622 1.60 0.026 6.71 0.034 7.35 0.021 7.60 0.055 8.03 0.032 7.33 0.060 7.89 -0.035 6.44 0.009 6.63 -0.036 8.28 0.008 8.89 -0.037 7.09 0.003 7.64 -0.031 7.90 -0.002 7.92 -0.013 6.55 -0.007 7.43 -0.002 5.96 0.009 6.39 -0.021 7.93 0.025 8.34 -0.033 7.27 -0.023 7.65 -0.017 7.37

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-0.014 C(11) 0.040 0.014 C(12) 0.053 0.053 C(13) 0.008 0.045 C(14) 0.002 0.017 C(15) -0.105 -0.026 C(16) 0.480 0.353 C(17) 0.444 0.455 C(18) 0.157 0.025 H(1O) 0.618 0.647 H(2O) 0.609 0.635


10.08 8.42 7.63 8.39 7.82 6.12 6.05 6.65 6.51 8.94 8.15 6.37 6.55 5.88 5.96 9.10 8.65 1.58 1.43 1.90 1.66

-0.034 -0.036 0.021 0.038 -0.012 -0.007 0.031 -0.005 0.070 0.143 0.001 0.365 0.389 0.424 0.415 0.176 -0.022 0.650 0.646 0.604 0.597

10.43 8.50 7.80 8.73 8.11 6.22 6.02 6.89 6.60 9.19 8.76 7.06 6.77 5.84 6.09 10.33 9.36 1.45 1.39 1.80 1.89

-0.014 H(12B) -0.040 0.035 H(14) 0.014 -0.002 H(15A) 0.052 0.026 H(15B) 0.042 0.007 H(16) -0.009 0.017 H(17) -0.006 0.013 H(18A) -0.020 0.011 H(18B) -0.018 0.001 H(18C) -0.012 0.018 Total -0.009 -0.009

9.86 7.19 7.39 6.46 7.40 6.58 7.32 6.84 8.14 8.91 10.09 7.07 8.43 5.76 6.31 5.54 5.96 7.47 7.79 364.22 367.08

0.031 0.001 -0.028 -0.007 0.008 -0.020 0.016 -0.031 0.029 0.007 0.047 -0.002 0.004 -0.027 0.015 -0.006 0.014 -0.006 0.009 0.049 0.032

7.10 6.91 7.21 5.74 6.33 7.64 8.59 6.85 7.54 6.33 6.73 4.60 4.89 6.01 6.50 5.95 6.31 8.68 9.00 367.53 366.96

a

First line – experimental data, second line – solid state theoretical calculation at fixed geometry (multipole refinement). Lerr = (L2/Natoms)1/2 = 0.00017 a.u. (experiment), 0.00012 a.u. (theory), L = -1/4(2).62 Lmax= 0.00067 a.u. (experiment), 0.00030 a.u. (theory). Total molecular volume = 731.8 Å3 (experiment), 734.0 Å3 (theory) , unit cell volume / 2 =735.7 Å3.

Table 3 lists atomic charges and volumes integrated over the atomic basins delimited by the zero-flux surfaces.57 Low Lagrangian values, overall charge being reasonably close to zero and a total molecular volume being close to the unit cell volume/2 (0.5% difference for the experiment and 0.2% for the theory) indicate the overall high quality of the data and of the refinement. There is a significant random difference between atomic charges for several carbon atoms, and atomic volumes calculated from theory are generally smaller for 'heavy' atoms and bigger for hydrogens. The O(1) atom attached to the aromatic ring is slightly more negative than O(2) and O(3) as expected, but there is no correlation between the charge and the atomic volumes for these atoms. All hydroxyl hydrogen atoms have relatively high positive charge with H(O1) having the smallest volume. Dipole moments calculated for the two estriol molecules appeared to be quite different (Table 4). For molecule A, the dipole moment value is comparable with the dipole moments of other estrogen molecules, whereas for molecule B it is much smaller. This feature is consistent with the distribution of the electrostatic potential (see below).



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Table 4. Molecular dipole moments (Debye) of the estriol molecules and some other estrogens.16 Dmult, D Compound D , D estriol: molecule A 8.37 (4.53) 7.67 (5.64) estriol: molecule B 2.67 (4.43) 2.80 (3.02) 13.49 – 15.50 10.44 – 13.15 17-estradiol 9.75 8.36 17-estradiol estrone 9.27 11.17 genistein 7.69 7.87 Dmult is a molecular dipole moment calculated from the multipole model parameters; D is a molecular dipole moment calculated from atomic charges and dipole moments integrated over atomic basins. Values obtained from the multipole model refined with respect to theoretical structure factors are shown in parentheses.

Fig. 5. Electrostatic potential isosurfaces (-0.1 eÅ-1) of single estriol molecules taken from the crystal, 'experimental' maps are on the left, and theoretical maps are on the right. Drawn with the 3DPlot program.63


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Fig. 6. The electrostatic potentials of single estriol molecules taken from the solid state plotted on the molecular ((r) = 0.001 a.u.) surface, 'experimental' maps are on the left, and theoretical maps are on the right. Drawn with MolIso.64

C. Electrostatic potential and binding affinity Electrostatic potential (ESP) distributions for the estriol molecules taken from the crystal are shown in Figs. 5 and 6, either as an isosurface or projected onto the molecular surface. Negative ESP areas are more compact for the theoretically calculated electrostatic potential compared to the 'experimental' ESP. These negative ESP areas are also smaller for molecule B than for molecule A, which correlates with the smaller dipole moments observed for molecule B (Table 4). Based on the structures deposited in the PDB13 of E2 and related ligands bound to either ERα or ERβ, the most important ligand/receptor interactions are hydrogen bonds of the following types: i) the phenolic OH acting as a donor to a glutamate residue, and as an acceptor to an arginine, ii) a CH-π interaction of a phenylalanine residue65 with the π-cloud of the A ring, iii) an interaction of oxy-substituents on the D ring with N1 of a histidine residue. There is some ambiguity for the latter interaction. Whereas the keto-oxygen in estrone can only be a hydrogen bond acceptor, hydroxyl groups can be donors or acceptors, and the tautomerism of the ACS Paragon Plus Environment


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imidazole ring allows N1 to also be a donor or acceptor. Regions of highly negative ESP will clearly accept hydrogen bonds, whereas hydrogen atoms with a positive ESP will be potential donor sites. Hence, the proclivity to form hydrogen bonds can be seen from Figs. 5 and 6. However, for the OH groups on the D ring, there are three easily obtained rotamers, and for the phenolic OH there are two. From examination of the receptor/ligand complex, it is clear that the phenolic rotamer must be the opposite of what is observed for molecules A and B here. Rotation about the C-OH bond to obtain the correct rotamer for binding would also rotate the ESP features with little change in their value. Hence, we may obtain a measure of their likely hydrogen bond formation. For the CH-π interaction of the phenylalanine with the A ring, the approach in the receptor is toward the face opposite to the methyl substituent at C13, and the observed negative ESP over the aromatic ring is a measure of the potential importance of this interaction. For interactions concerning the OH groups on the D ring, the crystal structure of the receptor/ligand complex13 suggests that both OH groups interact with N1 of the imidazole ring of the histidine, but we cannot know if this represents two hydrogen bonds to a nitrogen acceptor, or a bifurcated hydrogen bond to two acceptor hydroxyl oxygens. As discussed above, rotating about the C-OH bond in each case should rotate the local features of the ESP without any major change in their values, hence we may obtain some measure of the likely strength of possible hydrogen bonding schemes. For both estriol molecules as well as for all other estrogen molecules previously studied in our group,16,17,18,19,20 large negative ESP areas around the aromatic hydroxyl group and above and below the aromatic ring are observed.66 The previously reported ESP maps have been redrawn so that all are on the same scale here. Negative ESP areas next to the D-ring (Fig. 1) of the estriol molecule are relatively small despite the presence of two oxygen atoms. From the ESP distributions of seven estrogen molecules (Figs. 7 and 8) the most striking observation is that for high binding affinity to either estrogen receptor, negative ESP areas on both ends of a ligand molecule have to be large and extensive. The synthetic estrogen molecules DES and DNS have large negative ESP areas enveloping the aromatic rings and the highest RBA to estrogen receptors ERα and ERβ. Estriol has one of the lowest RBA values, and its negative ESP areas are not as pronounced as in DES and


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DNS. Both estrone and α-estradiol molecules have large negative ESP areas only at one 'end' of the molecule and they have intermediate values of RBA.

Conclusion The asymmetric unit of the estriol crystal consists of two conformationally different estriol molecules. A number of hydrogen bonds and other bonding interactions have been found and characterized in the crystal, with the strongest hydrogen bonds having partially covalent character. No intra-molecular H···H interaction typical for other natural estrogens was found in either of the estriol molecules. The distribution of electrostatic potential and molecular dipole moments appear to be different for molecules A and B. Comparison of ESP distributions for seven estrogen molecules with different estrogen receptor binding affinities demonstrates that in order to get a high RBA, strong and extensive areas of negative ESP at each end of the molecule are important.



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Fig. 7. Electrostatic potential isosurfaces (-0.1 eÅ-1) of various estrogen molecules.16,17,18,19,20 RBA are relative binding affinities with respect to β-estradiol to the estrogen receptors ERα and ERβ, respectively. Drawn with the 3DPlot program.63


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Fig. 8. The electrostatic potentials of various estrogen molecules16,17,18,19,20 plotted onto the molecular ((r) = 0.001 a.u.) surface. RBA are relative binding affinities to the estrogen receptors ERα and ERβ. Drawn with MolIso.64



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Acknowledgement This work was supported by the National Science Foundation (grant NSF-CHE-1213329).

Supporting Information Available: statistical plots; residual and deformation electron density maps; figures with overlapped estriol molecules with the optimized and experimental geometry; table with intra-molecular critical points. This material is available free of charge via the Internet at http://pubs.acs.org.

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This effect is less pronounced for the molecule B in the case of theoretical calculation.



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TOC figure:


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Charge Density and Electrostatic Potential Study of 16Î±,17Î²-Estriol

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