Ab Initio Study of Hydroxyl Torsional Barriers and Molecular Properties

ARTICLE pubs.acs.org/JPCA

Ab Initio Study of Hydroxyl Torsional Barriers and Molecular Properties of Mono- and Di-iodotyrosine Luis F. Pacios* Unidad de Química y Bioquímica, Departamento de Biotecnología, ETS Ingenieros de Montes, Universidad Polit
ecnica de Madrid, 28040 Madrid, Spain

Pedro C. G
omez Departamento de Química Física I, Facultad de Química, Universidad Complutense de Madrid, 28040 Madrid, Spain

Oscar G
alvez Departamento de Física Molecular, Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain

bS Supporting Information ABSTRACT: Phenol rings with one or two iodine atoms bonded to ortho carbons are the essential organic source of iodine for living organisms. The salvage of this halogen fundamental for a variety of biological functions is accomplished through enzymatic processes that rely on recognition of monoand di-iodotyrosine (MIT and DIT, respectively). Ab initio quantum calculations are used to investigate molecular properties of MIT and DIT associated with their recognition by cognate proteins. Energies, electron density properties, atomic charges, and electrostatic potentials are analyzed in relation with the presence of one or two iodine atoms and internal rotation of hydroxyl hydrogen. The formation of an intramolecular hydrogen bond at some conformations has little effect on the properties that might affect the recognition and further deiodination of MIT and DIT. Polarizability of iodine and the reactive nature of iodinated tyrosines as nucleophilic targets are the essential features revealed in this work.

I. INTRODUCTION Iodine is an halogen of paramount importance for life. In vertebrates, thyroid hormones (THs, a collective term for thyroxine, T4, and triiodothyronine, T3) are critical molecules (Scheme 1) for regulating a wide range of biological functions within and between cells. Invertebrates lack thyroid glands, yet many use ingested TH-related compounds necessary to initiate and/or sustain critical developmental stages. Other organisms including plants, algae, and zooplankton store iodine as TH precursors such as monoiodotyrosine (MIT) and di-iodotyrosine (DIT).1 Iodine in earth exists in common oxidation states, basically iodide (I
), iodate (IO3
) and to a much lesser extent, molecular iodine (I2), but most atmospheric iodine is released by marine organisms in various organic and inorganic forms.2 Halogenation is a strategy used in nature to increase the biological activity of a large number of compounds. Given that halides are particularly reactive when activated, typically by oxidation, most of methods developed by evolution rely on oxidative enzymes that require either H2O2 or O2 to introduce halogen atoms into organic compounds.3,4 However, tyrosine (T) is known to easily incorporate iodine without enzyme assistance. It is conjectured that this unique reactivity led to spontaneous formation of iodotyrosines in primitive cells about r 2011 American Chemical Society

three billion years ago. 5 When primitive marine animals emerged from the sea, which is rich in iodine, and transferred to the iodinedeficient mainland, terrestrial life evolved with cell functions that depended on iodotyrosine derivatives.1 In mammals, iodide homeostasis is critical for generating THs and is achieved by capturing and salvaging iodide. Both of these processes are critical for human health. A Naþ/I
pump located in the membrane of thyroid cells captures iodide from the circulatory system. Iodotyrosine deiodinase (IYD) in thyroid tissue accomplishes salvage of iodide by catalyzing deiodination of MIT and DIT.6,7 While dehalogenation is commonly achieved in living organisms by oxidative pathways, the IYD-catalyzed deiodination is unusual since the carbon
iodine bond is broken through a reductive process. IYD actually represents one of only two enzymes known to promote reductive dehalogenation in mammals.7 The other enzyme, iodothyronine deiodinase (ID), acts alternatively Special Issue: Richard F. W. Bader Festschrift Received: April 4, 2011 Revised: May 25, 2011 Published: June 14, 2011 12616

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The Journal of Physical Chemistry A Scheme 1. Structural Formula of the Main Thyroid Hormones T3 and T4

to activate and deactivate T4 by deiodinating the outer or inner ring (right or left in Scheme 1), respectively.7 ID acts through a tautomerization of iodotyrosines to their keto form followed by transfer of an iodonium to an active-site thus reducing cysteine.6 By contrast, IYD requires no cysteine for catalysis and uses instead a bound flavin monocleotide (FMN) to protonate the substrate and release iodide although its role in reductive dehalogenation is not yet fully characterized. 7 Intermolecular interactions between iodinated phenol moieties and active site amino acids and/or cofactors such as FMN play an essential role in the recognition of THs by their cognate proteins. The large number of short I 3 3 3 O contacts between TH substrates and their associated proteins identified in a comprehensive survey of protein structures,8 has been taken as evidence of halogen bonding.9 This noncovalent interaction is supposed to stabilize inter- and intramolecular interactions that can affect protein
ligand binding affinity.8
11 However, the effects that characterize a halogen bond are related with electrostatic potential and electron density features associated to carbon
halogen and oxygen
halogen bonds.8,9,11 While high-level quantum calculations exist for iodine-containing species of atmospheric interest (mostly iodine oxides and related small molecules),12 theoretical studies on iodinated organic compounds are far more scarce. Density functional theory (DFT) calculations of reaction profiles of protonated MIT have been reported in a study of fragmentation mechanisms of protonated MIT, DIT, and T4.13 In a computational investigation on iodine release catalyzed by vanadium iodoperoxidase, we have recently reported thermodynamic data obtained from MP2 and CCSD(T) calculations for iodination reactions of organic substrates.4 In the present work, we present an ab initio study of changes in electron density properties and electrostatic potentials associated to the presence of one or two iodine atoms in ortho positions and internal rotation of hydroxyl hydrogen in p-methylphenol used as a T model (Scheme 2). With this theoretical report, which, to the best of our knowledge, is the first ab initio study of iodotyrosines published so far, we aim to gain insight into molecular properties of MIT and DIT that might affect their recognition and dehalogenation by TH-processing enzymes.

II. METHODS T, MIT, and DIT were represented by their corresponding pmethylphenol models (Scheme 2). The methyl group represents the CβH2- group connecting the phenol ring to the protein backbone. Torsional barriers at 15° intervals of dihedral angle ϕ (Scheme 2) between 0 and 180 ° were calculated for the three molecules in gas phase and three different solvents. Besides the equilibrium structure, a full relaxation of the molecule for every value of ϕ was allowed for by carrying out a complete optimization

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Scheme 2. p-Methylphenol Structures Used as Models of Tyrosine and Its Mono- and Di-iodo Derivativesa

a

Planar conformation sketched sets ϕ = 0°. Tyrosine numbering.

of geometry parameters other than ϕ at the MP2 level. Shapeconsistent averaged relativistic effective potential (AREP)14 was used to replace the 46-electron core for iodine.15 A valence-only cc-pVTZ basis set optimized for use with AREP developed before for iodine4 and conventional cc-pVTZ basis sets for C, H, and O atoms were used in these calculations. Media other than the gas phase were accounted for by computing polarizable continuum model (PCM) energies at MP2/cc-pVTZ gas-phase geometries with ε = 2 to simulate protein interiors,16 ε = 40 to simulate charge interactions in protein sites,17 and ε = 78.36 for water. For all the conformations in gas-phase, atomic charges were computed at MP2/AREP(I)/cc-pVTZ single point calculations using the CHelpG scheme,18 taking for iodine the atomic van der Waals radius 2.2 Å.19 In order to obtain properties dependent on the electron density F(r), further gas-phase geometries were optimized in all-electron (AE) MP2 calculations for relevant conformations ϕ = 0 ° and 90 ° in T, MIT, and DIT as well as ϕ = 180 ° in MIT (section III.A). Given that no cc-pVXZ AE basis sets exist for iodine, we resorted to Ahlrichs TZVPP basis sets,20 as they have a similar quality to cc-pVTZ sets for obtaining AE electron densities at geometries also optimized at the MP2/TZVPP level of theory. The location and characterization of BCPs of F(r) according to the atoms-inmolecules (AIM) theory21,22 was accomplished with AIMAll 10.12.16.23 Finally, electrostatic potentials Vel(r) were computed with CheckDen24 at space grids with 0.2 Å steps to render twodimensional (2-D) maps at the phenol plane as well as threedimensional (3-D) plots. Due to the computational burden in using MP2 densities to calculate Vel(r) at a great number of grid points (more than 54 000 in each 3-D mesh) in systems with large basis sets (DIT, for instance, has 474 basis functions), B3LYP/TZVPP electron densities were obtained at the MP2/ TZVPP geometries for computing AE electrostatic potentials. All the geometries, energies, atomic charges, and electron densities were computed with Gaussian 09.25

III. RESULTS A. Internal Rotation Potentials. The analysis of the energetics of the torsional barriers of hydroxyl hydrogen (Figure 1 and Table 1) provides some interesting insights. In the case of MIT, there are two minima with different energies. The more 12617

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Figure 1. Potential energy curves for internal rotation of hydroxyl hydrogen in T, MIT, and DIT (Scheme 2).

Table 1. Energy Differences ΔE and Corresponding Dihedral Angles ϕmax for the Internal Rotation of Hydroxyl Hydrogen in T, MIT, and DIT ε=1 molecule

ε = 40

ε = 78.36

ϕmaxb

ΔEa

ϕmaxb

ΔEa

ϕmaxb

ΔEa

ϕmaxb

T

3.39

91.3

3.43

91.3

3.58

91.3

3.58

91.3

MIT

5.44

82.7

4.89

84.6

4.03

88.0

4.00

88.1

2.62

0

1.91

0

0.67

0

0.62

0

4.78

91.9

4.42

91.9

3.86

91.8

3.84

91.8

DIT a

ΔEa

ε=2

ΔE in kcal mol
1 b ϕmax in degrees

stable conformer is the one in which the iodine atom is closer to the hydroxyl hydrogen atom (ϕ = 180 ° in Scheme 2) and the other conformation corresponds to ϕ = 0°. The former conformer

(MIT-180) is more stable than the latter conformer (MIT-0) by 2.6 kcal mol
1 in the gas phase. This is the expected result if one considers the stabilizing effect of an intramolecular O
H 3 3 3 I hydrogen bond in MIT (section III.B). It should be mentioned that gas phase standard free energies of phenol iodination with HOI, I2, and I3
reagents obtained in MP2 and CCSD(T) calculations predict consistently lower ΔRGo values by about 2.7 kcal mol
1 for formation of the monoiodophenol conformer allowing for O
H 3 3 3 I bonding. 4 However, while the energy difference between the minima is 2.62 kcal mol
1 in the gas phase, it almost disappears (reaches a value of 0.62 kcal mol
1, close to RT at 298 K) in water. The conclusion is that water and polar solvents represented by ε = 40 will tend to diminish energetic preferences between both conformers, thus decreasing H-bonding effects. Simple calculations using the Maxwell
Boltzmann statistics show that in gas phase, nearly 1% of MIT molecules will be in the ϕ = 0 ° conformation, whereas this percentage increases to 26% in water. On the other hand, maxima (determined upon fitting polynomials in the 60
120 ° interval to curves in Figure 1) appear at ϕ angles noticeably lower than 90° in gas phase and ε = 2 that shift toward the perpendicular value at polar media. This result is understandable if one analyzes the torsional behavior of hydroxyl in the two other molecules. In the case of T and DIT, there are two isoenergetic conformers separated by moderate barriers, higher in DIT than in T. However, while in the diiodinated species the barrier also decreases by about 1 kcal mol
1 in polar solvents, the barrier in T remains practically unchanged (it even increases yet in a negligible amount of ∼0.2 kcal mol
1). If MIT results are also considered, it seems evident that this behavior must arise from the sensitivity toward a polar environment of the hydrogen bond between the rotatable H atom and the highly polarizable I atom. As it is known, hydrogen bonding is stronger in nonpolar media.26,27 Since the internal rotation in MIT turns the H-bond off and on as ϕ gets closer to 0° or 180° respectively, the environmental effect is stronger in MIT than in DIT where the motion simply switches one O
H 3 3 3 I bond near ϕ = 0° to other identical near ϕ = 180°. Also note that contrary to that found in MIT, both T and DIT molecules agree in displaying maxima at ϕ close to 90° irrespective of the medium, a result easily understandable in light of the symmetry in both structures. Given that the stability associated with hydrogen bonding is greater in nonpolar media and that MIT shows this effect only when ϕ gets closer to 180°, energy maxima moves slightly to the left of the middle value in MIT ε = 1 and ε = 2 potential energy curves (Figure 1). A main conclusion arising from these internal rotation energies is that symmetry in the charge distribution in the cases of T and DIT makes the position of the OH group scarcely relevant to activate or deactivate phenol ring atoms in the sight of electrophilic or nucleophilic attacks (section III.D). However, in the case of MIT, there are clear differences determined by the OH position in nonpolar media. B. Bond Lengths and Electron Density Effects. The current status of the AIM theory as well as established and successful analytical tools in Theoretical Chemistry allow one to address bonding using the topological properties of the electron density as prescribed by the theory. 21,22 In this section, we make use of this information together with structural data. All the geometries may be found in the Supporting Information, hence we discuss here only interatomic distances and bond lengths relevant to the properties analyzed (Table 2). Non-nuclear critical points of F(r) are displayed in Figure 2 while Table 3 gathers topological 12618

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Table 2. Bond Lengths and Interatomic Distances Involving Hydroxyl and Iodine Atoms in Relevant Conformations of T, MIT, and DIT a,b conformation c

OH

OC4

d

IC3

IC5

I3 3 3H

O 3 3 3 IC3

0.963 0.962

1.368 1.369

T-90 MIT-0

0.962 0.961 0.964 0.963

1.388 1.388 1.363 1.362

2.079 2.081

3.146 3.149

MIT-90

0.963 0.962

1.378 1.378

2.081 2.083

3.210 3.212

T-0

MIT-180

0.969 0.968

1.357 1.357

2.088 2.090

DIT-90

0.965 0.964

1.370 1.369

2.081 2.083

2.082 2.083

2.586 2.582

DIT-180 d

0.970 0.970

1.352 1.351

2.089 2.090

2.080 2.082

2.561 2.556

O 3 3 3 IC5

3.255 3.251 3.190 3.191

3.193 3.194

3.239 3.234

3.130 3.134

a

First column values: iodine valence-only MP2/AREP(I)/cc-pVTZ geometries; second-column values in italics: iodine AE MP2/TZVPP geometries. All values in Å. b Atom numbering refers to Scheme 2. c ϕ value in degrees after molecule symbol. d 0 and 180 equivalent geometries.

Figure 2. Molecular graphs of conformations ϕ = 0 and 180° of MIT, and 0° of T and DIT. Blue spheres indicate BCPs of F(r), green spheres are RCPs, and the dotted yellow line represents an I 3 3 3 H
O hydrogen bond.

descriptors of bond critical points (BCPs) involving hydroxyl hydrogen and iodine atoms. As the differences between results obtained using either AE or valence-only treatments for iodine are completely negligible (Table 2), we omit hereafter to mention the methodology underlying their calculation. As it is readily apparent, bond lengths and topological descriptors of F(r) show the features typical of hydrogen bonding21,22,26
28 at 180° conformers of MIT and DIT (Figure 2). Upon formation of the H-bond, the donor OH bond elongates, and its BCP exhibits slight loss of density (lower Fc), depletion of charge (less negative r2Fc), and higher (less negative) total energy densities Hc. The BCP at the intramolecular H 3 3 3 I bond path shows Fc ∼ 0.02 au and r2Fc ∼ þ0.052 au, values that are within those prescribed by the Koch
Popelier criteria to characterize H-bonds.22 Given the well-established relationship between the strength of a bond and the F(r) value at the corresponding BCP, Fc data suggest a slightly stronger OH 3 3 3 I interaction than hydrogen bonds in

H2O/ClO and H2O/BrO complexes having Fc ∼ 0.017 au.29 In fact, H-bond energies ∼ 2.6 kcal mol
1 (section III.A) compared with 1.4 and 1.7 kcal mol
1 for H2O/ClO and H2O/BrO, respectively (also MP2/cc-pVTZ data), illustrate the greater stability of the intramolecular H 3 3 3 I bond. This result should confirm previous suggestions on the stabilizing importance of polarizability effects in large halogens.28,29 It is worth noting that, although BCP parameters and both OH and IC3 bond lengths in 180 ° conformers of MIT and DIT are virtually indistinguishable, the I 3 3 3 H distance is significantly longer in MIT. This difference might be attributed to the smaller geometry relaxation in DIT caused by a second iodine atom as compared with a hydrogen atom at C5 in MIT, which in turn should yield a shorter I 3 3 3 H distance. As for the OC4 bond, its length is noticeably longer at 90 ° and shorter at conformations with H-bond, which is understandable upon considering the interactions of rotatable hydrogen with other atoms, H-bonding being a more stringent geometry constraint imposed onto the CO bond. We explored all the AIM information in search of hints of intramolecular halogen bonding. Our previous report on H2O/ XO complexes revealed the presence of intermolecular BCPs at O 3 3 3 X paths in both X = Cl and Br cases.29 However, F(r) properties show no hint of halogen bonding at any conformer of MIT and DIT despite the fact that O 3 3 3 I distances are well below 3.5 Å (Table 2), the threshold proposed for O 3 3 3 I interactions in proteins that bind MIT and DIT.8 It seems obvious that, although the interatomic distance is appropriate for intramolecular halogen bonding, bond angles are not. In fact, for the C
O 3 3 3 I-C case, C
O 3 3 3 I angles between 80 and 135 ° and O 3 3 3 I
C angles between 132 and 176 ° were observed in proteins of the thyroid hormonal system showing evidence of halogen bonding.8 For all the conformers of MIT and DIT studied here, while C
O 3 3 3 I angles are between 68 and 73 ° (which is not too far from the lower limit in proteins), O 3 3 3 I
C angles remain almost unchanged at ∼ 49 °, too small an angle for possible halogen bonding. Finally, the ring critical point (RCP) corresponding to the phenol ring shows nearly invariant values (a.u.) Fc ∼ 0.024, r2Fc ∼ 0.14, and Hc ∼ 0.0035, irrespective of the molecule and conformer (data not shown). The RCP appearing at the ring closed by the I 3 3 3 H bond (Figure 2) shows also very similar values (a.u.) Fc ∼ 0.020, r2Fc ∼ 0.075, and Hc ∼ 0.0014 in both MIT and DIT systems. Despite the rather distinct electronic nature of both rings, the comparison of their RCPs shows Fc values not too different even though r2Fc reveals a greater depletion of density in the aromatic ring than in the H-bond ring. Both results could be explained in terms of the large volume associated to the electron density of 12619

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Table 3. Topological Properties of BCPs of the Electron Density in Bonds Involving Hydroxyl Hydrogen and Iodine Atoms in Relevant Conformations of T, MIT, and DITa,b OH

IC3

Fc

2

r Fc

Hc

T-0

0.368


2.76


0.772

T-90

0.366


2.74


0.768

MIT-0 MIT-90

0.366 0.363


2.76
2.73

MIT-180

0.359


2.70

conformation

c

IC5

Fc

2

r Fc

Hc

Fc


0.770
0.764

0.132 0.131


0.0367
0.0394


0.0765
0.0763


0.754

0.130


0.0478


0.0747

I3 3 3H

r Fc 2

Hc

DIT-90

0.361


2.72


0.759

0.131


0.0362


0.0760

0.131


0.0358


0.0760

DIT-180

0.357


2.69


0.751

0.130


0.0449


0.0745

0.131


0.0328


0.0761

Fc

r2Fc

Hc

0.0200

þ0.0516

þ5.10
5

0.0210

þ0.0527


2.10
4

Fc is the electron density at the BCP, r Fc is the Laplacian of the electron density at the BCP, and Hc is the total energy density at the BCP All values in atomic units. b Atom numbering refers to Scheme 2. c ϕ value in degrees after molecule symbol. a

2

Table 4. Atomic Charges of Atoms Involved in Internal Rotation of Hydroxyl Hydrogen in Relevant Conformations of T, MIT, and DIT a conformation b

a

HOH

O

C4

C3

C5

IC3

T-0

0.3921


0.5594

0.3270


0.1525


0.2432

T-90

0.3291


0.5167

0.3495


0.1979


0.1994

T-180 MIT-0

0.3923 0.3100


0.5616
0.4866

0.3355 0.3375


0.2504
0.1076


0.1573
0.1846


0.0342

MIT-90

0.3216


0.5004

0.3833


0.0788


0.2350


0.0614

IC5

MIT-180

0.4031


0.5609

0.3666


0.0202


0.2936


0.0502

DIT-0

0.3146


0.4862

0.3955


0.0810


0.1650


0.0351


0.0144

DIT-90

0.3105


0.4793

0.4094


0.1069


0.1105


0.0443


0.0432

DIT-180

0.3151


0.4874

0.3983


0.1650


0.0836


0.0148


0.0347

Atom numbering refers to Scheme 2. b ϕ value in degrees after molecule symbol.

iodine atom, which in turn would yield a greater contribution than other smaller atoms. C. Atomic charges. The whole set of all atomic charges for conformations may be found in the Supporting Information. We address here only atoms directly involved in the torsional motion of hydroxyl at relevant conformations ϕ = 0, 90, and 180 ° (Table 4). In all the cases, the C4 atom is the most positive one in the molecule, whereas the bonded O atom is the most negative, its electron population being the largest when no I atoms occur. The third member of the internal rotor, hydroxyl H, has always fairly large positive charge displaying a tiny variability in T and DIT molecules where H can assume two symmetry-equivalent positions upon torsion. Both atoms in the CO bond lose electron charge (C4 becoming more positive and O less negative) and H atom gains it (becoming less positive) at ϕ = 90 ° where hydrogen is furthest from the aromatic ring. However, the formation of the intramolecular C3
I 3 3 3 H
O hydrogen bond at 180 ° yields a different picture for MIT. According to known atomic charges patterns, donor oxygen gains charge (increasing its negative value) and hydrogen loses charge (increasing its positive value) as they move toward H-bond formation,28 which in our case corresponds to ϕ = 0° f 90° f 180°. As a matter of fact, this Hδþ effect, one of the Koch
Popelier criteria,22 is one of the main characteristics of hydrogen bonding. Apart from H-bond effects, the largest relative atomic charge variations with ϕ occur in ortho carbon atoms (namely C3 and C5 close to 40%). In both T and DIT cases, C3 increases its negative value for ϕ larger than 90° and decreases it when going backward

from 90° to zero, whereas C5 displays the opposite trend like compensating the evolution of its partner atom. In the case of MIT, C3 and C5 bear atomic charges similar to those in C
I and C
H bonds respectively, showing also similar evolution with ϕ. Iodine atoms have overall small negative charges that change very little upon torsion of hydroxyl hydrogen. In the case of DIT, both I atoms exhibit a nearly exact symmetry in the variation of their atomic charges at ϕ = 0°, 90°, 180°. As for the C-I bonds that are essential to the biological processing of iodotyrosines, atomic charges show two interesting features. The first one is that C3 and C5 atoms have negative charges irrespective of whether they are bonded to H or to I although in the latter case the charges have a smaller magnitude. The second one is that the small atomic charges in I atoms agree with the role played by this large halogen, not due to its electronegativity but to its polarizability, as mentioned in other places of this work. D. Electrostatic Potentials. By definition, the electrostatic potential Vel(r) gives the energy acting on a unit positive charge located at r due to the net effect arising from positive point charges ZA of nuclei and the negative distribution of electrons F(r). Isocontour maps of Vel(r) at the phenol plane plotted in Figure 3 display a wide interval extending far beyond the molecular structure. Given the ZA/R behavior of the repulsive term with the distance R to nucleus ZA, positive electrostatic potentials dominate at regions close to nuclear positions (isocontours above þ1000 kT/e are omitted in Figure 3 for clarity). Regions of negative potential indicate dominance of electron effects, and for these molecules reveal some interesting points. Two negative 12620

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Figure 4. 3-D electrostatic potential isosurfaces for conformations ϕ = 0° of T, 0° and 180° of MIT, and 0° of DIT. Isovalues plotted are
100 (red solid surface) and þ 100 (blue wireframe surface) kT/e (T = 298 K) units. Figure prepared with PyMOL.30 Figure 3. Electrostatic potential isocontour maps at the phenol plane for conformations ϕ = 0° of T, 0° and 180° of MIT, and 0° of DIT. Contours scan the interval [
200, þ1000] in units of kT/e (T = 298 K) at proper intervals to outline different spatial regions around the molecules. Red contours represent negative values, blue contours are positive values, and thick black lines represent zero isocontours.

regions are present in the four conformations: the large one above oxygen and the small one below p-methyl. The small region corresponding to the π electron cloud at the center of the aromatic ring vanishes in DIT, while the small negative area suggesting lone electron pairs effects in oxygen are visible only in MIT-0 and DIT maps. Note, though, the tight accumulation of decreasing positive contours in both the bay below hydroxyl and the center of the ring, which suggests the presence of zero and then negative isocontours at close regions above and below the plane used in these maps (see next paragraph). Interestingly, the negative region revealing electron lone pairs effects in iodine is seen only for this atom participating into OH 3 3 3 I hydrogen bond (MIT-180 and DIT), whereas the other iodine atom in DIT lacks a similar contour pattern. 3-D Vel(r) = ( 100 kT/e isosurfaces (a quite large energy of 59 kcal mol
1 at 298 K) are plotted in Figure 4. This negative potential is restricted to a region over the aromatic ring and much smaller regions in the proximity of oxygen lone pairs. Note the reduction of the aromatic negative domain with the presence of an iodine atom and how the lenticular shape in T shrinks in the direction of iodine in MIT-0 and MIT-180. The effect is much amplified with a second iodine atom, and this isosurface has nearly vanished in DIT. This negative potential is restricted to tiny domains around electron lone pairs near oxygen atoms. Nonetheless, a word of caution seems appropriate regarding these plots. If the isosurface displayed had been Vel(r) =
80 kT/e, one had seen how oxygen and aromatic negative domains in DIT merge. At Vel(r) =
20 kT/e, this merged isosurface had extended slightly outside the positive isomesh of DIT in Figure 4. However, in these two cases, the equivalent positive surfaces would extend over far larger regions. In fact, our aim in choosing the isosurfaces plotted in Figure 4 was to illustrate how, even at a very high values

such as Vel(r) = þ100 kT/e, the positive electrostatic potential overwhelmingly dominates. Furthermore, the region of iodine atoms exhibits by far the largest extent of this domination. Note also how the I and OH domains merge upon hydrogen bonding, thus giving rise to a great spatial isosurface in MIT-180, which, however, is smaller in the equivalent region in DIT. A conclusion arising from electrostatic potentials is that iodine atoms in MIT and DIT are prone to nucleophilic attack irrespective of the position of the OH group. Moreover, as far as electrostatic potential is concerned, the presence of iodine atoms diminishes the importance of aromatic electron contributions, inhibiting in turn possible electrophilic attacks. This effect is much more pronounced in DIT than in MIT, regardless of the existence of hydrogen bonding in the 180 conformer of MIT.

IV. DISCUSSION The ability of halogen atoms to act as sites for directing recognition processes permits to optimize binding of halogenated ligands to receptors, thereby tuning properties central to protein function.8
11 Noncovalent recognition occurring with participation of halogen bonds should involve electron donation from lone pairs contributed by O, N, or S atoms in the protein to a halogen and, in particular, to highly polarizable iodine. Halogens show amphiphilic character in the sense that they may act as electrophilic targets because of their electron lone pairs and as nucleophilic targets because of their polarizability, which increases with atomic size. While fluorine remains electronegative and is more likely to act as a H acceptor in F 3 3 3 H hydrogen bonds, larger bromine and iodine may show dominant electropositive behavior, iodine forming the strongest X 3 3 3 O halogen bonds.8,9 Since the magnitude of polarization effects underlying these interactions depends on the local environment, halogen bonds associated with aromatic compounds are expected to be stronger than those formed with aliphatic molecules.8 The preceding considerations suggest that MIT and DIT are particularly suitable ligands to enhance affinity to receptor sites. 12621

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The Journal of Physical Chemistry A After binding, the key process is catalytic deiodination achieved by reductive break of carbon-iodine bond. Although the details of this process are far from being elucidated, it is believed to occur through transfer of iodonium to an electron donor at the catalytic site and hydrogen donation from bound cofactors such as FMN.6,7 One of the two possible reductive pathways involves tautomerization of the phenolic form of MIT or DIT to its corresponding keto form prior to iodonium release.6 In either event, the phenolic hydroxyl group of MIT or DIT plays a crucial role in either making H-bonds with FMN7 or transferring hydrogen to the carbon bound to iodine undergoing tautomerization to its keto form.6 In this work, we have studied the molecular properties of MIT and DIT that might affect their recognition and deiodination in relation with the internal rotation of hydroxyl hydrogen. The major conclusions are the following. Whereas torsional barrier heights in MIT and DIT differ in gas phase, both compounds show lower maxima with similar values about 4 kcal mol
1 for conformers with OH perpendicular to the phenol ring. Although planar conformations permit formation of an intramolecular OH 3 3 3 I hydrogen bond that represents a stability of ∼2 kcal mol
1 in gas phase MIT, polar media decrease this energy to only 0.6 kcal mol
1. This way, the constraint to internal rotation that might represent the H-bond is largely diminished in polar media and water. It is also found that T has a barrier height that changes negligibly under distinct media. This means that the sensitivity to the medium polarity is entirely due to the presence of iodine atoms, which in turn facilitates the rotation of hydroxyl groups in polar environments such as those in catalytic sites. AIM analyses of F(r) together with bond lengths and interatomic distances confirm the existence of an intramolecular OH 3 3 3 I hydrogen bond in planar conformers of both MIT and DIT suggested by torsional energy curves. These analyses indicate features of conventional hydrogen bonding, yet stronger than that existing in H2O/ClO and H2O/BrO complexes studied before.29 This lends support to previous conjectures on the importance of polarizability effects arising from voluminous iodine in stabilizing nonbonding interactions. However, no hints of intramolecular halogen bonding are found in MIT and DIT. Although O 3 3 3 I distances are below the threshold proposed for this interaction in proteins that bind iodinated ligands, bond angles are too small to allow for intramolecular halogen bonding. Therefore, the putative stabilizing effects played by iodine polarizability with regard to halogen bonding can completely be played in intermolecular interactions at protein receptor sites. Atomic charges mainly reflect effects associated to the intramolecular hydrogen bonding with the largest charges found for both atoms of the hydroxyl group. Carbon atoms ortho to the OH group show negative charges irrespective of whether they are bonded to H or to I, which suggests a role essentially similar with regard to their reactivity. Iodine atoms exhibit very small negative charges that remain almost unchanged upon hydrogen bonding. This result agrees with the role played by iodine, not because of its electronegativity as a halogen but because of its polarizability as a large atom. Finally, electrostatic potentials provide clear evidence on the degree of reactivity of iodinated tyrosines as nucleophilic targets. Regardless of the position of hydroxyl hydrogen in both MIT and DIT, iodine atoms exhibit large positive electrostatic potential distributions surrounding them. It is also noteworthy that the presence of iodine atoms diminishes the spatial extent of negative electrostatic potentials associated with the aromatic π electron

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cloud in the phenol ring. This effect, which in turn suggests inhibition of possible electrophilic attacks, is much more pronounced in DIT than in MIT. Therefore, besides the obvious reason of having two iodine atoms, using DIT instead of MIT as a substrate for salvaging iodine should also be more favorable from a reactivity point of view.

’ ASSOCIATED CONTENT

bS

Supporting Information. AREP operators and AREPoptimized cc-pVTZ basis set for iodine. MP2/AREP(I)/ccpVTZ geometries, total energies at ε = 1, 2, 40, and 78.36, and CHelpG atomic charges for all the conformations of T, MIT, and DIT used in sections III.A and III.C. Iodine AE MP2/TZVPP geometries of ϕ = 0, 90, and 180 ° conformations of T, MIT, and DIT used in sections III.B and III.D. This material is available free of charge via the Internet at http://pubs.acs.org

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]

’ ACKNOWLEDGMENT L.F.P. acknowledges financial support from the Spanish Ministry of Science, Project BIO2009-07050 and Comunidad de Madrid, Grant EIADES S2009/ AMB-1478. P.C.G. acknowledges financial support from the Spanish Ministry of Science, Project CTQ2008-02578/BQU and Consolider Ingenio 2010 CSD20090038. O.G. acknowledges financial support from the Spanish Ministry of Science, Project FIS2010-16455 and “Ramon y Cajal” program. ’ REFERENCES (1) Crockford, S. J. Integr. Comp. Biol. 2009, 49, 155–166. (2) Vogt, R.; Sander, R.; Von Glasow, R.; Crutzen, P. J. J. Atmos. Chem. 1999, 32, 375–395. Gilfedder, B. S.; Lai, S. C.; Petri, M.; Biester, H.; Hoffmann, T. Atmos. Chem. Phys. 2008, 8, 6069–6084. Saiz-Lopez, A.; Boxe, C. S. Atmos. Chem. Phys. Discuss. 2008, 8, 2953–2976. Jones, C. E.; Hornsby, K. E.; Sommariva, R.; Dunk, R. M.; Von Glasow, R.; McFiggans, G.; Carpenter, L. J. Geophys. Res. Lett. 2010, 37 (L18804), 1–6. (3) Winter, J. M.; Moore, B. S. J. Biol. Chem. 2009, 284, 18577–18581. Butler, A.; Sandy, M. Nature 2009, 460, 848–854. (4) Pacios, L. F.; Galvez, O. J. Chem. Theory Comput. 2010, 6, 1738–1752. (5) La Barre, S.; Potin, P.; Leblanc, C.; Delage, L. Mar. Drugs 2010, 8, 988–1010. (6) Friedman, J. E.; Watson, J. A.; Lam, D. W. H.; Rokita, S. E. J. Biol. Chem. 2006, 281, 2812–2819. (7) Thomas, S. R.; McTamney, P. M.; Adler, J. M.; LaRonde-LeBlanc, N.; Rokita, S. E. J. Biol. Chem. 2009, 284, 19659–19667. (8) Auffinger, P.; Hays, F. A.; Westhof, E.; Ho, P. S. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 16789–16794. (9) Metrangolo, P.; Meyer, F.; Pilati, T.; Resnati, G.; Terraneo, G. Angew. Chem., Int. Ed. 2008, 47, 6114–6127. (10) Matter, H.; Nazar
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