Conformational Flexibility and Halogen Bonding in Thyroid Hormones and Their Metabolites

Article abstract of DOI:10.1021/acs.cgd.6b00945

Iodothyronamines (TAMs) are endogenous thyroid hormone (TH) metabolites, which are proposed to be biosynthesized by decarboxylation of the β-alanine side chain of THs. Iodothyronine deiodinases (DIOs) mediate phenolic and tyrosyl ring deiodinations of thy

Full text of DOI:10.1021/acs.cgd.6b00945

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Article
Conformational Flexibility and Halogen Bonding
in Thyroid Hormones and their Metabolites
Santanu Mondal, and Govindasamy Mugesh
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00945 • Publication Date (Web): 30 Aug 2016
Downloaded from http://pubs.acs.org on September 2, 2016
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Conformational Flexibility and Halogen Bonding in
Thyroid Hormones and their Metabolites
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Santanu Mondal and Govindasamy Mugesh*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangaloreꢀ560012,
India.
KEYWORDS. Conformation • Halogen bonding • Iodothyronamine • Thyroid hormone • XꢀRay
crystal structure
ABSTRACT. Iodothyronamines (TAMs) are endogenous thyroid hormone (TH) metabolites,
which are proposed to be bioꢀsynthesized by decarboxylation of βꢀalanine side chain of THs.
Iodothyronine deiodinases (DIOs) mediate phenolic and tyrosyl ring deiodinations of thyroid
hormones and play an important role in thyroid hormone homeostasis. These enzymes also
accept iodothyronamines as substrates for deiodination, but the binding affinities of TAMs to
DIOs are lower than that of THs. In this paper, we report, for the first time, the formation of
halogen bonding in various iodothyronamines by using single crystal Xꢀray studies. We also
describe the conformational changes that take place in the structures of THs and TAMs upon
deiodination. Density Functional Theory calculations were performed to understand the halogen
bonded assembly and their biological implications.
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INTRODUCTION
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Thyroid gland in the human endocrine system produces Lꢀthyroxine (T4) as a prohormone.
Three isoforms of the selenocysteineꢀcontaining enzymes, iodothyronine deiodinases (DIOs),
play an important role in maintaining the thyroid hormone levels throughout the body.1ꢀ5 Type 1
deiodinase (DIO1) catalyzes both the phenolic (5') and tyrosyl ring (5) deiodination of T4 to
form 3,3',5ꢀtriiodothyronine (T3) and 3,3',5'ꢀtriiodothyronine (rT3), respectively. In contrast,
type 2 (DIO2) and type 3 (DIO3) deiodinase catalyze selective phenolic and tyrosyl ring
deiodination of T4, respectively. Both T3 and rT3 undergo further deiodinations by all three
enzymes to form the biologically inactive metabolite Lꢀthyronine (T0) as shown in Scheme 1.6ꢀ11
In addition to the enzymeꢀcatalyzed deiodination pathways, thyroid hormones (THs) also
undergo decarboxylation of βꢀalanine side chain to form iodothyronamines (TAMs). Although a
total of nine iodothyronamine derivatives are possible, only 3ꢀiodothyronamine (3ꢀT1AM) and
thyronamine (T0AM) were detected so far in the serum of various organisms.12 These
metabolites have been shown to induce a decrease in the body temperature (hypothermia) and
cardiac output (bradycardia), when injected to mouse in pharmacological doses.12,13 3ꢀT1AM is
also known to activate trace amineꢀassociated receptor, an orphan Gꢀprotein coupled receptor.14
Recently, TAMs have been identified as substrates for DIOs.15 However, some of the THs and
TAMs do not undergo deiodination by these enzymes (Scheme 1),15,16ꢀ21 indicating that the
change in the structure and conformations of THs upon decarboxylation may alter the binding of
TAMs at the enzyme active site.
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Recently, we reported that T4 can exist in different conformations in the solid state.22 These
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two different solid forms have been studied by single crystal and powder Xꢀray diffraction, C
NMR and Raman spectroscopy. Interestingly, the two forms showed different solubility and
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Scheme 1. Deiodination of thyroid hormones and iodothyronamines by DIOs.
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I
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HO
O
I
DIO 1, DIO 3
R= CO2H / H
DIO 1, DIO 2
R= CO2H
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I
I
H R
NH2
T4, R= CO2H
T4AM, R= H
I
I
H
HO
O
H
HO
I
O
I
DIO 1, DIO 2
DIO 1, DIO 3
R= CO2H / H
I
I
R= CO2H / H
H R
H R
NH2
T3, R= CO2H
T3AM, R= H
rT3, R= CO2H
rT3AM, R= H
NH2
DIO 1 R= CO2H
R= CO2H / H DIO 1, DIO 3
I
H
H
HO
O
I
HO
O
H
HO
I
O
H
H
I
I
H
I
H R
NH2
H R
NH2
H R
NH2
3',5'-T2, R= CO2H
3',5'-T2AM, R= H
3,5-T2, R= CO2H
3,5-T2AM, R= H
3,3'-T2, R= CO2H
3,3'-T2AM, R= H
DIO 1, DIO 3 R= CO2H / H
H
R= CO2H / H DIO 1, DIO 2
H
R= CO2H / H
DIO 3
R= CO2H / H
DIO 1
HO
O
H
HO
O
H
H
I
I
H
H R
NH2
H R
NH2
3-T1, R= CO2H
3-T1AM, R= H
H
3'-T1, R= CO2H
3'-T1AM, R= H
HO
O
H
DIO 3
R= H
DIO 1
H
H
H R
NH2
R= CO2H
T0, R= CO2H
T0AM, R= H
optical rotation in different solvents. The relative orientations of two iodinated phenyl rings and
the amino acid moiety in T4 have been shown to affect the halogen bonding ability of the iodine
atoms.22 The conformationꢀdependent halogen bonding may also play an important role in the
binding of T4 to its transport protein transthyretin (TTR).23 Although iodothyronamines exhibit
different binding behavior to their transport protein, apolipoprotein Bꢀ100 (apoBꢀ100),24 the
involvement of iodine atoms of iodothyronamines in halogen bonding is not known. In this
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paper, we report on the Xꢀray structures and solid state conformations of different
iodothyronamines and compare them with that of thyroid hormones. We also discuss the
different types of halogen bonding interactions and its biological implications.
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RESULTS AND DISCUSSION
Iodothyronamines were synthesized by following a procedure reported by Scanlan and coꢀ
workers with minor modifications (Scheme S01, Supporting Information).25 All the
iodothyronamines and T3 were recrystallized as trifluoroacetate salts. The diiodo derivative 3,5ꢀ
T2 was recrystallized in the zwitterionic form, but our attemps to crystalize T4AM were
unsuccessful. Needleꢀshaped crystals of trifluoroacetate salt of T3 were obtained by slow
evaporation of a mixture of T3 and trifluoroacetic acid (TFA) in ethyl acetate and diethylether.
The crystals were of monoclinic space group P 21.26 The torsional angles, which are mostly used
to describe the conformations of thyroid hormones and their metabolites, are shown in Figure
1A. The torsional angles χ1 and χ2 determine the orientation of the βꢀalanine side chain with
respect to tyrosyl ring and phenolic ring. In T4ꢀdiethanolamine salt, both the cisoid and transoid
conformers of T4 were observed in the unit cell.27 The βꢀalanine side chain and phenolic ring of
thyroid hormones are placed in the same face and opposite faces of the plane of tyrosyl ring in
the cisoid and transoid conformers, respectively. In contrast to the structure of T4, only transoid
conformer is observed in the single crystal Xꢀray structure of trifluoroacetate salt of T3 (Figure
1B). An interesting characteristic of the structure of thyroid hormones is the mutual
perpendicularity of the tyrosyl ring and phenolic ring. This unique conformation of the diphenyl
ether linkage is important for the biological activity of T4.28,29 The two torsional angles, φ and φ'
(Figure 1A), appear to control the perpendicular arrangement of the phenolic and tyrosyl rings,
with one iodine atom in the phenolic ring occupying a position close to the tyrosyl ring in T4.
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Figure 1. (A) Different torsional angles used for defining the conformations of T3. The atomꢀ
numbering scheme, shown for T3, is applicable for all the thyroid hormones and their
metabolites. Hydrogen atoms are omitted for clarity. CꢀI bond lengths (Å) and C4ꢀO3ꢀC1' bond
angles (⁰) in the ball and stick model of single crystal Xꢀray structures of T3 (B), T3AM (C) and
rT3AM (D). Only one conformer of rT3AM has been shown. Trifluoroacetate and water are
omitted for clarity.
Although for an ideal mutual perpendicularity between the two rings, the values of φ and φ' are
expected to be 90⁰ and 0⁰, respectively, both the values deviate from the ideal ones. Therefore,
the values of φ and φ' indicate the “swinging” and “twisting” of the phenolic ring about the C4ꢀ
O3 and O3ꢀC1' axes, respectively. The values of φ and φ' for the trifluroacetate salt of T3 are
88.4⁰ and 20⁰, respectively (Table 1). Unlike T4, the 3'ꢀiodine atom in T3 can occupy both the
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positions, which are close to the tyrosyl ring (proximal conformer) or away from the tyrosyl ring
(distal conformer). Both the proximal and distal conformers of zwitterꢀionic form of T3 have
been reported earlier.30,31 In contrast, only the distal conformer was observed for 3,5,3'ꢀtriiodoꢀLꢀ
thyronine methyl ester and only the proximal conformer has been observed for 3,5,3'ꢀtriiodoꢀLꢀ
thyronine hydrate hydrochloride.32,33 For the trifluoroacetate salt of T3, we observed only the
proximal conformer (Figure 1B).
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Table 1. Conformational parameters (χ1, χ2, φ and φ') for different thyroid hormone metabolites.
χ1 (⁰)
ꢀ72.8
χ2 (⁰)
ꢀ78.8
φ (⁰)
88.4
φ' (⁰)
20.0
T3
T3AM
ꢀ164.7
ꢀ73.4
97.7
ꢀ2.0
rT3[a][b]
52, 52
105, 104
ꢀ56.7, ꢀ94.2
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8, ꢀ6
86, 87
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ꢀ30.7
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3,5ꢀT2
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ꢀ57.3
109.8, 107.2
114.1
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3,3'ꢀT2AM
3',5'ꢀT2AM
3ꢀT1AM
3'ꢀT1AM[b]
T0AM
ꢀ171.5
ꢀ69.0
104.4
ꢀ33.9
ꢀ176.9
ꢀ48.8
112.8
ꢀ20.7
ꢀ179.4
89.6
119.2
ꢀ40.8
178.1
ꢀ69.8
102.3
ꢀ22.8
ꢀ174.7, 173.0
179.3
ꢀ88.2, ꢀ92.2
ꢀ87.1
164.2, 49.9
142.8
ꢀ72.7, 45.9
ꢀ73.1
[a] The values have been taken from previously reported structure.34 [b] Two values for
each parameter correspond to two independent conformers observed in the asymmetric unit.
It is interesting to note that T3AM, the decarboxylated metabolite of T3, also adopts more
stable transoid geometry similar to T3 (Figure 1C).26 However, on removal of the carboxyl
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group from T3, the aminoethyl side chain in T3AM becomes almost parallel to the plane of
tyrosylꢀring (Figure 1B and 1C). In agreement with this observation, χ1 changes from ꢀ72.8⁰, in
T3, to ꢀ164.7⁰, in T3AM (Table 1). Although the “swinging” of the phenolicꢀring about C4ꢀO3
axis is almost similar in T3AM (97.7⁰) and T3 (88.4⁰), the “twisting” of the phenolicꢀring about
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the O3ꢀC1' axis is significantly less in T3AM (ꢀ2⁰) than in T3 (20⁰) (Table1). These observations
indicate that the decarboxylation of βꢀalanine side chain of T3 increase the mutual
perpendicularity between the two iodinated phenyl rings in T3AM. T3AM has been previously
crystallized as a bis(salicylato)borate salt.35 Although the structure of this salt exhibits twistꢀ
skewed geometry of the tyrosyl and phenolic ring as well as proximal conformation of the 3'ꢀ
iodine atom as observed in the structure of trifluoroacetate salt, the aminoethyl side chain and the
phenolic ring exhibited cisoid geometry with respect to the tyrosyl ring. This observation
indicates that the counter anion has significant effect on the conformation of T3AM. The tyrosyl
ring CꢀI bonds in T3AM are slightly elongated (average bond length 2.092 Å) than in T3
(average bond length 2.084 Å) whereas the phenolic ring CꢀI bond is significantly elongated in
T3AM (2.131 Å) than in T3 (2.078 Å). Interestingly, unlike T3, T3AM does not undergo
phenolic ring deiodination by DIO1.15
The conformational flexibility and internal dynamics in thyroid hormones in solution has been
studied earlier by NMR spectroscopy.36ꢀ38 Similar to the threeꢀdimensional structures of thyroid
hormones observed in the Xꢀray structures, these studies also indicated the roughly perpendicular
arrangement of the two iodinated rings and the rapid movement of the phenolic ring about the
diphenyl ether linkage. As discussed earlier, rotation of the phenolic ring about the O3ꢀC1' bond
in T3 gives rise to the formation of proximal and distal conformers. Temperatureꢀdependent
NMR spectroscopic investigation indicated that in solution, T3 adopts both the distal and
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proximal conformations with almost equal populations, which is contrast to our observation that
both T3 and T3AM exhibit proximal conformation in the solid state. The rotational energy
barrier between these two conformations was calculated to be about 36 kJmolꢀ1 and this energy
barrier was shown to be independent of the nature of the side chain functionalities.39,40 These
observations indicate that in solution, T3AM may also behave similarly to T3. Interestingly,
conversion between the cisoid and transoid conformers of thyroid hormones has also been
observed in solution and NMR experiments suggested that these two conformers are almost
equally populated in solution.38 It is possible that depending on the crystallization medium and
intermolecular interactions, the transoid conformer of trifluoroacetate salts of T3 and T3AM
crystallize out favorably. Although the conformations of tyrosyl and phenolic ring (φ and φ') of
T3 bound to the receptor and transport proteins are more or less similar to that observed in the
crystal structure of trafluoroacetate salt of T3, the conformations of amino acid side chain (χ1, χ2
and ψ) are observed to be significantly different in the bound and free forms of T3.41ꢀ44 It should
be noted that the tyrosl and phenolic ring of T3 bound to the receptor TRα1 (human) adopts a
conformation (φ = 86.3⁰ and φ' = 20.6⁰), which is almost similar to that observed in the crystal
structure of trafluoroacetate salt of T3.41
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It is known that rT3AM can be bioꢀsynthesized either by decarboxylation of rT3 or by tyrosyl
ring deiodination of T4AM by DIO1 or DIO3. Two independent conformations were observed in
the assymmetric unit of rT3AM.45 Both the conformers adopt transoid geometry (Figure 1D).
These observations are very similar to those observed in the crystal structure of rT3 by Okabe
and coworkers.34 Similar to the decarboxylation of T3, the decarboxylation of rT3 makes the
aminoethyl side chain in rT3AM almost parallel to the plane of tyrosyl ring (χ1=52⁰ in rT3; χ1=
167.7⁰ and 178.9⁰ in rT3AM, Table 1). The change in χ2 is almost negligible between T3 and
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T3AM, whereas χ2 changes significantly on decarboxylation of rT3 to rT3AM (Table 1). A
comparison of φ and φ' angles in rT3 with those in T3, T3AM and rT3AM reveal some
interesting structural features of rT3. In T3, T3AM or rT3AM, φ and φ' are close to 90⁰ and 0⁰,
whereas in rT3, they are close to 0⁰ and 90⁰ (Table 1). However, deviation in the values of φ and
φ' from the ideal values, that is 90⁰ and 0⁰, respectively, are higher in rT3AM than in T3AM
(Table 1), indicating that the mutual perpendicularity between two phenyl rings is more in
T3AM than in rT3AM. In rT3, phenolic ring CꢀI bonds (average CꢀI bond length 2.12 Å) are
more elongated than tyrosyl ring CꢀI bond (average bond length 2.00 Å).34 By contrast, in T3, the
tyrosyl ring CꢀI bonds (average bond length 2.084 Å) are more elongated than the phenolic ring
CꢀI bond (bond length 2.078 Å) (Figure 1B). These observations indicate that the tyrosyl and
phenolic ring iodine atoms of T3 and rT3, respectively, are easier to remove than the iodine
atoms on the other ring. In rT3AM, CꢀI bonds in both the tyrosyl (bond length 2.093 Å) and
phenolic ring (average bond length 2.094 Å) are of almost similar length. Comparison of these
CꢀI bond lengths with those in rT3 suggests that the removal of phenolic ring iodine atom from
rT3 is easier than from rT3AM but the removal of tyrosyl ring iodine atom from rT3AM is easier
than from rT3. These results are quite similar to the observed rates of deiodination of thyroid
hormones and iodothyronamines by DIOs.15 However, in addition to the CꢀI bond strength, the
positive charge on the iodine atom and the strength of Se···I halogen bonding also plays
important role in the CꢀI bond activation and deiodination of thyroid hormones.
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Solid state crystal packing of thyroid hormones and iodothyronamines shows extensive Hꢀ
bonding generally involving the amine group, 4'ꢀOH group, carboxylic acid group (for thyroid
hormones) and trifluoroacetate moiety (Figures S19B and S20, Supporting Information). In
addition to these Hꢀbonding interactions, the most interesting feature of the crystal packing of
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thyroid hormones and iodothyronamines is the I···I nonꢀcovalent interactions. Intermolecular
halogen···halogen (X···X) interactions are extensively studied in halogenated phenols,
halobenzoic acids, halogenated thiacalix[4]arenes and many other halogenated systems.46ꢀ54
Furthermore, X¯···X interactions have been discovered as the pillars for anion recognition
processes.52ꢀ54 Recently, our studies on the halogen bondꢀmediated deiodination of thyroid
hormones and dehalogenation of halogenated nucleosides have attracted significant research
attention.55ꢀ60 We have shown that in addition to the Se···I halogen bonding, I···I interactions can
also help in polarizing CꢀI bonds and in the deiodination of thyroid hormones by deiodinase
mimics.26 Depending on the angular contacts of the halogen atoms involved in the interaction,
two different kinds of X···X interactions have been proposed in the literature.61,62 In type I
interactions, the values of θ1 and θ2, that is the CꢀX1···X2 and X1···X2ꢀC angles (Figure 2A),
respectively, are almost identical. These contacts appear due to a close packing of the molecules
and it is common for all the halogens. Type I X···X interactions are generally not considered as
halogen bonding63ꢀ65 interactions. Type II contacts are generally recognized by the perpendicular
arrangements of the two CꢀX bonds i.e. θ1~180⁰ and θ2~90⁰ (Figure 2A). These interactions are
called halogen bonding because the lateral negative potential of one halogen interacts with the
positively charged σꢀhole66,67 of the other halogen atom. There are some X···X interactions that
lie in between the type I and type II interactions. These are termed as quasiꢀtype I/type II X···X
interactions. In a recent review, Desiraju and coꢀworkers have summarized the criteria for
different types of X···X interactions.62 For type I interactions, |θ1ꢀθ2| should be in between 0⁰ꢀ
15⁰, whereas for type II interactions, |θ1ꢀθ2| should be greater than 30⁰. X···X interactions having
|θ1ꢀθ2| between 15⁰ and 30⁰ can be termed as quasiꢀtype I/type II.
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Two types of I···I interactions are observed in the crystal packing of T3. Both of these
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Figure 2. (A) Schematic representations of type I and type II X···X interactions. (B) Capped
stick model of crystal packing of T3 indicating two types of I···I interactions. Hydrogen atoms
are omitted for clarity. (C) NBO analysis of the type II I···I interaction in T3 indicating the
natural charges on each iodine atoms.
interactions are observed between the adjacent layers of T3. Interestingly, only tyrosyl ring
iodine atoms form these I···I contacts. The orientations of the T3 molecules in the adjacent
layers appeared to be antiparallel to each other. Each 5ꢀiodine atoms of T3 in one layer form two
identical I···I contacts with the 5ꢀiodine atoms of two T3 molecules in the adjacent layer as
shown in Figure 2B. Similarly, 3ꢀiodine atoms of T3 in one layer also form two identical I···I
contacts with 3ꢀiodine atoms of two T3 molecules in the adjacent layer. However, I···I contacts
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formed by 3ꢀiodine atoms are weaker (I···I distance, 3.861 Å) than those formed by 5ꢀiodine
atoms (I···I distance, 3.643 Å). These distances are 2.5% and 8%, respectively, shorter than the
sum of the van der Waals radii of iodine (1.98 Å). These I···I interactions were categorized by
analyzing the values of θ1 and θ2 (Figure 2B, insets). The contacts formed by the 5ꢀiodine atom
(I···I distance, 3.643 Å) appear to be of type II category with a |θ1ꢀθ2| of 73.4⁰ whereas the
contacts formed by 5'ꢀiodine atom (I···I distance, 3.861 Å) appear to be of type III or quasi type
I/type II category with a |θ1ꢀθ2| of 26.6⁰. For these I···I interactions, two adjacent layers of T3
are brought close to each other by water mediated hydrogen bonds between 4'ꢀOH group and
carboxylic acid group, 4'ꢀOH group and trifluoroacetate, and salt bridge interaction between αꢀ
amine group and trifluoroacetate (Figure S19B, Supporting Information).
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To understand the electronic nature of the type II I···I halogen bonds, natural bond orbital
(NBO) analysis, using B3LYP hybrid functional, 6ꢀ311G** basis set for iodine and 6ꢀ31+G*
basis set for all other atoms, was carried out on the I···I bonded T3 molecules. As each T3
molecule, in one layer, forms two identical type II I···I halogen bonds with two T3 molecules in
the adjacent layer, coordinates of such three T3 molecules were taken from the crystal structure.
The overall positive charge on the iodine atoms IA, IB and IC are 0.296, 0.239 and 0.228
respectively (Figure 2C). The NBO calculations on the individual T3 molecules, involved in the
type II I···I interaction, using the same level of theory and basis sets afforded the charges on IA,
IB and IC as 0.261, 0.260 and 0.260, respectively. These results indicate that IA and IB act as
halogen bond acceptors (electron donors) in the IA···IB and IB···IC interactions, respectively. As
these I···I interactions extend through the threeꢀdimensional crystal packing, all the iodine atoms
involved in the type II interaction can be considered as a halogen bond donor as well as a
halogen bond acceptor. Although the I···I distances for all the type II interactions are same
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(3.643 Å), the strength of the halogen bonding between each pair of the iodine atoms appeared to
be slightly different. For the assembly shown in Figure 2C, the interaction energy between IA and
IB is 5.66 Kcal/mol whereas the energy between IB and IC is 6.14 Kcal/mol. By contrast to type II
interaction, in type III I···I interaction, both the iodine atoms donate electron density to the σꢀ
hole of the other iodine atom (Figure S26, Supporting Information). However, in each set of type
III interactions, the donation of electron density is more favoured in one direction. These type of
interactions are probably facilitated by relaxation from almost perpendicular arrangements of
two CꢀI units observed in type II interactions. Type III I···I interactions in T3 are 2.5ꢀ2.8 times
weaker than the type II I···I interactions.
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In contrast to T3, T3AM does not show I···I interactions in their solid state crystal packing.
However, rT3AM shows both type I and type II I···I interactions in the crystal packing.
Interestingly, only the phenolicꢀring iodine atoms take part in these I···I interactions. In the
asymmetric unit, two rT3AM molecules are hydrogen bonded via 4'ꢀOH groups (Figure S20B,
Supporting Information). Both the 3'ꢀ and 5'ꢀiodine atoms of rT3AM, in each layer, form a type
II I···I contact with the 5'ꢀ and 3'ꢀiodine atoms, respectively, of rT3AM molecules in the adjacent
layers as shown in Figure 3A. However, the geometries (θ1, θ2 and I···I distance) of these two
type II contacts are not identical. One set of type II interactions are identified by I···I distance of
3.662 Å and |θ1ꢀθ2| of 58.8⁰ whereas the other set is recognized by I···I distance of 3.727 Å and
|θ1ꢀθ2| of 57.4⁰. These two interatomic distances are 7.5% and 5.9%, respectively, shorter than
the sum of the Van der Waals radii of iodine (1.98 Å). Type I contacts, as shown in Figure 3A,
has an I···I distance of 3.747 Å and |θ1ꢀθ2| of 12.8⁰.
The NBO analysis on the halogenꢀbonded assembly has been carried out by following a similar
procedure as described for T3. In Figure 3B, the interactions among the four iodine atoms are
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Figure 3. (A) Capped stick model of crystal packing of rT3AM indicating the geometries of type
I and type II I···I interactions. Trifluoroacetate and water molecules are omitted for clarity. (B)
NBO analysis of the type I and type II I···I interactions in rT3AM. Hydrogen atoms are omitted
for clarity. The table indicates the natural charges on each iodine atoms at the halogen bonded
(Xꢀbonded) and isolated state, and the halogen bonding (Xꢀbonding) energies. (C) Space filling
model of crystal packing of rT3AM indicating that both the I···I halogen bonding and Hꢀbonding
involving rT3AM, trifluoroacetate and water molecules help in close packing of rT3AM
molecules.
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shown. The directions of arrow indicate the donation of electron density from one iodine atom to
the σꢀhole of other. The positive charge on IA and IB, 0.222 and 0.214, respectively, in the
isolated state changes to 0.188 and 0.221, respectively, after halogen bonding interaction (Figure
3B, see table). Similarly, the positive charge on IC and ID, 0.210 and 0.231, respectively, in the
isolated state changes to 0.208 and 0.200, respectively, after halogen bonding interaction. The
direction of transfer of electron density from one iodine atom to another has been derived from
the NBO calculation. The interaction energy between IA and IB is 5.05 Kcalmolꢀ1 whereas the
same between IC and ID is 4.24 Kcalmolꢀ1. Type I interaction between IB and IC involves a very
weak electron donation from IC to IB with interaction energy of 1.3 Kcal.molꢀ1. Together with Hꢀ
bonding among rT3AM, trifluoroacetate and water molecules, I···I halogen bonding interaction
helps in close packing of layers of rT3AM (Figure 3C).
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The diiodo derivative, 3,5ꢀT2, with both the iodine atoms in the tyrosyl ring, is an important
metabolite of thyroid hormones. Chronic administration of 3,5ꢀT2 in wistar rats has been shown
to exhibit selective thyromimetic activity68 with a decrease in thyroidal iodide uptake,
thyroperoxidase (TPO)ꢀactivity, NADPH oxidase 4 (NOX 4)ꢀactivity, Dio1ꢀactivity and with an
increase in expression of thyroid stimulating hormone (TSH) receptor and dual oxidase
(DUOX)ꢀactivity.69 3,5ꢀT2 treatment of rats also resulted in decrease in TSH, T3 and T4
concentration in a dose dependent manner. 3,5ꢀT2 administration to rats receiving fat enriched
diet reduced both the adiposity and bodyꢀweight gain without affecting the serum TSH, T3 and
T4 concentration.70 This compound has also been shown to have significant thyromimetic
activity in in vitro assays involving nuclear receptors (TRs) and GH3 cells.71 However, the
biosynthesis of 3,5ꢀT2 is still not clear. Although in many experiments T3 has been excluded as
substrate for phenolicꢀring deiodination by DIO1, homogenates of NCLPꢀ6E monkey
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hepatocarcinoma cells expressing DIO1 enzymatic activity were reported to mediate phenolicꢀ
ring deiodination of T3 to produce 3,5ꢀT2.16,17 Solution state structure of 3,5ꢀT2 was investigated
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earlier by spinꢀlattice relaxation time analysis of H and 13C NMR spectra, and 13C nuclear
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overhauser enhancement factors.38 Cody and coꢀworkers reported the first crystal structure of a
1:1 complex of 3,5ꢀT2 and Nꢀmethylacetamide.72
In the present study, we obtained the crystal structure of the zwitterionic form of 3,5ꢀT2.
Similar to the Nꢀacetamide salt,72 the zwitterionic form of 3,5ꢀT2 also exhibits a transoid
geometry of the phenolic ring and the amino acid backbone with respect to the tyrosyl ring
(Figure 4A). Removal of one iodine atom from the phenolic ring of T3 changes the values of χ1
and χ2 from ꢀ72.8⁰ to ꢀ57.3⁰ and from ꢀ78.8⁰ to ꢀ55.1⁰, respectively (Table 1). Interestingly, in
3,5ꢀT2 both the swinging and twisting of the phenolic ring about C4ꢀO3 and O3ꢀC1' axis,
respectively, is significantly different from those in T3 (see the values of φ and φ' from Table 1).
Furthermore, it should be noted that in 3,5ꢀT2, the twisting of phenolic ring is in the opposite
direction compared to that in T3. Removal of carboxyl group from 3,5ꢀT2 makes the aminoethyl
side chain of 3,5ꢀT2AM coplanar with the tyrosyl ring (Figure 4A and 4B). In agreement, similar
to the decarboxylation of T3 to T3AM, the value of χ1 changes from ꢀ57.3⁰ in 3,5ꢀT2 to ꢀ171.5⁰
in 3,5ꢀT2AM. By contrast to the differences in the values of χ1 and χ2 between T3 and 3,5ꢀT2,
these torsional angles were found to be almost similar in T3AM and 3,5ꢀT2AM (Table 1).
Although φ is almost similar in T3AM and 3,5ꢀ T2AM, φ' values are significantly different in
these two thyroid hormone metabolites. It should be noted that the average CꢀI bond lengths for
tyrosyl ring iodine atoms are almost similar in 3,5ꢀT2AM (2.089 Å) and T3AM (2.084 Å).
3,3'ꢀT2AM and 3',5'ꢀT2AM are two diiodinated iodothyronamines which can be bioꢀ
synthesized by tyrosyl ring deiodination of T3AM and rT3AM, respectively, by DIO1.15
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Figure 4. CꢀI bond lengths (Å) and C4ꢀO3ꢀC1' bond angles (⁰) in the ball and stick model of
single crystal Xꢀray structures of 3,5ꢀT2 (A), 3,5ꢀT2AM (B), 3,3'ꢀT2AM (C) and 3',5'ꢀT2AM
(D). Trifluoroacetate and water molecules are omitted for clarity. (E) Ball and stick model of
solid state crystal packing of 3,5ꢀT2AM indicating the geometries of type II I···I and F···I
interactions. Hydrogen atoms and water molecules are omitted for clarity.
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Phenolic ring deiodination of rT3AM by DIO1 can also produce 3,3'ꢀT2AM. Like T3AM and
rT3AM, 3,3'ꢀT2AM also exists in the transoid geometry (Figure 4C). Interestingly, the 3'ꢀiodine
atom in 3,3'ꢀT2AM adopts distal position with respect to the tyrosyl position. This observation
indicates that the removal of one tyrosyl ring iodine from T3AM, in which 3'ꢀiodine adopts
proximal position, results in rotation of the phenolic ring around O3ꢀC1' bond to place the 3'ꢀ
iodine in a distal position. This structural rearrangement results in change in swinging and
twisting of the phenolic ring of 3,3'ꢀT2AM by about 15ꢀ20⁰ compared to T3AM (Table 1).
However, the arrangement of the phenolic ring of 3,3'ꢀT2AM is almost identical to that in
rT3AM (see the values of φ and φ' from Table 1), indicating that the phenolic ring deiodination
of rT3AM to 3,3'ꢀT2AM does not affect the mutual perpendicularity of the two substituted
phenyl rings. By contrast, tyrosyl ring deiodination of rT3AM to 3',5'ꢀT2AM changes the φ and
φ' by almost 12⁰ and 27⁰, respectively (Table 1). Surprisingly, unlike all other iodothyronamine
derivatives, 3',5'ꢀT2AM exhibits a cisoid geometry of phenolic ring and aminoethyl side chain
with respect to tyrosyl ring (Figure 4D). In agreement with this, the sign of χ2 is opposite to that
observed in other iodothyronamine metabolites (Table 1).
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All the diiodo thyroid hormone derivatives and their metabolites show extensive Hꢀbonding
involving water molecules and trifluoroacetate (for iodothyronamines) moieties (Figures S21ꢀ
S24, Supporting Information). However, among the diꢀiodinated iodothyronamines, only 3,5ꢀ
T2AM forms I···I interactions in the crystal packing. These interactions are type II in nature with
I···I distances of 3.779 Å and |θ1ꢀθ2| of 44.2⁰ (Figure 4E). This nonꢀcovalent I···I distance is
4.6% shorter than the sum of the van der Waals radii of the iodine (1.98 Å). In the isolated state,
the positive charge on IA and IB (Figure 4E) are 0.257 and 0.254, respectively. After formation of
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halogen bonding by donation of electron density from IA to the σꢀhole of IB, the charges on IA
and IB become 0.267 and 0.226, respectively. NBO calculations, using the similar basis sets and
hybrid functional as discussed earlier, on this halogenꢀbonded assembly gave the interaction
energy between IA and IB as 3.61 Kcal/mol. Another interesting feature of the crystal packing of
3,5ꢀT2AM is the interꢀhalogen F···I halogen bonding interaction between trifluoroacetate and
3,5ꢀT2AM (Figure 4E). These contacts are also type II in nature with F···I distance of 3.261 Å
and |θ1ꢀθ2| of 44.8⁰. These F···I distances are 5.5% shorter than the sum of van der Waals radii of
fluorine (1.47 Å) and iodine (1.98 Å). For these interactions, the electron density is transferred
from fluorine atoms (halogen bond acceptor) to the σꢀhole of iodine atoms (halogen bond donor).
A similar type II O···I halogen bonding was also observed in the crystal packing of 3,3'ꢀT2AM
(Figure S23, Supporting Information).
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Phenolic ring deiodination of 3,3'ꢀT2AM by DIO1 as well as tyrosyl ring deiodination of 3,5ꢀ
T2AM by DIO1 and DIO3 produces 3ꢀiodothyronamine (3ꢀT1AM).15 The other monoiodo
metabolite 3'ꢀT1AM is produced by tyrosyl ring deiodination of 3,3'ꢀT2AM by DIO3 as well as
by phenolic ring deiodination of 3',5'ꢀT2AM by DIO1 and DIO2. 3ꢀT1AM further undergoes
tyrosyl ring deiodination by DIO3 to produce the completely deiodinated metabolite T0AM
whereas 3'ꢀT1AM does not undergo any further deiodination to form T0AM (Scheme 1).15 By
contrast, 3'ꢀT1 undergo phenolic deiodination by DIO1 to form T0 whereas 3ꢀT1 does not
undergo deiodination to form T0.16ꢀ21 These observations clearly indicate that the
decarboxylation of βꢀalanine side chain of the thyroid hormones changes the substrate specificity
of the DIOs. Recently, Scanlan and coꢀworkers have identified apolipoprotein Bꢀ100 (apoBꢀ
100), the protein component of several low density lipoprotein particles, to be a specific
transporter (KD = 17 nM) of 3ꢀT1AM with a 3ꢀT1AM/apoBꢀ100 stoichiometry of 1:1.24 They
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found 3'ꢀT1AM and T0AM to be very weak competitive inhibitors of binding of 3ꢀT1AM to
apoBꢀ100 with IC50 values of 259 nM and 268 nM, respectively. However, the three dimensional
structures and conformations of 3ꢀT1AM, 3'ꢀT1AM and T0AM are not known. It is possible that
3ꢀT1AM has the optimized structure and conformational parameters for its maximum transport
and physiological activity.
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Trifluoroacetate salt of 3ꢀT1AM crystallizes in monoclinic space group P 21/C. The transoid
conformation of 3ꢀT1AM exhibits antiparallel arrangement of the amino group (χ1 = 178.1⁰ and
χ2 = ꢀ69.8⁰) with respect to the tyrosyl ring (Figure 5A and Table 1). The “swinging” (φ) of the
phenolicꢀring about C4ꢀO3 axis is 102.3⁰ whereas the twisting (φ') of the same about O3ꢀC1' axis
is ꢀ22.8⁰ (Table 1). Removal of the tyrosyl ring iodine from 3,5ꢀT2AM decreases the φ' by
almost 11⁰ without affecting φ much (Table 1). In contrast, removal of phenolic ring iodine from
3,3'ꢀT2AM decreases the φ by almost 10⁰ without affecting φ' much. These observations
indicate that the removal of tyrosyl and phenolic ring iodine atoms from 3,5ꢀT2AM and 3,3'ꢀ
T2AM, respectively, have nearly opposite effects on the conformations of the deiodinated
product 3ꢀT1AM. The other monoiodo metabolite 3'ꢀT1AM crystallizes in monoclinic space
group P 21/n. Interestingly, two independent conformations were observed in the asymmetric unit
of 3'ꢀT1AM. Both the conformations have transoid geometry of the phenolic ring and the
aminoethyl side chain with respect to the tyrosyl ring (Figure 5B).
However, analysis of the positions of the iodine atom revealed that in one conformation 3'ꢀ
iodine atom is close to tyrosyl ring (proximal conformer), whereas in the other conformer, the
same is away from the tyrosyl ring (distal conformer). The swinging of phenolic ring about C4ꢀ
O3 axis is 164.2⁰ in the proximal conformer whereas the same in distal conformer is 49.9⁰
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(Table1). The twisting of the phenolic ring about O3ꢀC1' axis is ꢀ72.7⁰ in the proximal conformer
whereas the same in the distal conformer is 45.9⁰. Therefore, the presence of the iodine atom in
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Figure 5. Bond lengths (Å) and C4ꢀO3ꢀC1' bond angles (⁰) in the ball and stick model of Xꢀray
crystal structures of 3ꢀT1AM (A), 3'ꢀT1AM (B) and T0AM (C). Both the proximal and distal
conformers of 3'ꢀT1AM are shown. (D) Ball and stick model of crystal packing of 3ꢀT1AM
indicating the geometries of type I I···I interaction. Trifluoroacetate and water molecules are
omitted from all the structures for clarity.
the phenolic ring significantly affects the “twisting” and “swinging” of the same ring compared
to 3ꢀT1AM and other iodothyronamine metabolites. These significant structural differences
between 3ꢀT1AM and 3'ꢀT1AM may account for different substrate specificities of DIO1
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towards monoiodothyroanmines described earlier (Scheme 1). Completely deiodinated
thyronamine (T0AM) also shows transoid conformation (Figure 5C). The “twisting” and
“swinging” of the phenolicꢀring are 142.8⁰ and ꢀ73.1⁰, respectively (Table 1). In the crystal
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packing of 3ꢀT1AM, type I I···I interactions, with a I···I distance of 3.708 Å and |θ1ꢀθ2| of 0⁰,
were observed (Figure 5D). This nonꢀbonded distance between two iodine atoms is 6.4% shorter
than the sum of the van der Waals radii of iodine (1.98 Å). However, 3'ꢀT1AM does not form
I···I contacts in the crystal packing.
Although the completely deiodinated species T0AM shows similar physiological activities like
3ꢀT1AM, T0AM was found to be less potent than 3ꢀT1AM in inducing hypothermia, bradycardia
and hyperglycemia.12,13 Structural and conformational differences between T0AM and 3ꢀT1AM
may affect their binding to the receptors and transport proteins. It is known that the backbone
nitrogen atom of Leu110 in transthyretin plays an important role in the binding of thyroxine by
forming a N···I halogen bond.23 Iodine atom in the 3ꢀposition of 3ꢀT1AM may form halogen
bonding interaction with receptor and transport protein and this may increase the affnity of 3ꢀ
T1AM to receptors and transport proteins compared to T0AM. However, further studies
including coꢀcrystal structures of 3ꢀT1AM and T0AM with transport protein or receptor are
requirred to understand the different physiological activites of these interesting decarboxylated
thyroid hormone metabolites.
CONCLUSIONS
In this paper, we have discussed the structures and conformations of iodothyronamines and some
of the thyroid hormones. Iodothyronamines are decarboxylated thyroid hormone metabolites and
we have shown that the decarboxylation of βꢀalanine side chain of thyroid hormones affects the
threeꢀdimensional structure of iodothyronamines significantly. Iodothyronamines are recognised
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as substrates of iodothyronine deiodinses (DIOs), although some of the deiodinated
iodothyronamines are not accepted by DIOs as substrates. We have shown that the deiodination
of iodothyronamines results in significant alterations in the conformation of the deiodinated
product which can account for the different substrate specificities of DIOs. These crystal
structures would help in understanding the importance of conformational parameters in the
binding of thyroid hormones and iodothyronamines to their transport proteins and cellular
receptors. The presence of I···I halogen bonding in the crystal packing of thyroid hormones and
iodothyronamines uncovers the potential of the iodine atoms in the these hormones to form
halogen bonding with donor atoms from receptors or enzymes.
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EXPERIMENTAL SECTION
Synthesis:
Iodothyronamines have been synthesized using a similar procedure reported by Scanlan et al.
with minor modifications.25
T4AM. NꢀtꢀBOCꢀ3,3',5,5'ꢀtetraiodothyronamine (200 mg, 0.24 mmol) was stirred with 1:4 v/v
mixture of trifluoroacetic acid and dichloromethane for 1h. The solvent was evaporated under
vacuum to yield a yellowish white sticky solid. T4AM was purified from the crude reaction
mixture by reverseꢀphase HPLC using a C18 column (Atlantis, 250 × 19 mm, 10ꢁm) and 70%
methanol/water as mobile phase. T4AM containing fractions were lyophilized to yield a white
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solid in 90% yield. H NMR (d4ꢀMeOH) δ (ppm): 7.89 (s, 2H), 7.10 (s, 2H), 3.22 (t, J = 7.6 Hz,
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2H), 2.94 (t, J = 7.6 Hz, 2H); C NMR (d4ꢀMeOH) δ (ppm): 153.3, 151.6, 150.7, 141.0, 138.6,
126.2, 91.1, 84.7, 40.4, 31.8; ESIꢀMS (m/z) calculated for C14H11NO2I4 [M+H]+: 733.7047,
found 733.9314.
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T3AM. This compound was synthesized by using NꢀtꢀBOCꢀ3,3',5ꢀtriiodothyronamine (200 mg,
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0.28 mmol) as described earlier for T4AM. Yield: 92%. H NMR (d4ꢀMeOH) δ (ppm): 7.89 (s,
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2H), 7.00 (d, J = 2.8 Hz, 1H), 6.78 (d, J = 8.8 Hz, 1H), 6.64ꢀ6.67 (dd, J = 6.4 Hz, 1H), 3.23 (t, J
= 8 Hz, 2H), 2.95 (t, J = 7.2 Hz, 2H); 13C NMR (d4ꢀMeOH) δ (ppm): 153.9, 152.5, 149.9, 141.0,
138.2, 125.6, 116.8, 115.0, 91.4, 83.4, 40.5, 31.8; ESIꢀMS (m/z) calculated for C14H13NO2I3
[M+H]+: 607.8080, found 607.7704.
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rT3AM. This compound was synthesized by using NꢀtꢀBOCꢀ3,3',5'ꢀtriiodothyronamine (200 mg,
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0.28 mmol) as described earlier for T4AM. Yield: 95%. H NMR (d4ꢀMeOH) δ (ppm): 7.85 (s,
1H), 7.31 (s, 3H), 6.91 (d, J = 8.4 Hz, 1H), 4.64 (br, s, 2H), 3.19 (t, J = 7.6 Hz, 2H), 2.94 (t, J =
7.6 Hz, 2H); 13C NMR (d4ꢀMeOH) δ (ppm): 156.0, 152.6, 151.3, 140.5, 135.0, 130.7, 129.0,
119.8, 88.8, 84.5, 40.7, 32.3; ESIꢀMS (m/z) calculated for C14H13NO2I3 [M+H]+: 607.8080,
found 607.7794.
3,3'-T2AM. This compound was synthesized by using NꢀtꢀBOCꢀ3,3'ꢀdiiodothyronamine (200
mg, 0.34 mmol) as described earlier for T4AM. Yield: 93%. 1H NMR (d4ꢀMeOH) δ (ppm): 7.81
(d, J = 2 Hz, 1H), 7.22ꢀ7.24 (q, J = 4 Hz, 2H), 6.84 (t, J = 2.8 Hz, 1H), 6.78 (d, J = 8.4 Hz, 1H),
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3.16 (t, J = 8 Hz, 2H), 2.91 (t, J = 7.6 Hz, 2H); C NMR (d4ꢀMeOH) δ (ppm): 157.0, 153.9,
149.9, 140.3, 133.9, 130.4, 129.4, 120.2, 118.5, 115.1, 88.0, 83.4, 40.8, 32.3; ESIꢀMS (m/z)
calculated for C14H14NO2I2 [M+H]+: 481.9114, found 481.8453.
3,5-T2AM. This compound was synthesized by using NꢀtꢀBOCꢀ3,5ꢀdiiodothyronamine (200 mg,
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0.34 mmol) as described earlier for T4AM. Yield: 90%. H NMR(d4ꢀMeOH) δ (ppm): 7.86 (s,
2H), 6.70 (d, J = 8.8 Hz, 2H), 6.56 (d, J = 8.8 Hz, 2H), 4.63 (s, br, 2H), 3.31 (t, J = 1.2 Hz, 2H),
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2.93 (d, br, J = 6.8 Hz, 2H); C NMR (d4ꢀMeOH) δ (ppm): 154.3, 152.5, 150.0, 140.9, 137.8,
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116.3, 115.9, 91.6, 40.5, 31.8; ESIꢀMS (m/z) calculated for C14H13NO2I2 [M+H]+: 481.9114,
found 481.8681.
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3',5'-T2AM. This compound was synthesized by using NꢀtꢀBOCꢀ3',5'ꢀdiiodothyronamine (200
mg, 0.34 mmol) as described earlier for T4AM. Yield: 94%. 1H NMR (d4ꢀMeOH) δ (ppm): 7.39
(s, 2H), 7.29 (d, J = 8.4 Hz, 2H), 6.96 (d, J = 8.4 Hz, 2H), 3.19 (t, J = 7.6 Hz, 2H), 2.96 (t, J = 8
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Hz, 2H); C NMR (d4ꢀMeOH) δ (ppm): 157.1, 152.6, 151.5, 132.1, 130.5, 130.0, 118.9, 84.4,
41.0, 32.9; ESIꢀMS (m/z) calculated for C14H13NO2I2 [M+H]+: 481.9114, found 481.8962.
3-T1AM. This compound was synthesized by using NꢀtꢀBOCꢀ3ꢀiodothyronamine (200 mg, 0.56
1
mmol) as described earlier for T4AM. Yield: 95%. H NMR (d4ꢀMeOH) δ (ppm): 7.79 (d, J = 2
Hz, 1H), 7.19 (dd, J = 6.4 Hz, 1H), 6.77ꢀ6.83 (m, J = 9.2 Hz, 4H), 6.71 (d, J = 8.4 Hz, 1H), 3.14
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(t, J = 8 Hz, 2H), 2.88 (t, J = 7.6 Hz, 2H); C NMR (d4ꢀMeOH) δ (ppm): 157.7, 154.3, 149.3,
140.1, 133.1, 130.1, 120.5, 117.6, 116.3, 87.5, 40.8, 32.3; ESIꢀMS (m/z) calculated for
C14H14NO2I1 [M+H]+: 356.0147, found 356.0094.
3'-T1AM. This compound was synthesized by using NꢀtꢀBOCꢀ3'ꢀiodothyronamine (200 mg,
0.56 mmol) as described earlier for T4AM. Yield: 91%. 1H NMR (d4ꢀMeOH) δ (ppm): 7.30 (d, J
= 2.8 Hz, 1H), 7.23 (d, J = 8.8 Hz, 2H), 6.88ꢀ6.91 (m, J = 2.8 Hz, 3H), 6.83 (d, J = 8.8 Hz, 1H),
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3.15 (t, J = 8.4 Hz, 2H), 2.92 (t, J = 8 Hz, 2H); C NMR (d4ꢀMeOH) δ (ppm): 158.0, 153.8,
150.0, 131.2, 130.2, 130.1, 120.9, 118.2, 115.0, 83.3, 41.0, 32.8; ESIꢀMS (m/z) calculated for
C14H14NO2I1 [M+H]+: 356.0147, found 356.0525.
T0AM. This compound was synthesized by using NꢀtꢀBOCꢀthyronamine (200 mg, 0.87 mmol)
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as described earlier for T4AM. Yield: 96%. H NMR (d4ꢀMeOH) δ (ppm): 7.21 (d, J = 8.4 Hz,
2H), 6.87 (t, J = 10.4 Hz, 4H), 6.79 (d, J = 8.8 Hz, 2H), 3.15 (t, J = 8 Hz, 2H), 2.91 (t, J = 8 Hz,
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2H); 13C NMR (d4ꢀMeOH) δ (ppm): 158.6, 154.1, 149.4, 130.6, 130.1, 121.1, 117.7, 116.2, 41.1,
32.8; ESIꢀMS (m/z) calculated for C14H15NO2 [M+H]+: 230.1181, found 230.0648.
Computational Method:
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All the DFT calculations were performed in Gaussian03 and Gaussian09 suites of quantum
chemical programme using the hybrid Becke 3ꢀLeeꢀYangꢀParr (B3LYP) exchange correlation
functional.73ꢀ76 For the analysis of the I···I halogen bonding, coordinates have been taken from
the crystal structure of the thyroid hormones and iodothyronamines. Natural Bond Orbital (NBO)
calculations77,78 were performed using 6ꢀ31+G* basis sets except iodine for which 6ꢀ311G**
basis set was used.
Single-crystal X-Ray crystallography:
T3, T3AM, 3'ꢀT1AM were recrystallized as trifluoroacetate salt from a 1:1 mixture of ethyl
acetateꢀdi ethyl ether by slow evaporation method. rT3AM, 3,5ꢀT2AM, 3,3'ꢀT2AM, 3',5'ꢀT2AM,
3ꢀT1AM and T0AM were recrystallized as trifluoroacetate salt from a 3:1 methanolꢀwater
solution. 3,5ꢀT2 was recrystallized as zwitterꢀion from an ammoniacal mixture of water and
methanol (1:3). The single crystal Xꢀray diffraction data were collected on a Bruker SMART
APEX CCD diffractometer utilizing SMART/SAINT software.79 Intensity data were collected
using graphiteꢀmonochromatized MoꢀKα radiation of wavelength 0.71073 Å at room
temperature. All the structures were solved by SHELXꢀ97 program incorporated in WinGX.
Empirical absorption corrections were applied with SADABS.80 81
,
T3.CF3CO2. C17H13NO8F3I3, Fw = 796.98, Monoclinic P21, a = 15.124(3) Å , b = 5.0451(10) Å,
c = 16.052(3) Å, α= 90⁰, β= 103.409(12)⁰, γ = 90⁰, V = 1191.5(4) Å3, Z = 2, MoKα radiation (λ
= 0.71073 Å), T = 296(2) K, ρcalcd (gcmꢀ3) = 2.222, µ (MoKα) (mmꢀ1) = 4.000, Collected
reflections = 7017, Unique reflections = 3357, GOF (F2) = 0.957, R1a = 0.0578, wR2b = 0.1373.
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T3AM.CF3CO2. C16H13NO4F3I3, Fw = 720.97, Monoclinic P 21/C, a = 6.565(4) Å , b =
11.892(8) Å, c = 27.565(18) Å, α= 90⁰, β= 89.92(4)⁰, γ = 90⁰, V = 2152(2) Å3, Z = 4, MoKα
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radiation (λ = 0.71073 Å), T = 296(2) K, ρcalcd (gcmꢀ3) = 2.225, µ (MoKα) (mmꢀ1) = 4.403,
Collected reflections = 5070, Unique reflections = 1967, GOF (F2) = 0.901, R1a = 0.0732, wR2b
= 0.2089.
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rT3AM.CF3CO2. C16H15N1O5F3I3, Fw = 738.99, Triclinic P ꢀ1, a = 8.6684(5) Å , b = 9.1810(5)
Å, c = 31.3546(18) Å, α= 88.003(2)⁰, β= 83.224(2)⁰, γ = 62.305(2)⁰, V = 2193.5(2) Å3, Z = 4,
MoKα radiation (λ = 0.71073 Å), T = 296(2) K, ρcalcd (gcmꢀ3) = 2.238, µ (MoKα) (mmꢀ1) =
4.325, Collected reflections = 11565, Unique reflections = 7322, GOF (F2) = 1.029, R1a =
0.0446, wR2b = 0.1090.
3,5-T2AM.CF3CO2. C16H16NO5F3I2, Fw = 613.10, Monoclinic C 2/c, a = 14.9418(2) Å , b =
10.3139(15) Å, c = 26.3434(4) Å, α= 90⁰, β= 92.614(4)⁰, γ = 90⁰, V = 4055.5(10) Å3, Z = 8,
MoKα radiation (λ = 0.71073 Å), T = 296(2) K, ρcalcd (gcmꢀ3) = 2.008, µ (MoKα) (mmꢀ1) =
3.154, Collected reflections = 6294, Unique reflections = 5023, GOF (F2) = 1.055, R1a = 0.0370,
wR2b = 0.0822.
3,5-T2. C15H13NO4I2, Fw = 525.06, orthorhombic P212121, a = 6.0404(17) Å , b = 12.8126(3)
Å, c = 24.8761(5) Å, α= 90⁰, β= 90⁰, γ = 90⁰, V = 1925.3(8) Å3, Z = 4, MoKα radiation (λ =
0.71073 Å), T = 296(2) K, ρcalcd (gcmꢀ3) = 1.811, µ (MoKα) (mmꢀ1) = 3.281, Collected
reflections = 3851, Unique reflections = 2613, GOF (F2) = 1.079, R1a = 0.0703, wR2b = 0.2385.
3,3'-T2AM.CF3CO2. C16H15NO5F3I2, Fw = 612.09, Monoclinic C 2/c, a = 28.792(5) Å , b =
8.390(5) Å, c = 16.654(5) Å, α= 90⁰, β= 92.033(5)⁰, γ = 90⁰, V = 4021(3) Å3, Z = 8, MoKα
radiation (λ = 0.71073 Å), T = 296(2) K, ρcalcd (gcmꢀ3) = 2.022, µ (MoKα) (mmꢀ1) = 3.181,
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Collected reflections = 6162, Unique reflections = 4960, GOF (F2) = 1.013, R1a = 0.0306, wR2b
= 0.0758.
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3',5'-T2AM.CF3CO2. C16H14NO4F3I2, Fw = 595.08, Triclinic P ꢀ1, a = 5.5691(6) Å , b =
10.3724(11) Å, c = 17.6633(18) Å, α= 78.671(6)⁰, β= 85.729(5)⁰, γ = 79.229(5)⁰, V =
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982.06(18) Å3, Z = 2, MoKα radiation (λ = 0.71073 Å), T = 296(2) K, ρcalcd (gcmꢀ3) = 2.012, µ
(MoKα) (mmꢀ1) = 3.250, Collected reflections = 6149, Unique reflections = 4278, GOF (F2) =
1.056, R1a = 0.0365, wR2b = 0.0977.
3-T1AM.CF3CO2. C16H17NO5F3I1, Fw = 487.21, Monoclinic P 21/c, a = 15.4185(4) Å , b =
14.7521(4) Å, c = 8.7484(19) Å, α= 90⁰, β= 106.186(13)⁰, γ = 90⁰, V = 1910.9(8) Å3, Z = 4,
MoKα radiation (λ = 0.71073 Å), T = 296(2) K, ρcalcd (gcmꢀ3) = 1.694, µ (MoKα) (mmꢀ1) =
1.729, Collected reflections = 6081, Unique reflections = 3093, GOF (F2) = 1.006, R1a = 0.0493,
wR2b = 0.1414.
3'-T1AM.CF3CO2. C16H15NO4F3I1, Fw = 469.19, Monoclinic P21/n, a = 12.158(2) Å , b =
9.3054(17) Å, c = 32.979(6) Å, α= 90⁰, β= 92.357(12)⁰, γ = 90⁰, V = 3728(11) Å3, Z = 8, MoKα
radiation (λ = 0.71073 Å), T = 296(2) K, ρcalcd (gcmꢀ3) = 1.672, µ (MoKα) (mmꢀ1) = 1.766,
Collected reflections = 11436, Unique reflections = 3013, GOF (F2) = 0.931, R1a = 0.0831, wR2b
= 0.3050.
T0AM.CF3CO2. C16H16NO4F3, Fw = 343.30, Monoclinic P21/c, a = 16.9525(2) Å , b =
11.7547(14) Å, c = 8.0149(10) Å, α= 90⁰, β= 93.763(4)⁰, γ = 90⁰, V = 1593.7(3) Å3, Z = 4,
MoKα radiation (λ = 0.71073 Å), T = 296(2) K, ρcalcd (gcmꢀ3) = 1.431, µ (MoKα) (mmꢀ1) =
0.125, Collected reflections = 4979, Unique reflections = 2326, GOF (F2) = 1.008, R1a = 0.0630,
wR2b = 0.2062.
ASSOCIATED CONTENT
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Supporting Information.
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Synthesis schemes, 1H and 13C NMR spectra, mass spectra, ORTEP diagrams, crystal packing,
Hꢀbonding interactions have been included. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Email: mugesh@ipc.iisc.ernet.in
Funding Sources
This study was supported by Science and Engineering Research Board (SERB), New Delhi,
India.
ACKNOWLEDGMENT
This study was financially supported by Science and Engineering Research Board (SERB), India.
S. Mondal thanks Indian Institute of Science, Bangalore for a research fellowship, Division of
Chemical Sciences, IISc for singleꢀcrystal XꢀRay diffraction (SCXRD) facility and Dr. Arun
Kumar Bar for his training on solving crystal structures. G. Mugesh thanks SERB for the J. C.
Bose National Fellowship (Grant No. SB/S2/JCBꢀ067/2015).
REFERENCES
(1)
(2)
Behne, D.; Kyriakopoulos, A.; Meinhold, H.; Köhrle, J.; Biochem. Biophys. Res.
Commun. 1990, 173, 1143ꢀ1149.
Berry, M. J.; Banu, L.; Larsen, P. R. Nature 1991, 349, 438ꢀ440.
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