Biomimetic deiodination of thyroid hormones and iodothyronamines-a structure-activity relationship study

Article abstract of DOI:10.1039/c6ob01375a

Mammalian selenoenzymes, iodothyronine deiodinases (DIOs), catalyze the tyrosyl and phenolic ring deiodination of thyroid hormones (THs) and play an important role in maintaining the TH concentration throughout the body. These enzymes also accept the decarboxylated thyroid hormone metabolites, iodothyronamines (TAMs), as substrates for deiodination. Naphthalene-based selenium and/or sulphur-containing small molecules have been shown to mediate the regioselective tyrosyl ring deiodination of thyroid hormones and their metabolites. Herein, we report on the structure-activity relationship studies of a series of peri-substituted selenium-containing naphthalene derivatives for the deiodination of thyroid hormones and iodothyronamines. Single crystal X-ray crystallographic and ⁷⁷Se NMR spectroscopic studies indicated that the intramolecular Se?X (X = N, O and S) interactions play an important role in the deiodinase activity of the synthetic mimics. Furthermore, the decarboxylated metabolites, TAMs, have been observed to undergo slower tyrosyl ring deiodination than THs by naphthyl-based selenium and/or sulphur-containing synthetic deiodinase mimics and this has been explained on the basis of the strength of Se?I halogen bonding formed by THs and TAMs.

Full text of DOI:10.1039/c6ob01375a

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Biomimetic deiodination of thyroid hormones and
iodothyronamines – a structure–activity
relationship study†
Cite this: DOI: 10.1039/c6ob01375a
Santanu Mondal and Govindasamy Mugesh*
Mammalian selenoenzymes, iodothyronine deiodinases (DIOs), catalyze the tyrosyl and phenolic
ring deiodination of thyroid hormones (THs) and play an important role in maintaining the TH
concentration throughout the body. These enzymes also accept the decarboxylated thyroid hormone
metabolites, iodothyronamines (TAMs), as substrates for deiodination. Naphthalene-based selenium
and/or sulphur-containing small molecules have been shown to mediate the regioselective tyrosyl
ring deiodination of thyroid hormones and their metabolites. Herein, we report on the structure–
activity relationship studies of a series of peri-substituted selenium-containing naphthalene derivatives
for the deiodination of thyroid hormones and iodothyronamines. Single crystal X-ray crystallographic
and 77Se NMR spectroscopic studies indicated that the intramolecular Se⋯X (X = N, O and S)
interactions play an important role in the deiodinase activity of the synthetic mimics. Furthermore,
the decarboxylated metabolites, TAMs, have been observed to undergo slower tyrosyl ring
deiodination than THs by naphthyl-based selenium and/or sulphur-containing synthetic deiodinase
mimics and this has been explained on the basis of the strength of Se⋯I halogen bonding formed by
THs and TAMs.
Received 25th June 2016,
Accepted 9th August 2016
DOI: 10.1039/c6ob01375a
www.rsc.org/obc
Introduction
Regioselective monodeiodination of L-thyroxine or 3,3′,5,5′-
tetraiodothyronine (T4), the major prohormone secreted by
the thyroid gland, plays an important role in thyroid hormone
homeostasis. T4 can undergo monodeiodination at its pheno-
lic and tyrosyl ring to produce the biologically active and in-
active metabolites 3,3′,5-triiodothyronine (T3) and 3,3′,5′-
triiodothyronine (rT3), respectively.1 Similarly, removal of one
iodine from the tyrosyl- and phenolic rings of T3 and rT3,
respectively, is also known to produce a biologically inactive
metabolite 3,3′-diiodothyronine (3,3′-T2) (Scheme 1). These
regioselective deiodinations of thyroid hormones (THs) are
catalysed by three isoforms of a selenoenzyme, iodothyronine
deiodinase type 1 (DIO1), type 2 (DIO2) and type 3 (DIO3).
While DIO1 can remove iodine from both the tyrosyl and
phenolic rings, DIO2 and DIO3 are selective towards phenolic
Department of Inorganic and Physical Chemistry, Indian Institute of Science,
Bangalore 560012, India. E-mail: mugesh@ipc.iisc.ernet.in
†Electronic supplementary information (ESI) available: NMR characterisation
data, mass spectra, HPLC chromatograms, coordinates of optimized geometries.
CCDC 1486467 and 1486468. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c6ob01375a
Scheme 1 (A) Biochemical deiodination of thyroid hormones and
iodothyronines by iodothyronine deiodinases (DIOs). (B) Chemical struc-
tures of compounds 1–4.
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and tyrosyl ring deiodinations, respectively.2–4 Although all the
three isoforms have a very similar amino acid sequence in the
active site, the origin of the regioselectivity of deiodination is
still not clear. Although DIO-catalysed biochemical de-
iodinations have been discovered as the major metabolic
pathways, THs also undergo decarboxylation to form iodo-
thyronamines (TAMs).
Although there can be a total of nine iodothyronamines
depending on the number and position of the iodine atoms,
only 3-iodothyronamine (3-T1AM) and thyronamine (T0AM)
could be detected so far in the blood plasma of various orga-
nisms.5a These two metabolites have been shown to induce
hypothermia, hyperglycemia and bradycardia in mice when
injected in pharmacological doses.5a,b They are also known to
activate trace amine-associated receptor 1 (TAAR1), an orphan
G-protein coupled receptor, and inhibit the neoronal reuptake
of the neurotransmitter dopamine and the norepinephrine by
dopamine transporter (DAT) and the norepinephrine trans-
porter (NET), respectively.6,7 Recently, TAMs have been shown
to be isozyme-specific substrates of iodothyronine deiodinases
(DIOs) (Scheme 1) although some of the TAMs do not undergo
deiodination by DIOs.8
Recently, our efforts in the development of naphthyl-based
selenium and/or sulphur-containing compounds (1–4) as func-
tional mimics of DIO3 have attracted significant research
attention.9 Compounds 1–4 mediate regioselective tyrosyl ring
deiodination of T4 and T3 to produce rT3 and 3,3′-T2, respec-
tively. These compounds are also found to mediate the deiodi-
nation of thyroid hormone metabolites such as iodothyronine
sulphates and iodothyronamines, and iodinated nucleo-
sides.9d–f Deiodinase activity of these compounds has been
explained by the cooperative action of Se⋯I halogen bonding
and the chalocogen bonding interaction between two nearby
chalocogen atoms. Noncovalent Se⋯N interaction between the
amine group and the nearby selenium atom in compound 4
was shown to increase the deiodinase activity of the com-
pound. Herein, we report the effect of different heteroatom-
containing substituents on the deiodination activity of
naphthalene derivatives for the deiodination of thyroid
hormones and iodothyronamines. We also show that the
carboxylic acid group in the β-alanine side chain of thyroid
hormones has a significant effect on the biomimetic de-
iodination reactions.
Scheme 2 Chemical structures of 5–10 and 12–17.
produce the Schiff bases 20 and 21, which were further
reduced with NaBH4 to afford the amine-substituted com-
pounds 12 and 13, respectively. ortho-Substituted aldehyde 18
was reduced with NaBH4 to produce compound 14.
Compounds 15–17 were synthesized by borontrifluoride–
diethyl etherate (BF3–Et2O) catalysed thioacetal formation by
18 in the presence of thiols.
The deiodination reactions were monitored by high
pressure liquid chromatography (HPLC) and the deiodinated
products were quantified by comparing the peak area with that
of standard samples. Due to the low solubility of the TAMs in
sodium phosphate buffer (100 mM) at physiological pH, de-
iodination reactions were performed in a mixture of sodium
phosphate buffer (100 mM, pH 7.00) and 20% v/v acetonitrile.
Under these assay conditions, T4 readily underwent deiodi-
nation by all the compounds 5–10 to form rT3. The initial rate
of deiodination was determined by monitoring the initial
10–15% conversion of the starting material to the product. The
initial rates of deiodination of T4 by compound 5 and 6 are
2.6 and 1.9 times, respectively, higher than that of the parent
compound 3 (Fig. 1 and Table S1, ESI†). These results are in
agreement with our previous observation that the introduction
of a secondary amine group in the close proximity of one of
the selenols in 3 increases the deiodination activity. The
deiodination of T4 by compound 7 (Fig. 1 and Table S1, ESI†)
Results and discussion
Deiodination of THs and TAMs in the presence of dithio-
threitol was carried out by naphthyl-based diselenols 5–10
having a lone pair-containing atom such as nitrogen, oxygen
and sulphur in the close proximity of one of the selenols
(Scheme 2). Compounds 5–10 were freshly generated in the
assay mixture by reducing the corresponding diselenides
12–17 (Scheme 2) with sodium borohydride (NaBH4). All of
these compounds were synthesized from the aldehyde 18 or 19
Fig. 1 Comparison of initial rates of deiodination of T4 by compounds
3 and 5–10. Assay conditions: T4 (0.3 mM), mimics (1.2 mM), dithiothrei-
tol (DTT) (15 mM), sodium borohydride (NaBH4) (30 mM), sodium phos-
(Scheme S1, ESI†). 18 was refluxed with the primary amines to phate buffer (100 mM, pH 7.0), 20% acetonitrile (v/v), 37 °C.
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is also faster than that by the parent compound 3, indicating (δ = 156 ppm) (Table 1). This indicates the more nucleophilic
that the presence of an alcohol moiety can also facilitate the character of the selenol adjacent to the secondary amine
deiodination reactions. Interestingly, compound 8, with thio- moiety as compared to the other selenol moiety. Similarly, the
acetal substitution, exhibits the highest deiodination activity signal for the selenol adjacent to the oxygen atom in 7 and
in the series for the deiodination of T4. The initial rate of de- sulphur atoms in 11 appear at more upfield regions δ =
iodination of T4 by compound 8 is almost 3 times higher than 52 ppm and δ = 72 ppm, respectively (Table 1). These results
that of the parent compound 3. However, the deiodination indicate that the Se⋯O and Se⋯S interactions can increase the
activities of the other thioacetal-based compounds 9 and 10 nucleophilicity of the proximal selenol more than the Se⋯N
are 1.4 and 1.5 times, respectively, lower than that of com- interaction does in 5 and 6. In compound 9, the selenol
pound 8 (Fig. 1 and Table S1, ESI†). In contrast to the de- moiety adjacent to the thioacetal ring appears at δ = 102 ppm,
iodination of T4, T4AM does not undergo deiodination by indicating that the Se⋯S interaction is weak due to the rigidity
compounds 3 and 5–10 (Fig. S29A, ESI†). As the addition of of the five-membered ring (Table 1). Although compound 10
T4AM to phosphate buffer (100 mM, pH 7.0) makes the solu- does not contain an intramolecular Se⋯S interaction, the
tion turbid, the deiodination of T4AM has been carried out in 77Se signals for this compound indicate the possible inter-
acetonitrile in which it is completely soluble. In fact, T4AM molecular Se⋯S interactions in solution.
undergoes both the tyrosyl and phenolic ring deiodination to
However, it is not clear why compound 5 exhibits two
form rT3AM and T3AM, respectively, by all the compounds in upfield-shifted peaks (δ = 91 ppm and 86 ppm), whereas the
acetonitrile, indicating that the solvent has a significant effect corresponding oxidized compound 12 exhibits only one
on the regioselectivity of deiodination reactions (Fig. S29B, upfield-shifted peak at δ = 345 ppm in the 77Se NMR.
ESI†).
The presence of Se⋯X (X = N, O and S) interactions was
Deiodination of THs by naphthyl-based diselenols was also observed in the X-ray structure of the oxidized precursors
explained by the co-operative action of halogen bonding and of deiodinase mimics. The single crystal X-ray structure of
chalcogen bonding.9c While the Se⋯I halogen bonding inter- compound 13 exhibited a strong Se⋯N interaction with a
action elongates the C–I bond, chalcogen bonding between Se⋯N nonbonded distance of 2.563 Å, which is almost 25.8%
two selenium atoms increases the strength of the halogen shorter than the sum of van der Waals radii of selenium and
bonding between selenium and iodine. Strong Se⋯N noncova- nitrogen (3.45 Å) (Fig. 2A).10 Similarly, a strong Se⋯O non-
lent interaction in the amino-substituted naphthyl-based di- covalent interaction was observed in the single crystal X-ray
selenol 4 increases the deiodination activity by supplying structure of compound 14 (Fig. 2B). The non-bonded distance
electron density to the selenium atom close to nitrogen and between the selenium and oxygen atom in 14 has been
therefore, further strengthening the halogen bonding as well observed to be 2.818 Å, which is almost 17.6% shorter than
as chalcogen bonding interactions.9c Compounds 5–10 were the sum of van der Waals radii of selenium and oxygen
designed on the basis of Se⋯X (X = N, O and S) interactions (3.42 Å). One of the sulphur atoms in the thioacetal-based
that can enhance the activity of these compounds in deiodinat- compound 16 interacts strongly with the nearby selenium with
ing thyroid hormones and iodothyronamines. The presence of a Se⋯S non-bonded distance of 3.090 Å.9d The other sulphur
Se⋯X (X = N, O and S) interactions in compounds 5–10 and atom in the five-membered ring of compound 16 interacts very
their oxidized precursors 12–17 can be understood from the weakly with the nearby selenium atom.
77Se chemical shifts. The signal for one of the selenols in 6
The strength of the Se⋯X (X = N, O and S) interaction in
(δ = 91 ppm) is shifted upfield significantly relative to compounds 5–10 correlates well with the observed de-
the other selenol (δ = 212 ppm) in the same compound or 3 iodination activity. Compounds 5–7, with the secondary amine
and hydroxymethyl substitutions, exhibit higher deiodination
activity compared to the parent compound 3. Thioacetal-based
Table 1 Chemical shifts (δ) in the 77Se NMR spectra of compounds 3
and 5–17
Chemical shiftb
(δ, ppm)
Chemical shiftb
(δ, ppm)
Compounda
Compounda
3
5
6
7
8
9
10
156
11
12
13
14
15
16
17
415
86, 91, 217
91, 212
52, 211
73, 198
102, 205
96, 199
345, 426
342, 439
406, 411
390, 446
382, 453
381, 453
a Selenols 3 and 5–10 were generated by reducing the corresponding
diselenides 11–17 with sodium borohydride (NaBH4). The chemical
structure of compound 11 has been shown in the ESI (Scheme S1).
b 77Se chemical shifts of the selenols and diselenides were recorded in
a mixture of 1 : 1 chloroform and methanol.
Fig. 2 ORTEP representation of the single crystal X-ray structures of
compounds 13 (A) and 14 (B) showing the Se⋯N and Se⋯O interactions,
respectively. The thermal ellipsoids are drawn at 50% probability.
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compound 8 exhibits the highest deiodination activity in the compounds 9 and 10 exhibit lower deiodination activity than
series for the deiodination of T4 (Fig. 1 and Table S1, ESI†). As compound 8 for the deiodination of T3.
sulphur is more polarisable than nitrogen and oxygen, it may
Deiodination of T3AM by compounds 3 and 5–10 follow
interact more strongly with the nearby selenium atom, which almost similar trends to that described for T3. Compounds
becomes electron-deficient due to halogen bonding and chal- 5–7, with amino and hydroxymethyl substitutions, mediate
cogen bonding interactions. Furthermore, a closer orbital faster deiodination of T3AM than the parent compound 3
matching between the sulphur and selenium atom may also (Fig. 3 and Table S1, ESI†). The thioacetal-based compound 8
lead to more effective electron donation from sulphur than exhibits the highest activity in the series for the deiodination
from nitrogen or oxygen to the nearby selenium atom. of T3AM (Table 1). Interestingly, the deiodination of T3 by
Compound 9, with a rigid five-membered ring, exhibits weaker compounds 3 and 5–10 has been observed to be almost 4–11
Se⋯S interaction and therefore, a lower deiodination activity times faster than that of T3AM, indicating that the removal of
than compound 8 is expected. Compound 10, which lacks the the carboxylate group from the β-alanine side chain of T3
intramolecular Se⋯S interaction but could potentially have decreases the rate of deiodination (Fig. 3 and Table S1, ESI†).
intermolecular Se⋯S interactions, also exhibits lower de- These results are quite similar to our previous observations
iodination activity than compounds 8 and 9 for the deiodination that T3AM undergoes slower deiodination by synthetic organo-
of T4 (Fig. 1 and Table S1, ESI†).
selenium compounds than T3.9d It should be noted that T3AM
Deiodination of the triiodo-derivatives, T3 and T3AM, were also undergoes slower tyrosyl ring deiodination by DIO3 than
also checked with compounds 3 and 5–10. Interestingly, all T3.8 Interestingly, the other triiodo derivative, rT3AM does not
these compounds mediate the tyrosyl ring deiodination of T3 undergo deiodination by the naphthyl-based deiodinase
and T3AM, to form 3,3′-T2 and 3,3′-T2AM, respectively. Similar mimics (Fig. S30B, ESI†), which is very similar to our previous
to the deiodination of T4, the initial rates of deiodination of observation that rT3 does not undergo deiodination by de-
T3 by compounds 5 and 6 are higher than that by the parent iodinase mimics.
compound 3 (Fig. 3 and Table S1, ESI†). In contrast to the
The diiodo derivative, 3,5-diiodothyronine (3,5-T2), with
deiodination of T4, the activity of compound 7, exhibiting both the iodine atoms in the tyrosyl ring (Fig. 4A), exhibits
Se⋯O interaction, is found to be more than that of 5 and 6, interesting pharmacological and thyromimetic11 properties.
which exhibits Se⋯N interaction, for the deiodination of T3.
Furthermore, the rate of deiodination of T3 by compound 7 is
1.5 times more than that by the parent compound 3 (Fig. 3
and Table S1, ESI†). Thioacetal-based compound 8 with a
sulphur-containing flexible side chain exhibits the highest
deiodination activity of the series. The rate of deiodination of
T3 by compound 8 has been found to be 2.3 times more than
that by the parent compound 3 indicating that the presence of
polarizable sulphur atoms in the close proximity of selenium
facilitates the deiodination reaction by peri-substituted
naphthalene derivatives. Similar to the deiodination of T4,
Fig. 4 (A) Chemical structures of 3,5-T2 and 3,5-T2AM. (B) HPLC chro-
matogram of the deiodination of 3,5-T2AM by compound 3. (C)
Fig. 3 Comparison of initial rates of deiodination of T3 and T3AM by
compounds 3 and 5–10. Assay conditions: T3 or T3AM (0.3 mM), mimics
(1.2 mM), dithiothreitol (DTT) (15 mM), sodium borohydride (NaBH4)
(30 mM), sodium phosphate buffer (100 mM, pH 7.0), 20% acetonitrile
(v/v), 37 °C.
Comparison of initial rates of deiodination of 3,5-T2 and 3,5-T2AM by
compounds and 5–10. Assay conditions: 3,5-T2 or 3,5-T2AM
(0.3 mM), mimics (1.2 mM), dithiothreitol (DTT) (15 mM), sodium boro-
hydride (NaBH4) (30 mM), sodium phosphate buffer (100 mM, pH 7.0),
20% acetonitrile (v/v), 37 °C.
3
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Chronic administration of 3,5-T2 in Wistar rats has been 3-iodothyronamine (3-T1AM) and the completely deiodinated
shown to significantly reduce the thyroidal iodide uptake, thyro- derivative, thyronamine (T0AM) are detected in the blood
peroxidase (TPO)-activity, NADPH oxidase 4 (NOX 4)-activity, plasma of several organisms.5a Biosynthesis of 3-T1AM from
DIO1-activity and to increase the expression of thyroid stimu- T4 has recently been shown to occur in the intestinal tissue,
lating hormone (TSH) receptor and dual oxidase (DUOX)- and this process involves both the decarboxylation by
activity.12 Regular dosage of 3,5-T2 in rats also leads to ornithine decarboxylase (ODC) and deiodination by DIOs.16
decrease in TSH, T3 and T4 concentration. Intraperitoneal Although 3,5-T2AM undergoes deiodination by the deiodinase
(i.p.) administration of 3,5-T2 in Wistar rats, receiving fat mimics to produce 3-T1AM, 3-T1AM has been found to be
enriched diet, results in a significant reduction in the adi- completely stable in the presence of small molecule deiodi-
posity and body-weight gain without altering the serum TSH, nase mimics (Fig. S30E, ESI†). The other monoiodo derivative,
T3 and T4 concentration.13 3,5-T2 also exhibits remarkable 3′-iodothyronamine (3′-T1AM) also does not undergo bio-
thyromimetic activity in the in vitro assays involving nuclear mimetic deiodination to produce T0AM (Fig. S30F, ESI†). It
receptors (TRs) and GH3 cells.14 However, the biosynthesis of should be noted that 3-T1AM undergoes tyrosyl ring deiodina-
3,5-T2 is still not clear. Homogenates of NCLP-6E monkey tion by DIO3 to form T0AM but 3′-T1AM does not undergo
hepatocarcinoma cells expressing DIO1 enzymatic activity has phenolic ring deiodination by DIO1.8
been reported to mediate phenolic ring deiodination of T3 to
As Se⋯I halogen bonding was proposed to be responsible
produce 3,5-T2, although T3 has been excluded as the sub- for the deiodination of thyroid hormones and their metaboli-
strate for phenolic ring deiodination by DIO1 in many tes,9c we carried out detailed density functional theory (DFT)
experiments.15
calculations to understand the strength of the halogen bond
Herein, we show, for the first time, that 3,5-T2 and its de- formed by THs and TAMs, and to understand the origin of
carboxylated metabolite, 3,5-T2AM (Fig. 4A) can undergo regio- differences in the reactivity of THs and TAMs towards deiodi-
selective tyrosyl ring deiodination by naphthyl-based nation reactions. Theoretical studies and experimental evi-
deiodinase mimics to form 3-iodothyronine (3-T1) and dence suggest that the electron density is anisotropically
3-iodothyronamine (3-T1AM), respectively. These deiodination distributed around the halogen atom in organic halides and
reactions were also monitored by HPLC (Fig. 4B) and the for- because of this, a region of positive electrostatic potential is
mation of deiodinated products was confirmed by comparing present on the tip of the C–X (X = halogen) covalent bond
the HPLC chromatogram with that of a standard sample of the (σ-hole). The σ-hole is surrounded by an electro-neutral ring
expected deiodinated product as well as mass spectral analysis. followed by a belt of negative potential coaxial with the C–X
Comparison of the initial rates of deiodination of 3,5-T2 by bond.17 The presence of the σ-hole on the halogen atoms is
compounds 3 and 5–10 indicates that compounds 5–7 exhibit responsible for the formation of the halogen bond in the pres-
higher deiodination activity than the parent compound 3 ence of electron donors.18 Natural bond orbital (NBO) calcu-
(Fig. 4C and Table S1, ESI†). Although compound 8 exhibits lations on 3,5-T2 indicated the positive charges on the tyrosyl
the highest activity in the series, the activity of compounds 9 ring iodine atoms at 0.201 and 0.199 arbitrary units (a.u.). To
and 10 has been found to be almost similar to or slightly lower understand the potential of the iodine atoms to form halogen
than that of compound 8, which is in contrast to the deiodina- bonding with selenium, 3,5-T2 was optimized with a model
tion of T4, T3 and T3AM by compounds 8–10 (Table S1, ESI†). selenolate, methyl selenolate (MeSe−), using B3LYP hybrid
Interestingly, unlike the deiodination of 3,5-T2, compounds 5 functional and 6-31+G* basis sets for all the atoms except
and 6 exhibit almost similar deiodination activity as the parent iodine for which the 6-311G** basis set was used. As expected,
compound 3 for the deiodination of 3,5-T2AM (Fig. 4C and the optimised geometry exhibited a strong Se⋯I halogen
Table S1, ESI†). Furthermore, in contrast to the deiodination bonding between 3,5-T2 and MeSe−. The non-bonded distance
of 3,5-T2, the deiodination activity of compound 9 is signi- between selenium and iodine atoms in the halogen-bonded
ficantly lower than that of compound 8 for the deiodination of geometry (3,5-T2·MeSe−), 2.962 Å, has been found to be almost
3,5-T2AM. These results indicate that in addition to the Se⋯X 23.7% shorter than the sum of their van der Waals radii
(X = N, O and S) interactions, the deiodinase activity of com- (3.880 Å) (Fig. 5A). Furthermore, the C–I bond in 3,5-T2
pounds 5–10 also depends on the nature of the substrates. (2.127 Å) is elongated by almost 0.199 Å in 3,5-T2·MeSe−
Similar to the differences in the rate of deiodination of T3 and (2.326 Å). Similar DFT calculations on 3,5-T2AM indicated the
T3AM, deiodination of 3,5-T2 by compounds 3 and 5–10 has positive charge on both the tyrosyl ring iodine atoms at
been found to be 3–10 times faster than that of 3,5-T2AM 0.199 a.u. Interestingly, the Se⋯I distance in the halogen-
(Fig. 4C and Table S1, ESI†). Furthermore, the deiodination of bonded
T3 and T3AM by the naphthyl-based deiodinase mimics is (3,5-T2AM·MeSe−) has been found to be 2.975 Å (Fig. 5B),
faster than that of 3,5-T2 and 3,5-T2AM, respectively.
which is higher than that observed in 3,5-T2·MeSe−. In agree-
geometry
between
3,5-T2AM
and
MeSe−
Interestingly, the other diiodothyronamines, 3,3′-diiodo- ment with this, the C–I bond elongation in 3,5-T2AM·MeSe−,
thyronamine (3,3′-T2AM) and 3′,5′-diiodothyronamine (3′,5′- 0.191 Å (C–I bond length: 2.127 Å in 3,5-T2AM; 2.318 Å in
T2AM) do not undergo deiodination by naphthyl-based de- 3,5-T2AM·MeSe−) is less than that observed in 3,5-T2·MeSe−.
iodinase mimics under physiologically relevant conditions NBO calculations on the halogen-bonded geometries afforded
(Fig. S30C and D, ESI†). One of the monoiodo derivatives, the Se⋯I interaction energies at 57.79 kcal mol−1 and
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the cisoid conformations of THs and TAMs. Interestingly, the
interaction energies are slightly different for the cisoid and
transoid conformations. For example, the strength of the
halogen bond formed by the cisoid geometries of 3,5-T2 and
3,5-T2AM (Fig. 5E and F) is found to be 57.66 kcal mol−1 and
55.4 kcal mol−1, respectively. Nevertheless, the calculations
both in the cisoid and transoid geometry of THs and TAMs
follow a similar trend in the strength of Se⋯I halogen
bonding, and these interaction energies may be attributed to
the faster deiodination of thyroid hormones (T3, 3,5-T2) than
iodothyronamines (T3AM, 3,5-T2AM) as well as the faster de-
iodination of triiodo derivatives (T3, T3AM) than the diiodo
derivatives (3,5-T2, 3,5-T2AM) (Fig. 3, 4C and Table S1, ESI†).
In addition to the Se⋯I interaction energies, intermolecular
dimerisation of T3 in solution with the help of hydrogen
bonding and I⋯I halogen bonding has been shown to
enhance the rate of deiodination of the same by organosele-
nium compounds.9d Similarly, intermolecular interactions in
solution may also play an important role in the initial rate of
deiodination of 3,5-T2 and 3,5-T2AM by deiodinase mimics.
Enzymatic deiodinations of THs are known to be highly pH-
sensitive. pH can alter the regioselectivity as well as the
rate of deiodination of THs by DIOs. For example, while DIO1
catalyzes the phenolic ring deiodination of T4 to produce T3 at
physiological pH, it catalyzes the tyrosyl ring deiodination of
T3 to produce 3,3′-T2 at alkaline pH. In contrast, DIO3 and its
Cys mutant do not catalyze the phenolic ring deiodination of
T4, T3 and rT3 over a wide pH range although the rate of deio-
dination by DIO3 has been found to be enhanced at alkaline
pH. In our previous report on the biomimetic deiodination of
THs, we have also shown that the tyrosyl ring deiodination of
T3 by compound 3 is sensitive to pH of the reaction
mixture.9b However, the effect of pH on the regioselectivity
and the rate of deiodination of TAMs by DIOs is not
Fig. 5 A comparison of the Se⋯I distances, C–I bond lengths (Å) and
C–I–Se and C–Se–I angles (°) in the halogen bonded adducts formed
by the tyrosyl ring iodine atoms of transoid conformations of 3,5-T2 (A),
3,5-T2AM (B), T3 (C), T3AM (D), and cisoid conformations of 3,5-T2 (E)
and 3,5-T2AM (F) with MeSe−.
55.07 kcal mol−1 for 3,5-T2 and 3,5-T2AM, respectively, indicat- known. In the present study, we have investigated the effect of
ing that the decarboxylated metabolite, 3,5-T2AM forms pH on the deiodination of T3AM and 3,5-T2AM by com-
weaker halogen bonding interactions with selenium than 3,5- pound 9. Deiodination of both T3AM and 3,5-T2AM by
T2. Similar results were obtained for the halogen-bonded compound 9 was found to be extremely slow at acidic pH.
geometries formed by the tyrosyl ring iodine atoms of T3 and However, the initial rate of deiodination of T3AM and 3,5-
T3AM (Fig. 5C and D). The Se⋯I non-bonded distances were T2AM by compound 9 to produce 3,3′-T2AM and 3-T1AM,
found to be 2.958
Å and 2.964 Å
in T3·MeSe− and
T3AM·MeSe−, respectively. NBO calculations on these halogen-
bonded geometries afforded the Se⋯I halogen bonding inter-
action energies at 58.68 kcal mol−1 and 57.33 kcal mol−1 for
T3·MeSe− and T3AM·MeSe−, respectively, suggesting that T3
and T3AM form stronger halogen bonding with selenium than
3,5-T2 and 3,5-T2AM, respectively.
Although thyroid hormones are known to exist both in the
cisoid and transoid conformations, transoid is the most stable
conformation.19 The amino acid side chain and phenolic ring
of THs remain in the same face and opposite faces of the
plane of the tyrosyl ring in the cisoid and transoid conformers,
respectively. We have recently shown that the strength of
halogen bonding formed by the iodine atoms of thyroid
hormones can be altered by changing the conformation.20
Fig. 6 Variation in the initial rate of deiodination of T3AM (A) and 3,5-
T2AM (B) by compound 9 with increasing pH. Assay conditions: T3AM or
3,5-T2AM (0.3 mM), compound 9 (1.2 mM), dithiothreitol (DTT) (15 mM),
sodium borohydride (NaBH4) (30 mM), sodium phosphate buffer
Therefore, we calculated the Se⋯I interaction energies also in (100 mM), 20% acetonitrile (v/v), 37 °C.
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respectively, was observed to significantly increase over the pH acetate–acetic acid buffer (15 mM, pH 4). The formation of
range 7–11 (Fig. 6). Interestingly, in both cases, the regio- deiodinated products was monitored at λ = 275 nm and they
selectivity of deiodination was not altered at alkaline pH. were quantified from the corresponding peak areas. The final
These observations are very similar to those observed for the assay mixture contained 0.3 mM thyroid hormones or
deiodination of T4 and T3 by DIO3 and deiodinase mimics. iodothyronamines, 1.2 mM mimic, 15 mM dithiothreitol
Increase in the rate of deiodination at alkaline pH may be (DTT), 30 mM sodium borohydride (NaBH4), phosphate buffer
attributed to the increased nucleophilicity of selenolate and (100 mM, pH 7.0) and 20% v/v acetonitrile.
thiolate groups, which are produced by sequential deprotona-
Synthesis
tion of selenol groups in 9 and thiol groups in dithiothreitol
(DTT), respectively, at higher pH.
Compounds 11 and 18 (Scheme S1, ESI†) were synthesized by
following the literature procedure.9a–c
Synthesis of 19. Compound 19 (Scheme S1, ESI†) was
obtained as a side-product during the synthesis of 18 by using
the Vilsmeier–Haack formylation of naphthalene-1,8-disele-
Conclusions
In this paper, we have discussed the biomimetic deiodination nide, 119b and was purified by column chromatography using
of THs and their decarboxylated metabolites, TAMs by a series 230–400 mesh silica gel as the stationary phase and toluene as
of naphthalene-based organoselenium compounds. The sele- the mobile phase. 1H NMR (CDCl3) δ (ppm): 10.17 (S, 1H),
nium-containing compounds used in the present study have 8.97–8.99 (dd, J = 6.8 Hz, 1H), 7.69 (d, J = 7.6 Hz, 1H),
secondary amine, primary alcohol and thioacetal substitutions 7.46–7.53 (m, 3H); 13C NMR (CDCl3) δ (ppm): 191.7, 153.5,
in the close proximity of one of the selenium atoms. The 142.4, 138.9, 138.3, 135.5, 130.9, 128.5, 123.0, 121.6, 120.5;
activity of all of these compounds in deiodinating THs and 77Se NMR (CDCl3) δ (ppm): 455, 437.
TAMs is found to be higher than the unsubstituted compound,
Synthesis of 12. 2-Amino-2-methylpropan-1-ol (1.43 g,
naphthalene-1,8-diselenol (compound 3). The deiodinase 16 mmol) was added dropwise to a solution of 18 (100 mg,
activity of these compounds has been explained on the basis 0.32 mmol) in 25 mL dry acetonitrile and the resulting mixture
of Se⋯X (X = N, O and S) noncovalent interactions although it was refluxed for 24 h. The solvent was evaporated under
is also found to be dependent on the nature of iodinated sub- reduced pressure and the resulting Schiff base (20) was
strates. THs undergo faster deiodination than TAMs by these reduced without further purification. The Schiff base was dis-
naphthyl-based small molecules. Furthermore, the deiodina- solved in 1 : 1 chloroform/methanol (30 mL) and was treated
tion of the triiodo derivatives is found to be faster than that of with sodium borohydride (242 mg, 6.4 mmol) in small por-
the diiodo derivatives. Theoretical investigations on the for- tions. Then the reaction mixture was heated at 50 °C for 5 h
mation of Se⋯I halogen bonding indicated that THs form followed by evaporation of solvent under reduced pressure.
stronger halogen bonding with selenium than TAMs. Also the The reaction mixture was dissolved in ethyl acetate and
triiodo derivatives form stronger Se⋯I halogen bonding than washed with water twice. The organic layer was collected, dried
the diiodo derivatives. Altogether, the deiodination of THs and over anhydrous sodium sulphate and evaporated in vacuo to
TAMs by naphthyl-based selenium-containing small molecules afford a viscous liquid. The crude product was then purified by
depends on the strength of the Se⋯I halogen bond and intra- flash chromatography using chloroform/ethyl acetate as the
molecular Se⋯X (X = N, O and S) noncovalent interactions.
eluent to give 12 as a red coloured viscous liquid in 60%
overall yield. 1H NMR (CDCl3) δ (ppm): 7.33–7.41 (m, 3H), 7.13
(t, J = 7.6 Hz, 1H), 6.89 (d, J = 8 Hz, 1H), 3.80 (s, 2H), 3.48
(s, 2H), 2.67 (s, br, 2H), 1.13 (s, 6H); 13C NMR (CDCl3) δ (ppm):
141.3, 140.3, 139.4, 136.7, 134.9, 127.5, 126.7, 125.0, 123.4,
123.1, 69.6, 55.9, 46.9, 30.2, 23.4; 77Se NMR (CDCl3) δ (ppm):
Experimental section
Deiodination assays
The deiodination reactions of thyroid hormones and iodothyro- 426, 345. ESI-MS (m/z) calcd for C15H17NOSe2 [M + H]+:
namines were performed in a mixture of 20% v/v acetonitrile 387.9719, found 387.9876.
and phosphate buffer (100 mM, pH 7.00) at 37 °C in the pres-
Synthesis of 13. 2-Amino-2-methylpropane-1,3-diol (1.68 g,
ence of 15 mM dithiothreitol (DTT). Due to the low solubility 16 mmol) was added dropwise to a solution of 18 (100 mg,
of iodothyronamines in phosphate buffer at physiological pH, 0.32 mmol) in 25 mL dry acetonitrile and the resulting mixture
20% acetonitrile was added to the buffer. The selenols was refluxed for 24 h. After removing the solvent under
(1.2 mM) were freshly generated in the assay mixture by reduc- reduced pressure, the resulting Schiff base (21) was reduced
ing the corresponding diselenides with sodium borohydride without further purification. The Schiff base was dissolved in
(NaBH4). Initial 10–15% conversion of thyroid hormones or 1 : 1 chloroform/methanol (30 mL) and was treated with
iodothyronamines to the deiodinated product was monitored sodium borohydride (242 mg, 6.4 mmol) portionwise. Then
to determine the initial rate of deiodination. The deiodinated the reaction mixture was heated at 50 °C for 5 h after which
products were separated by reverse-phase HPLC using a pre- the solvent was evaporated under reduced pressure. The
packed cartridge (Princeton C18 column, 5 µm, 150 mm × mixture was dissolved in ethyl acetate and washed with water
5 mm) and gradient elution with acetonitrile/ammonium twice, dried over anhydrous sodium sulphate and evaporated
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in vacuo to afford a yellowish-red solid. The crude product was dichloromethane, thoroughly washed with water, dried over
then purified by flash chromatography using chloroform/ethyl anhydrous sodium sulphate and concentrated in vacuo to yield
acetate as the mobile phase to yield 13 as a yellowish red a dark brown coloured liquid. The crude product was then
coloured solid in 55% overall yield. 1H NMR (d6-DMSO) purified by column chromatography using petroleum ether/
δ (ppm): 7.53 (d, J = 8 Hz, 1H), 7.41 (d, J = 7.6 Hz, 2H), diethyl ether as the mobile phase to afford 16 as a dark brown
7.11–7.18 (m, 2H), 3.95 (s, 2H), 3.49 (s, 4H), 1.02 (s, 3H); solid in 35% yield. 1H NMR (CDCl3) δ (ppm): 7.51 (d, J =
13C NMR (d6-DMSO) δ (ppm): 141.5, 140.1, 139.4, 136.7, 136.5, 8.4 Hz, 1H), 7.44 (t, J = 6.8 Hz, 2H), 7.33 (d, J = 8 Hz, 1H),
128.0, 127.2, 125.2, 123.8, 123.3, 64.6, 59.7, 46.4, 18.1; 77Se 7.24–7.28 (q, J = 5.6 Hz, 1H), 5.87 (s, 1H), 3.61–3.67 (m, 2H),
NMR (d6-DMSO) δ (ppm): 439, 342. ESI-MS (m/z) calcd for 3.41–3.45 (m, 2H); 13C NMR (CDCl3) δ (ppm): 141.4, 141.3,
C15H17NO2Se2 [M + H]+: 403.9668, found 403.9364.
139.7, 137.6, 132.9, 129.1, 128.0, 125.2, 123.6, 122.8, 58.9, 41.0;
Synthesis of 14. Compound 14 was synthesized by following 77Se NMR (CDCl3) δ (ppm): 453, 382.
a similar procedure reported earlier.9e A solution of compound
Synthesis of 17. A mixture of 18 (100 mg, 0.32 mmol) and
18 (200 mg, 0.64 mmol) in a mixture of 1 : 3 chloroform and 1,2-ethane dithiol (46 mg, 0.49 mmol) in 20 mL dry dichloro-
methanol was treated with sodium borohydride (145 mg, methane was treated with borontrifluoride–diethyl etherate
3.84 mmol) in small portions. The reaction mixture was stirred (80 µL, 45–50% solution) dropwise at 0 °C. The reaction
for 1 h at room temperature after which the solvent was evap- mixture was stirred for 12 h at room temperature. Then
orated under reduced pressure. Then the viscous slurry was the mixture was diluted with excess dichloromethane,
dissolved in dichloromethane, washed thoroughly with water, thoroughly washed with water, dried over anhydrous sodium
dried over anhydrous sodium sulphate and evaporated in sulphate and concentrated in vacuo to yield a dark brown
vacuo to afford a brown solid. The crude product was then pur- liquid. The crude product was then purified by column chrom-
ified by flash chromatography using chloroform/ethyl acetate atography using petroleum ether/diethyl ether as the mobile
as the mobile phase to afford 14 as a brown solid in 95% yield. phase to afford 17 as a dark brown solid in 40% yield. 1H NMR
1H NMR (CDCl3) δ (ppm): 7.5 (d, J = 8 Hz, 1H), 7.46 (d, J = (CDCl3) δ (ppm): 7.49 (d, J = 8 Hz, 1H), 7.43 (t, J = 6.8 Hz, 2H),
8.4 Hz, 1H), 7.40 (d, J = 7.2 Hz, 1H), 7.25 (d, J = 7.2 Hz, 1H), 7.31 (d, J = 8.4 Hz, 1H), 7.22–7.27 (m, 1H), 5.86 (s, 1H),
7.21 (t, J = 8 Hz, 2H), 4.74 (s, 2H); 13C NMR (CDCl3) δ (ppm): 3.59–3.65 (m, 2H), 3.39–3.45 (m, 2H); 13C NMR (CDCl3)
140.9, 140.6, 138.8, 137.5, 134.0, 127.8, 127.1, 124.9, 123.7, δ (ppm): 141.5, 141.3, 139.7, 137.6, 132.9, 129.2, 128.0, 125.2,
122.0, 66.2; 77Se NMR (CDCl3) δ (ppm): 411, 406. ESI-MS (m/z) 123.6, 122.8, 58.9, 41.0; 77Se NMR (CDCl3) δ (ppm): 453, 381.
calcd for C11H8OSe2 [M + Na]+: 338.88, found 338.96.
ESI-MS (m/z) calcd for C13H10S2Se2 [M]+: 389.8554, found
Synthesis of 15. 3-Mercaptopropanoic acid (85 mg, 389.8551.
0.8 mmol) was added to a solution of 18 (100 mg, 0.32 mmol)
Synthesis of iodothyronamines. All the nine iodothyrona-
in 20 mL dry dichloromethane and the resulting solution was mines (T4AM, T3AM, rT3AM, 3,3′-T2AM, 3,5-T2AM, 3′,5′-T2AM,
cooled to 0 °C. Borontrifluoride–diethyl etherate (80 µL, 3-T1AM, 3′-T1AM and T0AM) were synthesized following a lit-
45–50% solution) was added dropwise to the mixture at 0 °C erature procedure reported by Scanlan et al.21 with minor
and the solution was allowed to attain room temperature over modifications.
12 h. Then the reaction mixture was extracted with ethyl
T4AM. N-t-BOC-3,3′,5,5′-tetraiodothyronamine (200 mg,
acetate (20 mL, 3×) and the combined organic layers were 0.24 mmol) was treated with a 1 : 4 v/v mixture of trifluoroace-
washed thoroughly with water, dried over anhydrous sodium tic acid and dichloromethane and the solution was stirred for
sulphate and evaporated under reduced pressure to afford a 1 h at room temperature. The solvent was evaporated in vacuo
dark brown-colored liquid. The crude product was then puri- to yield a yellowish white sticky solid, which was purified by
fied by reverse-phase flash chromatography using a pre-packed reverse-phase HPLC using a C18 column (Atlantis, 250 ×
C18 silica cartridge and water/methanol as the mobile phase 19 mm, 10 μm) and 70% methanol/water as the eluent, to
to yield 15 as a dark red liquid in 45% yield. 1H NMR afford T4AM as a white solid in 90% yield. 1H NMR (d4-MeOH)
(d4-MeOH) δ (ppm): 7.54 (d, J = 8 Hz, 1H), 7.44–7.47 (dd, J = δ (ppm): 7.89 (s, 2H), 7.10 (s, 2H), 3.22 (t, J = 7.6 Hz, 2H), 2.94
2.8 Hz, 2H), 7.36 (d, J = 8.4 Hz, 1H), 7.24 (t, J = 7.6 Hz, 1H), (t, J = 7.6 Hz, 2H); 13C NMR (d4-MeOH) δ (ppm): 153.3, 151.6,
5.43 (s, 1H), 2.84–2.95 (m, 2H), 2.70–2.79 (m, 2H), 2.62–2.67 150.7, 141.0, 138.6, 126.2, 91.1, 84.7, 40.4, 31.8; ESI-MS (m/z)
(m, 4H); 13C NMR (d4-MeOH) δ (ppm): 174.5, 141.6, 141.3, calculated for C14H11NO2I4 [M
+
H]+: 733.7047, found
139.6, 137.4, 133.5, 128.9, 127.9, 125.0, 123.2, 122.5, 55.6, 34.3, 733.9314.
28.2; 77Se NMR (CDCl3) δ (ppm): 446, 390. ESI-MS (m/z) calcd
for C17H16O4S2Se2 [M − H]−: 506.8742, found 506.7935.
T3AM. T3AM was synthesized using N-t-BOC-3,3′,5-tri-
iodothyronamine (200 mg, 0.28 mmol) by following a similar
Synthesis of 16. Compound 16 was synthesized following procedure described for T4AM. Yield: 92%. 1H NMR
the procedure reported earlier.9d A mixture of 18 (100 mg, (d4-MeOH) δ (ppm): 7.89 (s, 2H), 7.00 (d, J = 2.8 Hz, 1H), 6.78
0.32 mmol) and 1,2-ethanedithiol (46 mg, 0.49 mmol) in (d, J = 8.8 Hz, 1H), 6.64–6.67 (dd, J = 6.4 Hz, 1H), 3.23 (t, J =
20 mL dry dichloromethane was treated with borontrifluoride– 8 Hz, 2H), 2.95 (t, J = 7.2 Hz, 2H); 13C NMR (d4-MeOH)
diethyl etherate (80 µL, 45–50% solution) dropwise at 0 °C. δ (ppm): 153.9, 152.5, 149.9, 141.0, 138.2, 125.6, 116.8, 115.0,
The reaction mixture was allowed to attain room temperature 91.4, 83.4, 40.5, 31.8; ESI-MS (m/z) calculated for C14H13NO2I3
over 12 h. Then the mixture was diluted with excess [M + H]+: 607.8080, found 607.7704.
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rT3AM. rT3AM was synthesized using N-t-BOC-3,3′,5′-tri- 7.21 (d, J = 8.4 Hz, 2H), 6.87 (t, J = 10.4 Hz, 4H), 6.79 (d, J = 8.8
iodothyronamine (200 mg, 0.28 mmol) by following a similar Hz, 2H), 3.15 (t, J = 8 Hz, 2H), 2.91 (t, J = 8 Hz, 2H); 13C NMR
procedure described for T4AM. Yield: 95%. 1H NMR (d4-MeOH) δ (ppm): 158.6, 154.1, 149.4, 130.6, 130.1, 121.1,
(d4-MeOH) δ (ppm): 7.85 (s, 1H), 7.31 (s, 3H), 6.91 (d, J = 8.4 117.7, 116.2, 41.1, 32.8; ESI-MS (m/z) calculated for C14H15NO2
Hz, 1H), 4.64 (br, s, 2H), 3.19 (t, J = 7.6 Hz, 2H), 2.94 (t, J = 7.6 [M + H]+: 230.1181, found 230.0648.
Hz, 2H); 13C NMR (d4-MeOH) δ (ppm): 156.0, 152.6, 151.3,
Single crystal X-ray crystallography
140.5, 135.0, 130.7, 129.0, 119.8, 88.8, 84.5, 40.7, 32.3; ESI-MS
(m/z) calculated for C14H13NO2I3 [M + H]+: 607.8080, found
607.7794.
Compound 13 was recrystallized from a 1 : 1 mixture of chloro-
form–methanol by a slow evaporation method. Yellowish
orange coloured crystals were filtered, washed with petroleum
ether and dried under high vacuum. Compound 14 was recrys-
tallized from a 1 : 2 mixture of chloroform–methanol by a slow
evaporation method. Reddish-brown coloured crystals were fil-
tered, washed with petroleum ether and dried under high
vacuum. The single crystal X-ray diffraction data of 13 and 14
were recorded on a Bruker SMART APEX CCD diffractometer
utilizing SMART/SAINT software.22 All the diffraction data were
collected at room temperature using graphite-monochroma-
tized Mo-Kα radiation of wavelength 0.71073 Å. SHELX-97
program, incorporated in WinGX, was used to solve the struc-
tures. Empirical absorption corrections were assigned with
SADABS.23,24 All the non-hydrogen atoms were refined by an-
isotropic displacement coefficients. The hydrogen atoms on
the carbon, nitrogen and oxygen atoms were included in geo-
metric positions.
3,3′-T2AM. 3,3′-T2AM was synthesized using N-t-BOC-3,3′-
diiodothyronamine (200 mg, 0.34 mmol) by following a
similar procedure described for T4AM. Yield: 93%. 1H NMR
(d4-MeOH) δ (ppm): 7.81 (d, J = 2 Hz, 1H), 7.22–7.24 (q, J =
4 Hz, 2H), 6.84 (t, J = 2.8 Hz, 1H), 6.78 (d, J = 8.4 Hz, 1H), 3.16
(t, J = 8 Hz, 2H), 2.91 (t, J = 7.6 Hz, 2H); 13C NMR (d4-MeOH)
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; ESI-MS (m/z) calculated for
C14H14NO2I2 [M + H]+: 481.9114, found 481.8453.
3,5-T2AM. 3,5-T2AM was synthesized using N-t-BOC-3,5-di-
iodothyronamine (200 mg, 0.34 mmol) by following a similar
procedure described for T4AM. Yield: 90%. 1H NMR(d4-MeOH)
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), 2.93 (d, br,
J = 6.8 Hz, 2H); 13C NMR (d4-MeOH) δ (ppm): 154.3, 152.5,
150.0, 140.9, 137.8, 116.3, 115.9, 91.6, 40.5, 31.8; ESI-MS (m/z)
calculated for C14H13NO2I2 [M
481.8681.
+
H]+: 481.9114, found
Crystal data for 13
3′,5′-T2AM. 3′,5′-T2AM was synthesized using N-t-BOC-3′,5′-
diiodothyronamine (200 mg, 0.34 mmol) by following a
similar procedure described for T4AM. Yield: 94%. 1H NMR
(d4-MeOH) 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 Hz, 2H);
13C NMR (d4-MeOH) δ (ppm): 157.1, 152.6, 151.5, 132.1, 130.5,
130.0, 118.9, 84.4, 41.0, 32.9; ESI-MS (m/z) calculated for
C14H13NO2I2 [M + H]+: 481.9114, found 481.8962.
C15H17NO2Se2, Fw = 401.22, monoclinic P21/n, a = 12.201(4) Å,
b = 6.3287(16) Å, c = 19.477(5) Å, α = 90°, β = 96.736(10)°, γ =
90°, V = 1493.6(7) Å3, Z = 4, MoKα radiation (λ = 0.71073 Å), T =
296(2) K, ρcalcd (g cm−3) = 1.784, μ(MoKα) (mm−1) = 4.951, col-
lected reflections = 4519, unique reflections = 2688, GOF (F2) =
1.070, R1a = 0.0512, wR2b = 0.1005.
Crystal data for 14
3-T1AM. 3-T1AM was synthesized using N-t-BOC-3-iodothyro-
namine (200 mg, 0.56 mmol) by following a similar procedure
C11H8O1Se2, Fw = 314.09, orthorhombic P212121, a = 4.6505(5)
Å, b = 13.4476(13) Å, c = 16.1385(16) Å, α = 90°, β = 90°, γ = 90°,
V = 1009.27(18) Å3, Z = 4, MoKα radiation (λ = 0.71073 Å), T =
296(2) K, ρcalcd (g cm−3) = 2.067, μ(MoKα) (mm−1) = 7.286, col-
lected reflections = 2297, unique reflections = 1676, GOF (F2) =
1.208, R1a = 0.0783, wR2b = 0.2135.
1
described for T4AM. Yield: 95%. H NMR (d4-MeOH) δ (ppm):
7.79 (d, J = 2 Hz, 1H), 7.19 (dd, J = 6.4 Hz, 1H), 6.77–6.83 (m,
J = 9.2 Hz, 4H), 6.71 (d, J = 8.4 Hz, 1H), 3.14 (t, J = 8 Hz, 2H),
2.88 (t, J = 7.6 Hz, 2H); 13C NMR (d4-MeOH) 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; ESI-MS (m/z) calculated for C14H14NO2I1 [M + H]+:
356.0147, found 356.0094.
Computational methods
3′-T1AM. 3′-T1AM was synthesized using N-t-BOC-3′- Gaussian03 and Gaussian09 suites of quantum chemical pro-
iodothyronamine (200 mg, 0.56 mmol) by following a similar gramme were used to perform all the DFT calculations.25 All
procedure described for T4AM. Yield: 91%. 1H NMR the hormones and hormone metabolites were optimized using
(d4-MeOH) δ (ppm): 7.30 (d, J = 2.8 Hz, 1H), 7.23 (d, J = 8.8 Hz, the hybrid Becke 3-Lee-Yang-Parr (B3LYP) exchange correlation
2H), 6.88–6.91 (m, J = 2.8 Hz, 3H), 6.83 (d, J = 8.8 Hz, 1H), 3.15 functional and 6-31+G* basis sets for all the atoms except
(t, J = 8.4 Hz, 2H), 2.92 (t, J = 8 Hz, 2H); 13C NMR (d4-MeOH) iodine for which the 6-311G** basis set was used.26 To ensure
δ (ppm): 158.0, 153.8, 150.0, 131.2, 130.2, 130.1, 120.9, 118.2, that it was a minimum on the potential energy surface, fre-
115.0, 83.3, 41.0, 32.8; ESI-MS (m/z) calculated for C14H14NO2I1 quency calculations were performed for each optimized geo-
[M + H]+: 356.0147, found 356.0525.
metry at the same level of theory using the same basis sets.
T0AM. T0AM was synthesized using N-t-BOC-thyronamine Natural Bond Orbital (NBO) calculations27 were carried out
(200 mg, 0.87 mmol) by following a similar procedure using 6-311+G** basis sets except iodine for which the
1
described for T4AM. Yield: 96%. H NMR (d4-MeOH) δ (ppm): 6-311G** basis set was used. All the theoretical calculations
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have been done on the S-configuration of the thyroid
hormones at the chiral centre.
9 (a) D. Manna and G. Mugesh, Angew. Chem., Int. Ed., 2010,
49, 9246; (b) D. Manna and G. Mugesh, J. Am. Chem. Soc.,
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Acknowledgements
This research work was financially supported by the Science
and Engineering Research Board (SERB), India. S. Mondal
thanks the Indian Institute of Science, Bangalore for a research 10 Crystallographic data of 13 and 14 have been deposited
fellowship and G. Mugesh thanks SERB for the J. C. Bose
National Fellowship (Grant No. SB/S2/JCB-067/2015).
with the Cambridge Crystallographic Data Centre as sup-
plementary publication no. 1486468 and 1486467,
respectively.
11 J. D. Baxter and P. Webb, Nat. Rev. Drug Discovery, 2009, 8,
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12 A. S. Padron, R. A. L. Neto, T. U. Pantaleão, M. C. de S. dos
Santos, R. L. Araujo, B. M. de Andrade, M. da S. Leandro,
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