Zinc (II) porphyrins as viable scaffolds to stabilize hydrogen sulfide binding at the metal center

Article abstract of DOI:10.1016/j.ica.2017.06.069

H₂S is an important biological messenger but its reactivity and physiological concentrations are largely controversial. On the other hand, reactivity of H₂S with biologically relevant metal scaffolds for the selective tracking of H₂S in biological fluids is underexplored. In this work we investigated interaction of monohydrogensulfide with a series of zinc porphyrins by ¹H NMR, UV–vis and ESI mass experiments. The results provide evidence that HS^? binds the metal center for all the investigated complexes.

Full text of DOI:10.1016/j.ica.2017.06.069

Accepted Manuscript
Research paper
Zinc (II) porphyrins as viable scaffolds to stabilize hydrogen sulfide binding at
the metal centre
Maria Strianese, Silvia Mirra, Marina Lamberti, Claudio Pellecchia
PII:
S0020-1693(17)30512-1
http://dx.doi.org/10.1016/j.ica.2017.06.069
ICA 17718
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
3 April 2017
25 May 2017
28 June 2017
Please cite this article as: M. Strianese, S. Mirra, M. Lamberti, C. Pellecchia, Zinc (II) porphyrins as viable scaffolds
to stabilize hydrogen sulfide binding at the metal centre, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/
10.1016/j.ica.2017.06.069
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Zinc (II) porphyrins as viable scaffolds to stabilize hydrogen sulfide binding at
the metal centre.
Maria Strianesea*, Silvia Mirraa, Marina Lambertib, Claudio Pellecchiaa
b
aDipartimento di Chimica e Biologia “Adolfo Zambelli”, Dipartimento di Fisica “E. Caianiello”
Università degli Studi di Salerno Via Giovanni Paolo II, 132 84084 Fisciano (SA) Italy
*Corresponding author. E-mail: mstriane@unisa.it
Abstract
H2S is an important biological messenger but its reactivity and physiological concentrations are
largely controversial. On the other hand, reactivity of H2S with biologically relevant metal scaffolds
for the selective tracking of H2S in biological fluids is underexplored. In this work we investigated
interaction of monohydrogensulfide with a series of zinc porphyrins by 1 H NMR, UV-vis and ESI
mass experiments. The results provide evidence that HS- binds the metal centre for all the
investigated complexes.
Introduction
Hydrogen sulfide (H2S) is one of the oldest molecules which continues to gain increased interest as
endogenous signaling molecule. This is the result of the fairly recent discovery that H2S joins CO
and NO as the third ‘gasotransmitter’. Biological processes attributed to H2S include vasodilation,
numerous cardiovascular effects, neuromodulation, and cytoprotection and by now its action in both
healthy and disease states is well acknowledged.1-5 Furthermore H2S has been recently shown to
induce various cytoprotective features in micromolar concentrations. However most part of the
complex biological roles H2S is involved in is still unknown.6 Thus selective tracking of this small
molecule in physiological conditions is particularly relevant to elucidate its complex contributions
to both healthy and disease states.7
To date, despite the renewed interest in the complex biology of H2S, the coordination of H2S to bio-
inspired transition-metal scaffolds remains still limited. This is probably due to the redox activity of
sulfide and to its propensity to form insoluble metal sulfides which inhibit isolation of metal
hydrosulfido adducts.8 Such coordination chemistry is highly interesting since the mechanisms of
action of H2S can be envisioned to involve interactions with transition metals. Among the multitude

of sites where H2S is believed to play a role, heme centers represent perhaps the most logical targets
of metal-based reactivity. Over the years, the different devised strategies (e.g. steric protection,
secondary-coordination sphere interactions) have been mostly focused on studying H2S reactivity
with iron-based systems,9-13 probably due to the importance of heme iron in biological targets.
However examples of transition metal complexes featuring H2S or HS_ binding to the metal center
(where the metal center is different than iron) have been isolated and fully characterized.14;15 Stable
8;16-19
zinc hydrosulfide complexes are not numerous
scaffolds.12;20-22
so as, in particular, studies on porphyrinoid
Aiming at shedding light on the biochemical reactions H2S is involved in, our studies on the
coordination of H2S to transition metals started some time ago and focused both on properly
tailored low molecular weight compounds and on natural metallo-proteins.23-28 In 2015 our research
group reported a simple copper porphyrin complex able to bind HS- in basic aqueous media.29 Very
recently we described binding of hydrogen sulfide to a zinc porphyrin complex and stabilization of
the related hydrosulfido adduct as assessed by high-resolution MALDI Fourier transform ion
cyclotron resonance mass spectrometry (HR MALDI-FT-ICR) and 1H NMR experiments.30
In this work, encouraged from the latter results,30 and considering that metalloporphyrins (and in
particular zinc protoporphyrinIX) have already an estabilished biological role as Heme Oxygenase
inhibitors,31-33we synthesized a series of zinc porphyrins complexes (see scheme 1) for studying
their propensity to bind HS-.
The modifications on the porphyrin moieties were realized to study the possible effect of different
substituents on the metal hydrosulfido stabilization. More specifically, as a starting point (complex
1, scheme 1) we selected protoporphyrin IX in analogy with the copper porphyrin complex we had
already found able to bind H2S.29 In the case of complex 3 (scheme 1) we aim at assessing whether
substituents on the meso-positions and the concomitant presence of acidic protons would influence
the HS- binding reactivity of the resulting complex. In the case of complex 2 (the methyl ester
derivative of complex 1) we want to investigate whether acidic protons affect the HS- binding
OH
reactivity.
CH2
CH3
H3C
CH2
N
N
N
N
N
N
N
N
Zn
Zn
OH
HO
CH3
H3C
O
Scheme 1
O
OR
RO
OH
complex 3
complex 1: R = H
complex 2: R = CH3

Results and discussion.
Complexes 1-3 were obtained in good yield by allowing to react an equimolar amount of the
porphyrin ligand and Zn(CH3COO)2 in DMF by following a slightly modified literature
procedure.34-36 The stoichiometry of complexes 1-3 was confirmed by ESI mass experiments (figure
S1-S3 in the SI). For all the complexes under investigation no peaks ascribable to binuclear species
were found.
Complexes 1-3 are fully soluble in dimethylsulfoxide and in basic aqueous solutions.
The proton NMR spectrum of complex 1 in DMSO-d6 (figure S4 in the SI) shows four singlets for
the meso-protons at 10.28, 10.19, 10.17 and 10.15 ppm, and three multiplets, each integrating for
two protons, for the vinyl units centered at 8.54, 6.42, and 6.16 ppm. The methylene protons of the
propionate groups display signals at 4.33 and 3.19 ppm, while the protons of the four methyls give
four singlets at 3.76, 3.75, 3.64 and 3.62 ppm. The broad band centered at 7.18 ppm is ascribed to
1
the protons of the OH groups on the carboxylic moieties. The H NMR spectrum of complex 2 in
DMSO-d6 (figure S5 in the SI) is quite similar to that of complex 1 (see experimental part for the
ppm values), but a broad signal attributable to the methyl protons of the ester moieties appears at
3.58 ppm instead of the signals of the OH protons. The 1H NMR spectrum of complex 3 in DMSO-
d6 (figure S6 in the SI) displays a singlet at 9,86 ppm ascribable to the hydroxylic protons, a singlet
at 8.80 ppm for the β-pyrrolic protons and two doublets at 7.96 and 7.17 ppm for the aromatic
protons. These assignments were made by comparison with spectra of similar porphyrin complexes
reported in the literature.36;37 A further characterization of complexes 1-3 was also performed via
UV-vis (figures S7- S9 in the SI). Electronic absorption spectra of complexes 1-3 feature both the
B-band (Soret band) and the Q-bands, as expected for metal porphyrin complexes.38;39 When
comparing the UV-vis spectra of the series of zinc porphyrins we are investigating, we observed
changes both in the region of the Soret bands (around 400 nm) and in the region of the Q-bands
(500-650 nm). More precisely, the positions of the Soret band undergo shifts due to variations in the
substituent groups on the porphyrin ring, for instance complex 1 features the Soret band centered at
421 nm whereas in the case of complex 3 this band shifts to 437 nm. Complex 3 exhibits a
bathochromic shift with respect to complex 1. In the region of the Q-bands we observed two bands
(called α and β) for all the complexes under investigation with different relative intensities.
Accordingly, when comparing the UV-vis spectrum of complex 3 to that of complex 1 also the Q-
bands result red-shifted most likely for the presence of the electron-donating phenoxy groups at the
meso-positions.
Interaction of HS- with complexes 1-3 and its possible binding to the metal center was first studied
via 1H NMR in DMSO-d6 solution. Addition of an excess of NaSH to the solution of the complex

resulted in an upfield shift of the proton resonances of the porphyrin moiety for all the three zinc
complexes. For complexes 1 and 3 the concomitant deprotonation of the ligand (with disappearance
of the signals of the acidic protons) occurred (see figure 1 and figure S10). The similar shift in the
pattern of resonances of complex 2 lacking acidic protons, upon HS- addition, suggests that acid-
base reactions are not responsible of the spectral change pointing to HS- binding to the metal center
(see figure 1 c). The Zn porphyrin-HS adducts resulted stable for about one week in DMSO-d6, as
assessed via NMR.
a
*
b
c
*
1
Figure 1. (a) H NMR spectrum of complex 1 in DMSO-d6 (lower trace) and after addition of an
excess of HS- (upper trace). [complex 1] = 10x10-3 M ; [NaSH] = 0,10 M. (b) 1H NMR spectrum of
complex 3 in DMSO-d6 (lower trace) and after addition of an excess of HS- (upper trace).
1
[complex 3] = 10x 10-3 M; [NaSH] = 0,10 M. (c) H NMR spectrum of complex 2 in DMSO-d6
(lower trace) and after addition of an excess of HS- (upper trace). [complex 2] = 10x10-3 M; [NaSH]
= 0,10 M. Peaks denoted with * belong to the acidic protons (ie. the hydrogens of the COOH
groups in complex 1 and the hydrogens on the phenolic moieties in complex 3).
To confirm that the observed shifts are not due to simple deprotonation of the porphyrin ligand, we
performed a control experiment. We first added 4 equivalents of KOH to the NMR solution of
complex 3 (i.e. to deprotonate the acidic groups on the porphyrin ring) and observed disappearance
of the acidic protons and then added an excess of NaSH. Again a visible shift of the proton
resonances of the porphyrin moiety was registered (figure 2), supporting coordination of HS- with
formation of the [Zn porphyrin-HS]- adduct.

Figure 2. 1H NMR spectrum of complex 3 in DMSO-d6 (lower trace) and after addition of 4 equiv.
of KOH (middle trace) and of 10 equiv. of HS- (upper trace). [complex 3] = 10x10-3 M; [NaSH] =
0,10 M.
To test whether HS- coordination is acid-labile (e.g. if the addition of a suitable proton source
results in chemically reversible coordination of HS-), as shown in the case of related systems,17;30
we prepared, as a test case, the 1-HS species in situ by adding 100 equiv of NaSH to the complex in
1
DMSO-d6 and then added an excess of acetic acid. The initial H NMR spectrum of complex 1
(lower spectrum), which undergoes an upfield shift upon addition of HS- (middle spectrum), was
restored when acetic acid was added (upper spectrum). Figure 3 shows the 1H NMR spectra.
Figure 3. 1H NMR spectrum of complex 1 in DMSO-d6 (lower trace), after addition of an excess of
HS- (middle trace) and after addition of CH3COOH (upper trace). [complex 1] = 10x10-3 M ;
[NaSH] = 0,10 M.
Interaction of HS- with complexes 1-3 was then studied via UV-vis spectroscopy in basic water
DMSO. Upon addition of an excess of NaSH to the solution of each complex under investigation,

sizeable changes of the initial absorption spectra of complexes 1-3 were observed (figures S11-
S13). In particular clear bathochromic shifts of the Soret band were observed for all the tested
complexes. In the case of complex 1 and complex 2 the two Q-bands (which were initially well
resolved and separated in the UV spectra of the free complexes) coalesce into a single peak with a
small shoulder. In the case of complex 3 the Q-bands result both red-shifted in the presence of HS-.
To obtain independent confirmation of HS- binding to the metal centers we examined the reaction
between complex 1-3 and HS- by electrospray ionization mass spectrometry. We measured the ESI
spectra of the reaction mixtures between complexes 1-3 and an excess of NaSH (~10 equivalents)
soon after the mixing (ionizing the product of the reaction in the negative ion mode) and compared
the spectrum with those of the HS--free complexes (under the same experimental conditions). In the
spectrum of the hydrogen sulfido adducts an additional peak matching the expected mass and
isotope pattern of the [complex-HS]- species was visible in the case of all the complexes tested. As
an example figure 4 displays a region of the ESI spectrum of the hydrogen sulfido adduct of
complex 2. The peak at 685.187 m/z units matches the expected mass (685.183 m/z) and isotope
pattern of [2-HS]- (see figure 4).

Figure 4. Enlargement of a region of the ESI Mass spectrum of 2 in the presence of 10 equiv. of
NaSH in methanol (ionizing the sample in the negative ion mode). The upper trace is the
experimental trace whereas the lower is the theoretical one.
Similar findings were found when studying the spectra of the hydrogen sulfido adduct of complexes
1 and 3 (figures S14-15). These findings provide evidence that HS- binding to the metal center
occurs for all the complexes tested and corroborates the hypotheses made on the basis of 1H NMR
and UV-vis spectroscopy.
Conclusions
1
ESI mass, H NMR and UV-vis experiments provide evidence that HS- coordination occurs at the
metal center of the synthesized porphyrin complexes and proved stabilization of the resulting zinc
hydrosulfide complexes for about one week by NMR.
These findings could help to understand how H2S interacts with a series of biologically relevant
zinc scaffolds. From our results it seems that the substituent groups on the porphyrin ligands do not
influence HS- binding at the metal center. We believe the target systems have the potential to be
implemented as HS- sensors and maybe helpful for insights on the selective tracking of H2S in
physiological conditions.
Experimental Section
Materials: All chemicals used for the synthetic work were obtained from Sigma-Aldrich or Strem
Chemicals and were of reagent grade. They were used without further purification.
General. Mass spectrometry analyses were carried out using a Bruker SolariX XR Fourier
transform ion cyclotron resonance mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany)
equipped with a 7 T refrigerated actively-shielded superconducting magnet (Bruker Biospin,
Wissembourg, France). The samples were ionized in negative ion mode using the ESI ion source
(Bruker Daltonik GmbH, Bremen, Germany). The mass range was set to m/z 150 – 3000. The mass
spectra were calibrated externally using a NaTFA solution in negative ion mode. A linear
calibration was applied. For the in situ experiments between complexes 1-3 and NaSH the reaction
was performed in either basic MilliQ water or in methanol in a 1:10 molar ratio. Room temperature
NMR spectra were recorded on a Bruker AVANCE 400 NMR instrument (1H, 400.13 MHz; 13C,

∂ (ppm) referenced
100.62 MHz) using 5 mm o.d. NMR tubes. The chemical shifts were reported in
to SiMe4. Typically, 5 mg of the complex in 0.5 mL of solvent were used for each experiment.
Synthesis of complex 1: 0.20 g (0.35 mmol) of protoporphyrin IX in 20 ml of DMF is refluxed
with 0.155 g (0.71 mmol) of zinc(II) acetate dihydrate. The solution is left under stirring and at
reflux for 2 hours. The reaction flask is removed from the hot plate and cooled in an ice bath for 15
minutes. 200 ml of chilled water is added and a crystalline precipitate forms. After checking that
conversion to the Zn(II)-porphyrin is complete, as indicated by the absence of free porphyrin
absorption bands in a visual spectroscope, the solution is filtrated via buckner. The solid is washed
with fresh water and evaporated to dryness dried under vacuum to yield the purple-brown solid.
Yield: 80%. C34H32ZnN4O4: calcd C 49.28, H 3.89, N 6.76; found C 49.30, H 3.90, N 6.73. MS
1
(ESI water): m/z (%) 623.164 (100) [C34H31N4O4Zn]-. H NMR [400 MHz, DMSO-d6]: δ =3.19
(4H, 2 x CH2, m), 3.62 (3H, CH3, s), 3.64 (3H, CH3, s), 3.75 (3H, CH3, s), 3.76 (3H, CH3, s), 4.33
(4H, 2 x CH2, m), 6.16 (2H, m), 6.42 (2H, m), 7.18 (OH, bs), 8.54 (2H, m), 10.15 (1H, s), 10.17
(1H, s), 10.19 (1H, s), 10.28 (1H, s) ppm.
Complex [1-HS]-. 1H NMR [400 MHz, DMSO-d6]: 2.84 (4H, 2 x CH2, m), 3.52 (3H, CH3, s), 3.54
(3H, CH3, s), 3.68 (3H, CH3, s), 3.69 (3H, CH3, s), 4.22 (4H, 2 x CH2, m), 6.04 (2H, m), 6.33 (2H,
m), 8.45 (2H, m), 9.88 (1H, s), 9.91 (1H, s), 10.02 (1H, s), 10.09 (1H, s) ppm. MS (ESI water): m/z
(%) 655.106 (100).
Synthesis of complex 2: 0.10 g (0.17 mmol) of protoporphyrin IX dimethyl ester in 10 ml of DMF
is refluxed with 0.075 g (0.34 mmol) of zinc(II) acetate dihydrate and worked up similarly to
complex 1. Yield: 75%. C36H36ZnN4O4: calcd C 66.11, H 5.55, N 8.57; found C 66.15, H 5.58, N
1
8.60. MS (ESI methanol): m/z (%) 651.195 (100) [C36H35N4O4Zn]-. H NMR [400 MHz, DMSO-
d6]: δ = 3.29 (4H, 2 x CH2, t, J = 7.35 Hz), 3.58 (6H, 2 x OCH3, bs), 3.60 (3H, CH3, s), 3.62 (3H,
CH3, s), 3.74 (3H, CH3, s), 3.75 (3H, CH3, s), 4.36 (4H, 2 x CH2, t, J = 7.35 Hz), 6.14 (2H, m), 6.41
(2H, m), 8.54 (2H, m), 10.08 (1H, s), 10.17 (1H, s), 10.18 (1H, s), 10.27 (1H, s) ppm.
Complex [2-HS]-. 1H NMR [400 MHz, DMSO-d6]: 3.26 (4H, 2 x CH2, t, J = 7.58 Hz), 3.596 (3H,
OCH3, s), 3.599 (3H, OCH3, s) 3.54 (3H, CH3, s), 3.56 (3H, CH3, s), 3.68 (3H, CH3, s), 3.70 (3H,
CH3, s), 4.28 (4H, 2 x CH2, bs), 6.06 (2H, m), 6.34 (2H, m), 8.45 (2H, m), 9.81 (1H, s), 9.93 (1H,
s), 9.95 (1H, s), 10.05 (1H, s) ppm. MS (ESI methanol): m/z (%) 685.187 (100).
Synthesis of complex 3: 0.25 g (0.37 mmol) of 5,10,15,20-Tetrakis(4-hydroxyphenyl)-21H,23H-
porphine ester in 20 ml of MeOH is refluxed with 0.808 g (3.7 mmol) of zinc(II) acetate dihydrate

and worked up similarly to complex 1. Yield: 80%. C44H30ZnN4O4: calcd C 71.02, H 4.06, N 7.53;
found C 71.05, H 4.08, N 7.56. MS (ESI water): m/z (%) 763.129 (60) [C44H28N4O4ZnNa]+;
779.103 (100) [C44H28N4O4ZnK]+ .1H NMR [400 MHz, DMSO-d6]: δ =9.86 (4H, hydroxyl-H, s),
13
8.80 (8H, pyrrole-H, s), 7.96 (8H, phenyl-H, d) 7.17 (8H, phenyl-H, d) ppm. C NMR [100 MHz,
DMSO-d6] : δ = 19.22, 26.24, 75.87, 113.89, 120.63, 131.53, 133.68, 135.40, 150.03, 157.20.
Complex [3-HS]-. 1H NMR [400 MHz, DMSO-d6]: δ = 8.64 (8H, pyrrole-H, s), 7.86 (8H, phenyl-
H, d) 7.18 (8H, phenyl-H, d) ppm. MS (ESI water): m/z (%) 771.161 (100).
Absorbance measurements. Absorption spectra were recorded on a Cary-50 Spectrophotometer,
using a 1 cm quartz cuvette (Hellma Benelux bv, Rijswijk, Netherlands) and a slit-width equivalent
to a bandwidth of 5 nm. Measurements were performed in basic water (pH = 12,6) at room
temperature.
NMR characterization of the [Zn porphyrins-HS]- adducts. The NMR tube was charged with
solutions of complexes 1-3 in DMSO-d6 then NaSH solid (to the end concentrations specified in the
figure captions) was added and the spectra registered.
ACKNOWLEDGMENT
The authors thank Dr. Patrizia Oliva and Dr. Patrizia Iannece of the Department of Chemistry and
biology (University of Salerno) for their technical assistance.

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Entry for the Table of Contents
Zinc (II) porphyrins as viable scaffolds to stabilize hydrogen sulfide binding at
the metal centre.
Maria Strianesea*, Silvia Mirraa, Marina Lambertib, Claudio Pellecchiaa
b
aDipartimento di Chimica e Biologia “Adolfo Zambelli”, Dipartimento di Fisica “E. Caianiello”
Università degli Studi di Salerno Via Giovanni Paolo II, 132 84084 Fisciano (SA) Italy
HS
N
N
N
N
HS-
N
N
N
N
Zn
Zn
H3O+
In this work we investigated interaction of monohydrogensulfide with a series of zinc porphyrins by
1H NMR, UV-vis and ESI mass experiments. The present data provide proof-of-principle that HS-
binds the metal centre for all the investigated complexes.
*Corresponding author. E-mail: mstriane@unisa.it

Highlights :
• Binding of HS- to a series of zinc porphyrins is explored
• The different substituents on the porphyrin skeleton do not seem to play a role
• HS- binding to the zinc centers is assessed via ESI Mass, UV-vis and NMR
experiments