Zinc(II) and copper(II) complexes with pheophytin and mesoporphyrin and their stability to UV-B irradiation: Vis spectroscopy studies

Article abstract of DOI:10.2298/JSC110310175Z

The stability of Zn(II) and Cu(II) complexes of porphyrin derivatives (pheophytin and mesoporphyrin) to UV-B irradiation was studied by absorbance spectroscopy in 95 % ethanol. The chosen porphyrins and their heavy metal complexes underwent first-order ph

Full text of DOI:10.2298/JSC110310175Z

J. Serb. Chem. Soc. 77 (2) 187–199 (2012)
JSCS–4260
UDC 546.472’562+547.979.733+
544.032.6:66.022.362
Original scientific paper
Zinc(II) and copper(II) complexes with pheophytin and
mesoporphyrin and their stability to UV-B irradiation:
Vis spectroscopy studies
*
JELENA B. ZVEZDANOVIĆ , DEJAN Z. MARKOVIĆ and SANJA M. MILENKOVIĆ
University of Niš, Faculty of Technology, Bulevar oslobodjenja 124, 16000 Leskovac, Serbia
(Received 10 March, revised 20 July 2011)
Abstract: The stability of Zn(II) and Cu(II) complexes of porphyrin derivatives
(pheophytin and mesoporphyrin) to UV-B irradiation was studied by absor-
bance spectroscopy in 95 % ethanol. The chosen porphyrins and their heavy
metal complexes underwent first-order photochemical decomposition. In ge-
neral, pheophytin was more stable than mesoporphyrin to UV-B irradiation.
Moreover, the stabilities of the Zn(II) complexes were lower than those of the
Cu(II)-complexes for both pheophytin and mesoporphyrin. However, while the
Cu(II)-complex with pheophytin was more stable than the one with mesopor-
phyrin, the situation was vice versa for the Zn(II)-complexes.
Keywords: pheophytin; mesoporphyrin; heavy metal; complexes; UV-B ki-
netics.
INTRODUCTION
Pheophytin and mesoporphyrin belong to the family of porphyrin compounds.
The structure of mesoporphyrin IX (MP), 7,12-diethyl-3,8,13,17-tetramethyl-por-
1,2
phyrin-2,18-dipropionic acid, is shown in Fig. 1A. Insertion of heavy metals
(HM) such as zinc or copper (in the form of ions) into the center of the porphyrin
structure leads to the formation of a heavy metal–mesoporphyrin (HM–MP) com-
3
plexes, in which the new metal coordinates the four symmetric pyrrole rings. On
the other hand, pheophytin (Pheo – shown in Fig. 1B) is a derivative of the major
photosynthesis pigment, chlorophyll (Chl), with an isocyclic cyclopentanone ring
fused to a C-pyrrole ring of the porphyrin core between the C-13 and C-15 posi-
tions. In chlorophyll, the central magnesium (Mg) atom is bonded with the N-atoms
from the four symmetric pyrrole rings by two covalent and two coordination
3
bonds. Pheophytin is formed when chlorophyll is depleted of magnesium (Fig.
1B). It is known that heavy metals can replace the labile bonded central Mg-atom
*Corresponding author. E-mail: jelite74@yahoo.com
doi: 10.2298/JSC110310175Z
187
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ZVEZDANOVIĆ, MARKOVIĆ and MILENKOVIĆ
of chlorophyll to form substitutional or “central” complex of chlorophyll, i.e. heavy
4,5
metal complex of pheophytin, HM–Pheo.
Fig. 1. Structure of mesoporphyrin (A), and pheophytin (B) with numerated C-positions;
R = CH3 in pheophytin a; R = CHO in pheophytin b.
It is well known that porphyrins and their metal derivatives have attracted
significant attention in many areas because of their spectral properties; their cha-
racteristic absorption spectrum consists of an intense (Soret or B) band close to
6,7
400 nm and a less intense (Q) band in the region 500–700 nm. They make a
core in many other types of molecules, such are chlorophyll and hemoglobin, as
8
well as in many enzymes where they play a heme role. In numerous biological
3,9
and solar energy applications, it is often important to examine the basic pro-
perties of porphyrins and their metal-complexes when they are irradiated by UV
light (e.g., UV-B). Treatment of chlorophyll with strong UV and visible light in
solutions and in isolated photosynthetic organelles results in irreversible break-
10–12
down of chlorophyll, i.e., bleaching.
This work deals with the stability of two porphyrin derivatives, i.e., pheo-
phytin and mesoporphyrin, and their heavy metal complexes to continuous UV-B
irradiation. The irradiation was performed in 95 % ethanol for different irradia-
tion periods, providing possibilities for kinetic analysis.
EXPERIMENTAL
All experiments, beginning with the extraction, were performed under dim light as far as
possible, and inside vessels and equipment covered with aluminum foil or black cloth, thus
preventing exposure of the pigments to light.13
Mesoporphyrin IX (Sigma Aldrich) was a gift from the Hugo Scheer Laboratory, Bo-
tanisches Institute, LMU University, Munich, Germany.
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STABILITY OF (METALLO)PORPHYRINS TO UV-B IRRADIATION
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Chlorophyll isolation and preparation of pheophytin
The chlorophylls were isolated from plant pigments (extracted from spinach leaves,
Spinacia oleracea) using an already published method.4,14 Chlorophyll fraction (containing
Chla and Chlb in molar ratio ≈ 6:1, respectively) was isolated by column chromatography
with silica gel as the adsorbent (silica gel 60, Merck, 0.063–0.200 mm) and an n-hexane/ace-
tone mixture as the eluent.15 The n-hexane/acetone ratio was changed from initial 1:0 to final
1:1, to permit easier elution of the polar fractions. Chlorophyll was eluted at an eluent compo-
sition 1:0.1 (n-hexane/acetone, respectively). To prove the purity of the Chl-fraction, high
pressure liquid chromatography (HPLC) was employed using a Chla-standard (Sigma–Al-
drich) in acetone; the chlorophyll fractions were then run through HPLC as reported, proving
large content of Chla in each of them and a minor contribution of Chlb (the peaks ratio
8.5:1).15 The total Chl content (Chla + Chlb) in the isolated Chl-fraction has been calculated
as reported.16
Pheophytins (a and b) were made from the collected chlorophyll in 95% ethanol by the
dropwise addition of 1.0 M HCl.17 Conversion was completed in approximately 2 h in the
dark as observed by the color change from green to olive brown. The freshly made Pheo
matter was then extracted by n-hexane and then dissolved in 95 % ethanol. The Pheoa and
Pheob content in the pheophytin fraction in 95% ethanol was calculated as reported pre-
viously.16
Preparation of pheophytin and mesoporphyrin heavy metal complexes
Zinc(II) and copper(II) complexes of pheophytin – denoted as Cu–Pheo and Zn–Pheo*,
were prepared from Chl using a modified method proposed by Kupper.18 Chlorophyll, dried at
room temperature, was dissolved in 95 % ethanol and a solution of CuSO4, or ZnSO4, was
then added. The substitution reaction of the central Mg in the Chl molecule by Zn, or Cu,
(formation of the central Zn– or Cu–Pheo complexes) was performed by heating the reaction
mixture in a reflux apparatus for 1 h at 40 °C, followed by 24 h at room temperature.
The copper and zinc complexes of mesoporphyrin (Cu–MP and Zn–MP) were prepared
using the patent method of Inoue.19 Mesoporphyrin was placed in 95 % ethanol and a 50 %
ethanolic solution of CuSO4, or ZnSO4, was then added. Formation of the Cu–, or Zn–MP,
complex was performed by heating the reaction mixture in a reflux apparatus for 1 h at 40 °C,
followed by 24 h at room temperature.
Before metal ions addition to the pigment solutions, control experiments were performed
in order to check the stability of the pigment solutions, and to assign all potential absorption
changes to other factors other than metal–porphyrin interactions. The controls were realized in
a period of 3 days (with porphyrin solutions only) by controlling their absorption spectra over
the indicated period.
*Strictly speaking, those complexes should be denoted as Cu(II)–Pheo and Zn(II)–Pheo.
However, essentially the pheophytin complex with Cu(II) or Zn(II) and chlorophyll (Chl)
complex with Cu(II) or Zn(II) are basically the same, and the manner of their formation is
practically the same: with chlorophyll, the formation these complexes is preceded by the
removal of the central Mg atom (i.e., pheophytin formation); then Cu(II) or Zn(II) ions (from
surrounding solution) fill “the hole” and the complexes are formed. Therefore in most cita-
tions, Cu(II)–Pheo and Zn(II)–Pheo are denoted like Chl-complexes with these metals, i.e.,
Cu(II)–Chl and Zn(II)–Chl, or just Cu–Chl and Zn–Chl.
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ZVEZDANOVIĆ, MARKOVIĆ and MILENKOVIĆ
UV-B Treatment
In all the experiments, the final concentration of pheophytin and mesoporphyrin was
adjusted to 2.4 µM. Continuous irradiation of Pheo and MP solutions, as well as their Zn(II)-
and Cu(II)-complexes in 95 % ethanol was performed in a cylindrical photochemical reactor
“Rayonnet” with 8 symmetrically placed UV-B lamps having an emission maximum at 300
nm. The samples were irradiated in quartz cells (1 cm×1 cm×4.5 cm) placed on a circular
rotating holder. The total measured energy flux (hitting the samples) was about 12.0 W m-2 at
a distance of 10 cm from the lamps.
Vis spectroscopy
The spectrophotometric measurements were made on a Varian Cary-100 spectrophoto-
meter equipped with 1.0 cm quartz cells. All spectra before and after irradiation were recorded
from 300 to 750 nm with a 1.0 nm bandwidth.
HPLC Analysis
HPLC analysis of the isolated chlorophyll was performed under isocratic conditions;
apparatus: Agilent 1100 series, Waldborn, Germany; column: Zorbax Eclipse XDB-C18; mo-
bile phase: acetonitrile/methanol/ethyl acetate, 60:20:20; flow rate: 0.5 cm3 min-1; tempera-
ture: 25 °C. The monitoring wavelengths were 430 and 660 nm.
RESULTS AND DISCUSSION
Vis (absorption) spectra of UV-B-irradiated porphyrin derivatives
Absorption spectra of pheophytin (A), Zn–Pheo complex (B) and Cu–Pheo
complex (C) following increasing periods of continuous UV-B irradiation in 95
% ethanol are shown in Fig. 2. Similarly, absorption spectra of mesoporphyrin
(A), Zn–MP complex (B) and Cu–MP complex (C) following increasing periods
of UV-B irradiation in 95 % ethanol are shown in Fig. 3*.
It is well known that porphyrins have two major absorption bands in the vi-
sible range, due to extended π-delocalization at the edge of the cyclic tetrapyrrole
6,7,20
(porphyrin) skeleton: a ‘‘red’’ (Q) band and a ‘‘blue’’ (Soret or B) band.
HM–Pheo complexes showed spectroscopic behavior different from those of Chl
and Pheo themselves, both qualitatively (shifts of the characteristic bands maxi-
mums, A
ing bands).
), as well as quantitatively (different intensities of the correspond-
Qmax
6,7,18,21
For example, the formation of the Cu–Pheo complex was fol-
lowed by a characteristic hypsochromic (“blue”) shift of the Q absorption band,
4,21
compared to Chl itself;
complex.
this effect was not clearly seen with the Zn–Pheo
The absorption maximum of the Q-band (A ) for Pheo a in 95 %
Qmax
4,21
ethanol was located at 665.0 nm, while same absorption maximum for Zn–Pheo
and Cu–Pheo in 95 % ethanol were located at 664.0 and 654.0 nm, respectively
16
(Fig. 1S of the Supplementary material and Fig. 2). On the other hand, the Cu–MP
and Zn–MP complexes expressed very different spectral behavior in comparison to
*The absorption spectra of: pheophytin (a), Zn–Pheo (b), Cu–Pheo (c) and mesoporphyrin
(d), Zn–MP (e), Cu–MP (f) in 95 % ethanol are shown in Fig. 1S of the Supplementary
material to this paper.
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STABILITY OF (METALLO)PORPHYRINS TO UV-B IRRADIATION
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Fig. 2. UV-B-induced decom-
position of Pheo (A), Zn–
–Pheo (B) and Cu–Pheo (C)
in 95 % ethanol – changes in
the absorption spectra fol-
lowed their exposure to UV-B
radiation. The exposure times
are displayed for all three ir-
radiated samples. The initial
samples concentration was 2.4
µM.
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ZVEZDANOVIĆ, MARKOVIĆ and MILENKOVIĆ
Fig. 3. UV-B-induced decom-
position of MP (A), Zn–MP
(B) and Cu–MP (C) in 95 %
ethanol – changes in the ab-
sorption spectra followed their
exposure to UV-B radiation.
The exposure times are dis-
played for all three irradiated
samples. The initial samples
concentration was 2.4 µM.
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STABILITY OF (METALLO)PORPHYRINS TO UV-B IRRADIATION
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MP itself; seen through the higher absorption in “blue” region, i.e., the Soret (B)
3,22,23
band (Fig. 1S of the Supplementary material and Fig. 3).
maximum of the Soret band, (A
The absorption
) of MP and the Zn– and Cu–MP com-
Soret-max
plexes in 95 % ethanol were found at ≈ 394 nm and the maxima were, respecti-
vely, ≈ 2 and ≈ 3 times higher for Zn– and Cu–MP, compared to MP. The metal
coordination of the four N-atoms leads to an increase of the porphyrin symmetry
(in comparison to the free base porphyrin) and consequently to a rise in the inten-
24,25
sity of the Soret band.
The UV-induced changes of chlorophylls were already detected in various
solvents, such as acetone and n-hexane, using the Q-band as a sensitive indi-
10
cator. Similar to this, in the present study, it was found that UV-B irradiation
induced a gradual decrease of the absorption over the whole measured spectral
range (300–750 nm), e.g., a hypochromic effect was clearly observed for irra-
diated pheophytin, and the Zn– and Cu–Pheo complexes in 95 % ethanol, as shown
in Figs. 2A–2C. In addition, it was also found that UV-B irradiation of mesopo-
rphyrin and its Zn– or Cu–MP complexes induced a clear decrease of the Soret
band intensity (A ), as shown in Fig. 3. Clearly, UV-B irradiation of pheo-
Soret-max
phytin and its Zn(II)- and Cu(II)-complexes, as well as of MP and its Zn(II)- and
Cu(II)-complexes in 95 % ethanol results in their irreversible photochemical de-
composition – bleaching.
Kinetics of the UV-B-induced photochemical decomposition the porphyrin
derivatives
Photochemical decomposition kinetics of pheophytin and HM–Pheo com-
plexes (Cu–Pheo and Zn–Pheo), as well as of MP and the HM–MP complexes
(Cu–MP and Zn–MP), i.e., logarithmic (ln) plots of the absorption vs. time of ir-
radiation with UV-B light, are shown in Figs. 4 and 5, respectively. The decom-
position (bleaching) rate constants, k, for Pheo and the HM–Pheo complexes, and
for MP and the HM–MP complexes were calculated from the slopes given in
Figs. 4 and 5:
y = n – kx
where y is the ln of the absorbance maximum, ln A
and ln A
, respec-
Soret-max
Qmax
tively, and x is the UV-B irradiation time (in min). The decomposition kinetics of
Pheo and the Zn– and Cu–Pheo complexes were followed by recording the value
of the absorption maximum of the Q band, A
(665 nm for Pheo, 664 nm for
Qmax
Zn–Pheo and 654 nm for Cu–Pheo, Fig. 2). On the other hand, the decomposition
kinetics of MP, and the Zn– and Cu–MP complexes were followed by recording
absorption the maximum of the Soret band, A
at ≈ 394 nm (Fig. 3). The
Soret-max
UV-B-induced decomposition (bleaching) of the porphyrin derivatives fits the
first order kinetic model, as shown in Figs. 4 and 5. The calculated rate constants,
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ZVEZDANOVIĆ, MARKOVIĆ and MILENKOVIĆ
Fig. 4. First order kinetic
plots for the photochemical
decomposition of pheophytin
(A), Zn–Pheo (B) and Cu–
–Pheo (C) in 95 % ethanol
under UV-B irradiation. The
absorbances were taken from
the Q-band maximum.
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STABILITY OF (METALLO)PORPHYRINS TO UV-B IRRADIATION
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Fig. 5. First order kinetic
plots for photochemical de-
composition of mesoporphy-
rin (A), Zn–MP (B) and Cu–
–MP (C) in 95 % ethanol
under UV-B irradiation. The
absorbances were taken from
the Soret-band maximum.
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ZVEZDANOVIĆ, MARKOVIĆ and MILENKOVIĆ
–1
k (in min ), for Pheo and MP as well as for their Zn- and Cu-complexes in 95 %
2
ethanol are given in Table I (correlation coefficient r ≈0.99).
TABLE I. First order rate constants (k / 10-3 min-1) for photochemical decomposition of pheo-
phytin, mesoporphyrin and their Zn(II)- and Cu(II)-complexes in 95 % ethanol under UV-B
irradiation
Compound
Derivatives of pheophytin
Derivatives of mesoporphyrin
No metal
Zn(II)-complex
Cu(II)-complex
7.2
34
4.7
30
17
9.6
The bleaching rate constants of the Zn–Pheo and Zn–MP complexes gene-
rally exhibited higher values than those for the Cu(II)-complexes (Table I). The
ratios of the UV-B induced decomposition rate constants of the Zn(II)- and Cu(II)-
-complexes of Pheo and MP, taken separately (i.e., k
/k
and
Zn–Pheo Cu–Pheo
k
/k
) were 7.3 and 1.8, respectively (Table I). In porphyrins, the ener-
Zn–MP Cu–MP
gy levels of the corresponding electronic transitions are determined by the elec-
trostatic interactions between the N (nitrogen)-tetrapyrolles-mediated π-electron
densities and the metal center, and hence the effect of the central metal is related
26
to its electronegativity. In general, the stability of the metal complexes of pheo-
phytin and mesoporphyrin in solution can be explained by theoretical analysis of
the Falk “stability factor” (including charge number of the metal ion, effective ra-
13
dius of the metal ion in Å and the Pauling electronegativity). According to the
Falk equation, the stability of the complexes is ordered as: Mg(II)-complex ≈
13
≈ Zn(II)-complex << Cu(II)-complex. In addition, a similar order for the ten-
dency of metal ions to fill “the hole“ in the center of the chlorophyll molecule
18,21
was already found in the cited reports.
On the other hand, metal complexes
of porphyrins can be divided into two groups based on the properties of their
2
0
electronic structure. Complexes which contain closed-shell metal ions (d or
10
d ), such as Zn(II), have relatively low energy in d–π metal-based orbitals.
These interactions have very little effect on the porphyrin π–π* energy gap in
electronic spectra. On the other hand, metal complexes of porphyrins which con-
m
tain metals such as Cu(II) with a d configuration of the corresponding energy
levels (m = 6–9, number of electrons) have significant metal to porphyrin orbital
interaction (i.e., metal to ligand d–π-backbonding), resulting in an increased π–π*
2–4
energy separation, which is seen as a blue-shift in the appropriate spectra.
A
heavy metal in the central position of the porphyrin structure (Fig. 1) seems to
play a significant role in the stability of HM–Pheo and HM–MP complexes to
UV-B irradiation; the affinity of heavy metals to the four nitrogen atoms (N-(21-
-24)) in the center of the porphyrin molecule could play the role of stability factor
for HM-complexes against UV-B irradiation.
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STABILITY OF (METALLO)PORPHYRINS TO UV-B IRRADIATION
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The photochemical decomposition rate constants for Pheo and its Cu-com-
plexes (Cu–Pheo) generally showed smaller values than the same rate constants
for MP and its Cu-complexes (Cu–MP), as given in Table I. The ratios of UV-B
induced decomposition constants for MP and Pheo (k /k
) and their Cu–MP
MP Pheo
and Cu–Pheo complexes (k
/k
) are 4.1 and 2.1, respectively. Meso-
Cu–MP Cu–Pheo
porphyrin differs from pheophytin by an additional double bond in the D pyrrole
ring (Fig. 1A). In the mesoporphyrin structure, the extended delocalization at the
edge of the cyclic tetrapyrrole (porphyrin) skeleton contains 11 π–bonds (Fig.
1A). In the pheophytin structure, the extended delocalization contains 10 π-bonds
and two more π-bonds belonging to: 1) the C=O carbonyl group in position C-
1
1
-13 and 2) the C=C bond in position C-3 ; the phytil chain is bonded to the pro-
3
pionic acid residue in position C-17 and isocyclic cyclopentanone ring fused to
a C-pyrrole ring between the C-13 and C-15 positions (Fig. 1B) makes substan-
tial differences between the Pheo and MP molecules. These differences between
the two chosen porphyrin molecules and the related energy distributions seems to
be related to the stability of the Pheo molecule against UV-B irradiation, com-
pared to MP.
Pheophytin is more stable compared to its Zn(II) complex, but, on the other
hand, the Zn–MP complex is more stable than MP itself to UV-B irradiation, as
shown in Table I. The ratios of UV-B induced decomposition constants for Zn–
–Pheo and Pheo (k
/k
), as well as of MP and the Zn–MP complex
Zn–Pheo Pheo
(k /k ) are 4.7 and 1.7, respectively (Table 1). It is known that the Zn–
MP Zn–MP
–Pheo complex is more labile than Chl itself in solution: Zn(II) was shown to be
involved in the formation of a peripheral complex at the right edge of the Chl
4,21
molecule that includes at least two C=O groups in different positions.
CONCLUSIONS
The chosen porphyrins, pheophytin and mesoporphyrin, and their heavy me-
tal complexes undergo UV-induced photochemical decomposition obeying first-
-order kinetics. Pheophytin and its Cu(II)-complex are more stable to UV-B irra-
diation than mesoporphyrin and its Cu(II)-complex; contrarily, the Zn(II)-com-
plex of mesoporphyrin is more stable than the same complex of pheophytin.
SUPPLEMENTARY MATERIAL
Absorbance spectra of porphyrin derivatives are available electronically from http://
//www.shd.org.rs/JSCS/, or from the corresponding author on request.
Acknowledgements. This work was supported by the Ministry of Education and Science
of the Republic of Serbia under project number TR 34012. We thank Dr Hugo Scheer who
donated the mesoporphyrin standard.
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ZVEZDANOVIĆ, MARKOVIĆ and MILENKOVIĆ
И З В О Д
КОМПЛЕКСИ ФЕОФИТИНА И МЕЗОПОРФИРИНА СА ЦИНКОМ(II) И БАКРОМ(II) И
ЊИХОВА СТАБИЛНОСТ ПРЕМА ДЕЈСТВУ UV-B ЗРАЧЕЊА:
VIS СПЕКТРОСКОПСКА АНАЛИЗА
ЈЕЛЕНА Б. ЗВЕЗДАНОВИЋ, ДЕЈАН З. МАРКОВИЋ и САЊА М. МИЛЕНКОВИЋ
Univerzitet u Ni{u, Tehnolo{ki fakultet, Bulevar oslobo|ewa 124, 16000 Leskovac
Стабилност Zn(II) и Cu(II) комплекса са дериватима порфирина (феофитином и мезо-
порфирином) према дејству UV-B зрачења у 95 % етанолу, анализирана је апсорпционом
спектроскопијом. Изабрани порфирини као и њихови комплекси са тешким металима под-
лежу деструкцији (деградацији), пратећи кинетику првог реда. Генерално посматрајући, у
односу на мезопорфирин, феофитин је стабилнији према дејству UV-B зрачења. Са друге
стране, Zn(II) комплекси су мање стабилни од Cu(II) комплекса са оба испитивана порфи-
рина – феофитином и мезопорфирином; ипак, док је Cu(II) комплекс феофитина стабилнији
од оног са мезопорфирином, у случају комплекса са Zn(II) ситуација је обрнута.
(Примљено 10. марта, ревидирано 20. јула 2011)
REFERENCES
1. H. Fisher, On haemin and the relationships between haemin and chlorophyll, Nobel
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