Understanding ion sensing in Zn(II) Porphyrins: Spectroscopic and computational studies of nitrite/nitrate binding

Article abstract of DOI:10.1021/ic300039v

The development of effective sensor elements relies on the ability of a chromophore to bind an analyte selectively and then study the binding through changes in spectroscopic signals. In this report the ability of Zn(II) Tetraphenyl Porphyrin (ZnTPP) to s

Full text of DOI:10.1021/ic300039v

Article
pubs.acs.org/IC
Understanding Ion Sensing in Zn(II) Porphyrins: Spectroscopic and
Computational Studies of Nitrite/Nitrate Binding
Christi L. Whittington, William A. Maza, H. Lee Woodcock,* and Randy W. Larsen*
Department of Chemistry, University of South Florida, 4202 E. Fowler Avenue, Tampa, Florida 33620, United States
ABSTRACT: The development of effective sensor elements
relies on the ability of a chromophore to bind an analyte
selectively and then study the binding through changes in
spectroscopic signals. In this report the ability of Zn(II)
Tetraphenyl Porphyrin (ZnTPP) to selectively bind nitrite over
nitrate ions is examined. The results of Benesi−Hildebrand
−
−
analysis reveals that ZnTPP binds NO2 and NO3 ions with
association constants of 739
70 M−1 and 134 15 M−1,
respectively. Interestingly, addition of a pyridine ligand to the
fifth coordination site of the Zn(II) center enhances ion binding
with the association constants increasing to 71,300 8,000 M−1
and 18,900
3,000 M−1 for nitrite and nitrate, respectively.
Density functional theory calculations suggest a binding
mechanism through which Zn(II)−porphyrin interactions are disrupted by ligand and base coordination to Zn(II), with
Zn(II) having more favorable overlap with nitrite orbitals, which are less delocalized than nitrate orbitals. Overall, these provide
new insights into the ability to tune the affinity and selectivity of porphyrin based sensors utilizing electronic factors associated
with the central Zn(II) ion.
is first reduced to nitrite followed by chemical modification and
optical detection.8 When a fluorescent sensor is used,
concentrations as low as 10 nM can be determined.
Electrochemical detection improves sensitivity over standard
colorimetric analysis as well as increasing selectivity. HPLC
methods coupled with optical techniques can improve
detection limits to as low as 10 pM.8
In terms of optical detection, porphyrins represent attractive
candidates for sensor elements. This is because the porphyrin
macrocycle exhibits rich spectroscopic features including: (1)
high molar extinction coefficients in both the near UV (up to
∼200 mM−1 cm−1) and visible regions (up to ∼75 mM−1
cm−1), (2) high fluorescence quantum yields (up to ∼0.2), and
(3) the porphyrin core can accommodate a broad array of
metals which in turn bind or catalytically degrade a wide variety
of analytes, forming the basis for both optical and
potentiometric sensor elements.12−16 In addition, both free-
base and metalloporphyrins can be functionalized at the
periphery to enhance binding specificity.17,18 Porphyrin based
potentiometric sensing platforms have primarily utilized
Mn(III) porphyrins for the detection of triiodie,19 penicillin-
G,20 thiocyanate,21 and diclofenac (a nonsteroidal anti-
inflammatory drug used for the treatment of rheumatoid
arthritis)22 to name only a few.
INTRODUCTION
The oxides of nitrogen including NO2 and NO3 are
important ions in biology, the environment, and the food
■
−
−
−
industry. In humans, NO2 has been identified as a biomarker
for NO-synthase activity as well as a storage form of nitric
oxide, being activated by deoxy hemoglobin.1,2 In bacterial
−
systems, NO2 is an important ion in the nitrogen cycle which
converts NO3− to ammonium or vice versa.3 The conversion of
+
−
NH4 to NO3 represents an intermediate step in nitrogen
fixation and involves enzymes which catalyze assimilatory,
−
−
respiratory, or dissimilatory reduction of NO3 to NO2 .
−
Environmentally, NO3 based fertilizers (typically ammonium
nitrate) have proven to be the most cost-effective method of
nitrogen delivery to plants. However, the high water solubility
−
−
of both NO3 and NO2 (produced as an intermediate in
bacterial ammonification) have led to significant groundwater
contamination.4,5 This contamination has been identified as a
−
serious health risk as NO3 consumption can lead to
methemoglobinemia, a disorder in which methemoglobin
builds up in the bloodstream.6 Despite the obvious health
risks, low concentrations of nitrates are commonly used in the
food industry as preservatives.7
The ability to selectively sense nitrogen oxides has obvious
environmental and health implications with a number of
technologies having been developed for their detection. These
include electrochemical detection, ion chromatography, HPLC,
and various optical techniques.8−11 One of the most common
nitrite detection techniques is derivitization to append a
chromophore that can then be identified colorimetrically or
through fluorescence methods. In the case of nitrate, the anion
Zn(II) porphyrins are of particular interest in sensor
development as these chromophores exhibit significant
fluorescence quantum yields, have long-lived triplet states,
Received: January 6, 2012
Published: April 5, 2012
© 2012 American Chemical Society
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Scheme 1. Possible Orientations of ZnP Complexes, 1−10
Restricted Kohn−Sham Density Functional Theory (DFT)
calculations were performed with Q-Chem.30 The coordinates for all
calculations were obtained from the crystal structure of ZnTPP
coordinated with pyridine (Pyr); the pyridine and functionality of the
porphyrin ring was removed in Avogadro,31 giving ZnP, which was
and can interact with a wide variety of axial ligands through the
Zn(II) ion.23−26 These chromophores have been shown to
exhibit selectivity toward nitrogen- (amines) and oxygen-
containing (tetrahydrofurans) compounds as well as other
small molecules and ions.27−29 The ability of Zn(II) porphyrins
to bind small molecules allow these chromophores to be
examined as sensor platforms for nitrite/nitrate detection. In
the present study, experimental results demonstrating the
preferential binding of NO2 over NO3 to the metal-
loporphyrin Zn(II) 5,10,15,20-tetraphenylporphyrin (ZnTPP)
as well as the increase in binding affinity by 2 orders of
magnitude in the presence of a proximal base, pyridine (Pyr),
are presented. Further, computational evidence, from electronic
structure calculations, is provided to elucidate the underlying
effects that govern the observed binding affinities; for example,
proximal base effects. Zn(II) porphine (ZnP) was used as a
model for ZnTPP to examine the role of the central Zn(II) ion,
the porphyrin macrocycle, and ligand orbital interactions on
binding selectivity and affinity.
−
−
then combinatorially complexed with ligands (NO2 and NO3 ) and
bases (Pyr and H2O), resulting in complexes 1−10 (Scheme 1). DFT
gas phase (GP) geometry optimization of each structure employed the
M06-L exchange-correlation functional32 with a 75,302 grid.33
LANL2TZ basis functions with Effective Core Potentials (ECPs)34
were applied to Zn(II), with the remaining atoms being described by
the 6-31G* basis set.35 This has been shown to be an effective
combination for studying metal-porphyrin complexes.36 To ensure
that the optimization was not biased by initial ligand orientation, 180
degree rotations of both ligands parallel and perpendicular to the plane
of the porphine ring were performed with optimizations carried out on
each of these structures. In each case, the ligand adopted the
orientations in Scheme 1.
−
−
Exclusion of complexes 3, 5, 8, and 10 will be discussed in the
Results and Discussion section. While analyzing minimized complexes
1, 2, 4, 6, 7, and 9, it became clear that GP modeling was insufficient
for describing water coordination (1, 2, 4). Thus, quantum
mechanical/molecular mechanical (QM/MM) geometry optimizations
were performed using the Q-Chem/CHARMM interface.30,37,38
Complexes 1, 2, and 4 were each solvated in a TIP3P water sphere
MATERIALS AND METHODS
■
All reagents and solvents including NaNO2, NaNO3, 2-propanol,
methanol, and Zn(II) acetate (Zn(OAc)2) were purchased and used as
received from Sigma-Aldrich. Free-base 5,10,15,20-tetraphenyl por-
phyrin (H2TPP) was synthesized according to literature methods.30
ZnTPP was synthesized by dissolving H2TPP in methanol with excess
Zn(OAc)2 and refluxing for 3−4 h. The progress of the reaction was
monitored by UV−vis spectroscopy. Once the reaction was complete
the methanol was evaporated off and the solid resuspended in ethyl
acetate. The solution was washed three times with water to remove
excess Zn(OAc)2 and the organic layer collected and dried by rotary
evaporation.
(∼700 H2O molecules).39,40 The ZnP chromophore, ligand (NO2 /
−
−
NO3 ), and base (Pyr/H2O) constituted the QM region, treated at the
B3LYP/6-31G* level of theory,41,42 with the MM region consisting of
the remaining water molecules. QM nonbonded parameters for ZnP
−
and NO2 /NO3− were obtained from ZN, C, N, H, and O atom types
from the standard CHARMM22 force field.43
Localized orbitals involved in binding were determined by Natural
Bond Orbital (NBO)44,45 analysis during the SP calculations. NBO
assigns localized orbitals to molecules based upon bonds and lone pair
electrons (lp), providing a bridge between molecular and atomic
orbitals and facilitating analysis. Orbital stabilization energies (Eorb
)
UV−vis absorption spectra were collected on a Shimadzu UV-
2401PC spectrophotometer. Steady-state emission spectra were
collected using an ISS PC1 single photon counting spectrofluorimeter.
Samples were excited at 422 nm and emission data collected between
550 nm and 750 nm. Samples were prepared by diluting a small
amount of ZnTPP stocks (prepared in 2-propanol) into a 7:3 2-
propanol/water mixture (v/v) (sample porphyrin concentration <10
μM). Stock solutions of NaNO2 (530 mM), NaNO3 (520 mM), and
pyridine (150 mM) stock solutions were prepared in water and titrated
into samples of ZnTPP while mixing, using a magnetic stir-bar, and the
spectra collected.
related to stability gained from electron delocalization were extracted
from NBO results. Specifically, interactions involved in ligand and base
coordination to ZnP were used to analyze binding modes and
affinities.
To probe solvation effects in 1, 2, and 4, waters with a 3.2 Å
donor−acceptor distance46 to NO2 were selected and used to
−
construct and evaluate a microsolvated complex. The GP quantum
binding energy (ΔEHB) was determined for each hydrogen bonded
water for 1, 2, and 4 by calculating the total energy, moving each water
out of interaction range (≥100 Å), and recalculating the total energy at
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−
−
Figure 1. Absorption spectra of 10 μM ZnTPP in a 70% 2-propanol/water (v/v) solution containing: no NO2 or NO3 (solid line), (a) 31 mM
−
NO2− (dotted line), and (b) 37 mM NO3− (dashed line). Insets show difference spectra of ZnTPP in the presence and absence of NO2− or NO3 .
Figure 2. Steady-state emission spectra of a 10 μM ZnTPP in a 70% 2-propanol/water (v/v) solution containing: no NO2− or NO3− (solid line), (a)
31 mM NO2− (dotted line), and (b) 37 mM NO3− (dashed line). Insets show difference spectra of ZnTPP in the presence and absence of NO2− or
−
NO3 .
the B3LYP/6-31G* level of theory with the Q-Chem/CHARMM
binding to ZnTPP(Pyr)), Io is the emission intensity in the
absence of the anion, I∞ is emission intensity at infinite
concentration of the anion, and Ka is the association constant of
a 1-to-1 interaction following the scheme:
interface.
RESULTS AND DISCUSSION
■
The absorption spectra of ZnTPP solubilized in the 7:3 (v/v)
2-propanol/water mixture displays transitions typical for Zn(II)
porphyrins (Figure 1, dotted trace). Specifically, a symmetry
allowed transition centered at 422 nm corresponding to the B-
band, or Soret band, is observed as well as two transitions at
Ka
ZnTPP(L) + X− ⇄ ZnTPP(L)(X−)
(2)
where L is a proximal base (either H2O or Pyr). In the absence
of pyridine as a proximal base the Ka values from fits to eq 1a
were found to be 739
70 M−1 (ΔG° = −3.9
0.1 kcal
−
−
557 and 596 nm (Q-bands). Addition of NO2 or NO3
mol−1) and 134 15 M−1 (ΔG° = −2.9 0.1 kcal mol−1) for
resulted in hyperchromic shifts in the absorption spectra
(Figure 1 solid and dashed traces, respectively). Changes in the
−
−
NO2 and NO3 , respectively (See Figure 3, Table 1). Steady-
−
−
state emission data for both NO2 and NO3 binding to
ZnTPP yields Ka values identical (within experimental error) to
those obtained from the absorption data: 516 193 M−1 and
−
absorption spectra were plotted as a function of NO2− or NO3
ion concentration and fit using,47
ΔA = {Ka[X−]/(1 + Ka[X−])}[ZnTPP]0Δελ
116 25 M−1 for NO2 and NO3 , respectively. Addition of
pyridine to the ZnTPP solution, to give predominately the
ZnTPP(Pyr) complex, resulted in an increase in binding affinity
toward both NO2− and NO3− with Ka values of 71,300 8,000
M−1 (ΔG° = −6.6 0.1 kcal mol−1) and 18,900 3,000 M−1
(ΔG° = −5.9 0.1 kcal mol−1), respectively (Figure 4). Again,
−
−
(1a)
or, for emission data (Figure 2),
ΔI = {Ka[X−]/(1 + Ka[X−])}(I∞ − Io)
(1b)
where [ZnTPP]0 is the initial sample porphyrin concentration;
Δελ is the change in molar extinction at probe wavelength λ
(424 nm for both NO2− and NO3− binding to ZnTPP, 420 nm
−
−
the association constants for both NO2 and NO3 binding to
ZnTPP in the presence of pyridine obtained using emission
data yields Ka values identical (within experimental error) to
−
−
for NO3 binding to ZnTPP(Pyr), and 422 nm for NO2
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Figure 4. Change in steady-state absorption of 10 μM ZnTPP in the
presence of 1 mM pyridine (to give predominantly the 5-coordinate
ZnTPP(Pyr) complex) as a function of (bottom, changes monitored at
422 nm) [NO2 ] and (top, changes monitored at 420 nm) [NO3 ].
Solid lines represent best fits to eq 1a. Insets for each show the change
in steady-state emission of ZnTPP at 622 nm (λexc 422 nm); solid lines
represent best fits to eq 1b.
Figure 3. Change in steady-state absorption of ZnTPP at 424 nm as a
−
−
function of (bottom) [NO2 ] and (top) [NO3 ]. In each case the
initial concentration of ZnTPP was 10 μM. Solid lines represent best
fits to eq 1a. Insets for each show the change in steady-state emission
of ZnTPP at 608 nm (λexc 422 nm); solid lines represent best fits to eq
1b.
−
−
those obtained from the absorption data: 79,200 7,800 M−1
−
−
and 13,400 5,200 M−1, for NO2 and NO3 , respectively.
−
To characterize the selectivity of ZnTPP toward NO2 ,
orbital interactions that govern binding affinity have been
examined computationally for the model ZnP complexes
depicted in Scheme 1. There are three possible binding
−
orientations for NO2 ; single oxygen bound (1, 6), nitrogen
bound (2, 7), and double oxygen bound (3, 8); while NO3− has
only two; single oxygen bound (4, 9) and double oxygen bound
(5, 10). Figure 5 displays the geometry optimized complex 6, in
which the Zn(II) lies out of the porphine plane by 0.3−0.5 Å
−
−
for both ligands (NO2 , NO3 ) and either proximal base (Pyr,
H2O).48
Figure 5. Geometry optimized structure of complex 6. Zn(II) is out-
−
of-plane toward NO2 .
Integral to understanding ion selectivity by Zn(II)
porphyrins is knowing the correct binding modes of the
ligands to the central Zn(II). The first modes examined were
more possible hydrogen bond acceptor sites for single oxygen
bound orientations; five sites for 4 and 9 compared to four sites
for 3 and 8.
Although the orientations containing two oxygen atoms
coordinated to the Zn(II) are energetically favorable, NO2− has
two additional binding modes (1, 2) that can be stabilized by
−
−
those with two oxygen atoms from the NO2 /NO3 bound to
Zn(II) (3, 5, 8, 10) in GP. Although this orientation allows for
favorable interactions between the anions and the Zn(II),
−
specifically electron donation from the π system on NO2 /
−
NO3 to the unfilled Zn(4s) orbital, efficient solvent
−
−
interactions seem unlikely. For example, the NO2 structures
solvent interactions. These modes along with NO3 (4) were
with two oxygen atoms bound to the Zn(II) (3, 8) have only
three likely hydrogen bond acceptor sites available to interact
with solvent (water in this case) while conformations with a
single oxygen atom bound to the Zn(II) (1, 2, 6, 7) should
examined through QM/MM minimization, with the QM region
−
−
defined as ZnP, ligand (NO2 /NO3 ) and the proximal base
H2O. In all three cases, the QM water molecule (i.e., base)
reorients to participate in a solvent hydrogen bonding network.
Additionally, the solvent interaction with the ligands differed
−
have at least four available sites. Likewise for NO3 , there are
a
−
−
Table 1. Association Constants and Free Energies for NO2 and NO3 Binding to ZnTPP
ZnTPP
ZnTPP(Pyr)
Ka (M−1
)
ΔG° (kcal/mol)
−2.9 0.1
Ka (M−1
)
ΔG° (kcal/mol)
−5.8 0.1
−
−
NO3
134 15
(116 25)
739 70
18,900 3000
(13,400 5,200)
71,300 8000
NO2
−3.9 0.1
−6.6 0.1
(516 193)
(79,200 7,800)
a
Data in parentheses were obtained from steady-state emission.
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Figure 6. QM/MM optimized complexes 1, 2, and 4 with TIP3P waters hydrogen bonded to ligand, porphine, and base water.
between the three complexes examined. As predicted, 2
displayed four hydrogen bonds between NO2 and solvent
competing effects; the L→H2O stabilization, which is ∼5
−
kcal/mol more favorable for 2, is offset by more favorable
H2O→H2O interactions (∼7 kcal/mol) for 1. These results
indicate that the oxygen bound structure (1) is preferred largely
because of enhanced solvation effects, not more favorable L→
Zn(II) interactions.
with 4 having five. Surprisingly, 1 also exhibited five hydrogen
bonds (only four were predicted) with the additional hydrogen
bond to the Zn(II) coordinated oxygen (Figure 6).
−
To determine the extent of solvent stabilization in NO2 /
−
−
NO3 binding the QM region and water molecules of interest
To understand the selectivity of ZnP for NO2 relative to
−
were isolated and the hydrogen bond energy (EHB) for each
NO3 , NBO analysis was again utilized. The Eorb results for 1,
water was computed in GP (See methods, Table 2). The
4, 6, and 9 are reported in Table 3 and are consistent with the
Table 2. Binding Energies (EHB) of Hydrogen Bonded Water
Table 3. Stabilization Energies (Eorb) from NBO Analysis of
Single Point Energy Calculations in Gas Phase
a
Molecules in Gas Phase and Stabilization Energies (Eorb
)
from NBO Analysis of Single Point Energy Calculations,
Depicted in Figure 6
Eorb (kcal/mol)
a
orbital interaction
1
4
6
9
structure
1
5
2
4
5
L(lp) → Zn(4s)
Zn(4s) → L(σ*)
B(lp,π) → Zn(4s)
Zn(4s) → B(σ*)
P(lp) → Zn(4s)
Zn(4s) → P(σ*)
P(π,lp) → B(σ*)
38.5
5.8
27.0
0.3
43.9
3.6
41.7
0.1
number of H2O (n)
4
EHB (kcal/mol)
N/A
N/A
160.0
38.4
6.6
N/A
N/A
173.3
2.9
4.4
4.0
nH2O
45.7
9.1
35.6
8.9
43.5
8.7
1.6
0.0
average
142.6
17.4
1.6
143.6
2.1
Eorb (kcal/mol)
8.3
nH2O(lp)-nH2O(σ*)
L(lp) → Zn(4s)
Zn(3d) → L(σ*)
L(lp) → nH2O(σ*)
total H2O
15.6
29.2
0.2
1.5
19.6
0.3
0.0
2.0
35.1
a
L is NO2− or NO3 , B is H2O or Pyr, and P is porphine. Positive Eorb
−
0.8
values are favorable.
79.7
95.3
85.0
77.2
78.7
93.3
a
−
−
−
Structures included no base (Pyr or H2O). L is NO2 or NO3 .
experimental observation of NO2 binding selectively over
−
Positive EHB and Eorb values are favorable.
NO3 . First, the NBO results suggest that H2O→Zn(II)
interactions do not contribute significantly to stabilization with
H2O as the proximal base (1, 4). Thus, the orbital interactions
between both the porphine (P) ring and the NO2 /NO3
hydrogen bonded waters contribute a total of 45.7, 35.6, and
43.5 kcal/mol of EHB stabilization for 1, 2, and 4, respectively.
As Zn(II) is a closed shell metal, the primary Zn(II)
interactions observed are with the Zn(4s) orbital; however,
some Zn(3d) orbital interactions are seen. Analysis of the
microsolvated structures reveal an Eorb of 29.2 and 35.1 kcal/
mol due to ligand (L) to Zn(II) electron donation (L→Zn(II))
for 1 and 2, respectively. This indicates that the nitrogen bound
structure (2) would be preferred in the absence of micro-
solvation. However, inclusion of microsolvation interactions
elucidates the stabilizing role solvent plays on the complexes.
For example, 1 has five hydrogen bonds involving solvent
waters relative to four in 2. This results in ∼10 kcal/mol of
additional EHB stabilization and shifts the predicted favorability
from 2 to 1 despite the ∼6 kcal/mol greater stabilization due to
L→Zn(II) interactions in 2 (Table 2). To further explain the
microsolvation interactions, the L→H2O and H2O→H2O
orbital stabilizations were examined via NBO analysis for
complexes 1, 2, and 4. Again, the results demonstrate
−
−
ligands with the Zn(II) ion are responsible for the observed
selectivity. In contrast, with pyridine as the proximal base (6, 9)
additional interactions between Zn(II) and both the pyridine
nitrogen lp and the aromatic ring π electrons are observed.
These interactions result in increased stabilization for both
−
−
NO2 and NO3 by ∼3 kcal/mol due to pyridine lp and ∼1−
1.5 kcal/mol from π electron donation into the Zn(4s). These
stabilizations combined with L→Zn(II) interactions described
above account for ∼10 kcal/mol of increased stability for 6
relative to 1 and ∼19 kcal/mol of increased stability for 9
relative to 4. The stability enhancement by pyridine as a
proximal base (ΔEPyr = ΔE9−4 − ΔE6−1 = ∼9 kcal/mol) can be
largely rationalized by examining L→Zn(II) stabilization
energies: Δ(L→Zn)6−1 = 5.4 kcal/mol and Δ(L→Zn)9−4
=
14.7 kcal/mol. The underlying cause of this shift is multifaceted
beginning with the P→Zn(II) interactions, where Δ(P→Zn)6−1
= −17.4 kcal/mol and Δ(P→Zn)9−4 = −29.7 kcal/mol. The
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P→Zn(II) interaction is disrupted by the binding of pyridine
thus allowing the Zn(II) ion to accept additional electron
density, increasing L→Zn(II) stabilization and therefore
binding affinity.
AUTHOR INFORMATION
Corresponding Author
*E-mail: hlw@mail.usf.edu (H.L.W.), rwlarsen@usf.edu
(R.W.L.).
■
To further examine this disruption, Zn(II) back-bonding to
the ligand, base, and porphine was examined. Upon reduction
of Zn(II)→P back-bonding, L−Zn(II) (L→Zn(II) and Zn-
(II)→L) stabilization was reduced as well as binding affinity.
For example, when comparing 1 and 4 the Zn(II)→P back-
bonding was decreased by ∼36 kcal/mol and the L−Zn(II)
interaction was reduced by ∼17 kcal/mol. Likewise for 6 and 9,
an ∼15 kcal/mol decrease in back-bonding was accompanied
by an ∼6 kcal/mol drop in L−Zn(II) stabilization. Zn(II)→L
(1, 6) and Zn(II)→B (6) back-bonding are more favorable for
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to acknowledge the use of the services
provided by Research Computing, University of South Florida,
the Department of Defense−Defense Threat Reduction Agency
(DoD-DTRA) through HDTRA1-08-C-0035, NIH/NHLBI
(1K22HL088341-01A1), and the University of South Florida
(start-up) for funding.
−
NO2 complexes. These trends mirror the experimental
−
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■
−
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CONCLUSION
■
It has been demonstrated that ZnTPP selectively binds nitrite
over nitrate because of more favorable ligand−Zn(II) orbital
overlap, both π-type interactions from the ligand to the Zn(II)
ion and subsequent back-bonding to the ligand. For either
ligand, the preferred binding mode is such that a single oxygen
is bound to the Zn(II) (1, 6). With water in the proximal base
position an orientation is established in which the water
molecule engages in a hydrogen bonding network with
neighboring solvent water molecules as well as interactions
involving porphine π orbitals. With either pyridine or water as a
proximal base, electron donation from the porphine ring to the
Zn(II) ion is less stabilizing for nitrite than nitrate, and
subsequently more back-bonding between Zn(II) and the
porphine ring takes place with nitrite coordinated to the Zn(II).
Greater ligand and/or base π electron donation onto the
Zn(4s) destabilizes the porphine−Zn(II) interaction. Interest-
ingly, the ligand→Zn(II) π interaction is more favorable with
pyridine than water, explaining why, in solution, there is higher
ligand binding affinity with pyridine addition. Therefore,
binding affinities should be maximized when favorable
ligand−Zn(II) and disrupting porphine−Zn(II) interactions
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