A Water-Soluble Cationic Zinc Lysine Precursor for Coating ZnO on Biomaterial Surfaces

Article abstract of DOI:10.1021/acs.inorgchem.6b01663

A novel water-soluble cationic zinc lysine coordination compound, [Zn[(C₆H₁₄N₂O₂)]₂Cl]Cl·2H₂O (1), has been designed and synthesized and its crystal structure determined. The aqueous solution of this coordination compound is not only transparent and stable at room temperature but it is also nearly neutral (pH ～ 7). It is worth noting that zinc oxide (ZnO) forms in situ upon dilution of a solution of the compound. The bioactivity of ZnO has been confirmed using an Alarma Blue assay. These unique properties allow the coordination compound to gently grow ZnO coating with excellent antibacterial benefits onto biomaterial surfaces in a facile and safe manner.

Full text of DOI:10.1021/acs.inorgchem.6b01663

Communication
pubs.acs.org/IC
A Water-Soluble Cationic Zinc Lysine Precursor for Coating ZnO on
Biomaterial Surfaces
†
†
†
†
‡
†
Shaotang Yuan, Shiri Nawrocki, Michael Stranick, Ying Yang, Chong Zheng, James G. Masters,
,†
and Long Pan*
†
Colgate−Palmolive Company, 909 River Road, Piscataway, New Jersey 08854, United States
‡
Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115, United States
*
S Supporting Information
homogeneously. Consequently, the development of a novel
alternative method to firmly deposit insoluble ZnO on the
surfaces of biomaterials at milder conditions, such as at room
temperature and neutral pH, remains as a major challenge for
researchers.
ABSTRACT: A novel water-soluble cationic zinc lysine
coordination compound, [Zn[(C H N O )] Cl]Cl·2H O
6
14
2
2
2
2
(
1), has been designed and synthesized and its crystal
structure determined. The aqueous solution of this
coordination compound is not only transparent and stable
at room temperature but it is also nearly neutral (pH ∼ 7).
It is worth noting that zinc oxide (ZnO) forms in situ
upon dilution of a solution of the compound. The
bioactivity of ZnO has been confirmed using an Alarma
Blue assay. These unique properties allow the coordination
compound to gently grow ZnO coating with excellent
antibacterial benefits onto biomaterial surfaces in a facile
and safe manner.
It is known that Zn ions tend to bind to many proteins found in
24
humans.2 Because most biological surfaces are negatively
5,26
charged,
the synthetic premise was to design a soluble
cationic zinc coordination compound that can bind to surfaces of
most biomaterials by electrostatic interaction and then generate
ZnO in situ via hydrolysis. As building blocks of proteins, amino
acids have been widely used in pharmaceutical, personal care, and
oral care products. Therefore, understanding the structure of
zinc−amino acid coordination compounds is of critical
importance. Since Kretsinger and Cotton determined the first
27
inc oxide (ZnO) is a remarkable semiconductor and
crystal structure of these types in 1963, many structures of
other zinc−amino acid coordination compounds have been
solved. However, it should be noted that a majority of these
zinc−amino acid compounds are neutral. In fact, only a few
Z
piezoelectric material that has attracted immense interest
because of its many applications in the electronic and
1
−12
optoelectronic field.
In addition, ZnO is also a very
important active ingredient in pharmaceutical and cosmetic
cationic zinc compounds containing organic ligands have been
products because of its antibacterial and antimicrobial benefits, as
28−31
reported.
Alagha et al. reported the first cationic zinc−
13−16
well as for providing UV protection.
In many commercial
32
arginine coordination compound, which was synthesized by
zinc-containing products, insoluble ZnO particles are suspended
in the formula and applied to the surfaces of interest in the form
of an ointment, cream, suspension, etc. In such cases, ZnO
cannot be retained on the surfaces for extended periods of time
because of its easy removal by water and/or other external
factors. This low retention of ZnO on the surfaces provides
impetus for us to seek a new method to firmly deposit ZnO on
surfaces, particularly those of biomaterials.
mixing zinc acetate [Zn(OAc) ] and L-arginine (L-Arg) in an
2
anhydrous methanol solution. The resulting compound, [Zn-
(
OAc)(L-Arg) ]OAc·3H O, was insoluble in water. The
2 2
insolubility not only renders uniform aqueous surface deposition
onto biological surfaces nearly impossible but also leverages
hydrolysis of the compound to form ZnO impractical. This
finding prompted the need to investigate the synthesis of a water-
soluble zinc−amino acid coordination compound that is
compatible with biomaterials. In this Communication, we report
the synthesis of a water-soluble, cationic, zinc−amino acid
coordination compound by reacting ZnO with L-lysine hydro-
Over the past few decades, significant efforts have been
devoted to obtaining uniform, high-quality ZnO films and then
depositing them onto other materials. These techniques include
1
7,18
metal−organic chemical vapor deposition,
plasma-enhanced
19
20
chemical vapor deposition, pulsed-laser deposition, atomic
chloride to produce [Zn(C H N O ) Cl]Cl·2H O (1). It will
2
1
22,23
6
14
2
2 2
2
layer deposition, etc.
However, all of these techniques
be further shown that 1 can generate ZnO in situ on the surfaces
of biomaterials through a facile hydrolysis reaction. In addition,
we also demonstrate its outstanding property as a precursor for
depositing an antibacterial ZnO film on arbitrary biomaterial
surfaces in situ utilizing a facile and safe route. The reaction
equation to form 1 is
require high energy input, and the deposition process normally
occurs at high temperatures (>300 °C). As a result, these severe
conditions hinder the practicality of such methods for the
deposition of ZnO onto biomaterials, such as human skin, teeth,
and hair. On the other hand, the wet chemistry method of
reacting soluble zinc salts with base can produce zinc hydroxide,
which, at low temperatures, gradually dissociates to ZnO.
Nevertheless, the basic pH of such a solution may cause
irritation, which also inhibits its practical use on biomaterials
Received: July 12, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b01663
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
In order to reveal the unique properties of 1, we explored and
compared its hydrolysis reaction to that of other zinc−amino
acid mixtures containing similar compositions under the same
conditions. Because the hydrochloride salt forms of arginine
ZnO + 2C H N O Cl + H O
6
15
2
2
2
→
[Zn(C H N O ) Cl]Cl·2H O
6
14
2
2 2
2
An aqueous solution of 1 is transparent and has a nearly neutral
pH (7.0−7.4). The crystal structure of 1 is determined using
single-crystal X-ray diffraction and shown in Figure 1. The
(
Arg), cysteine (Cys), and glycine (Gly) are commercially
available, we prepared ZnO-Arg·HCl, ZnO-Cys·HCl, and ZnO-
Gly·HCl solutions for comparison with 1. Details of the
preparations are summarized in the Supporting Information.
These solutions were diluted to a zinc concentration of 2% by
weight. The pH values of the solutions were 7.4, 7.3, 2.9, and 3.9
for 1, ZnO-Arg·HCl, ZnO-Cys·HCl, and ZnO-Gly·HCl,
respectively (see Table S2 for details). Among all of the samples,
the ZnO-Arg·HCl solution was the only one that appeared
turbid, although both arginine and lysine fall into the same amino
acid category. The instability of the ZnO-Arg·HCl solution
differentiates it from that of stable 1 even though both share a
similar final pH. 1, ZnO-Cys·HCl, and ZnO-Gly·HCl were then
further diluted 2-, 4-, 8-, 16-, and 32-fold by deionized water, and
each individual solution was placed in the turbidity meter to
monitor the formation of precipitates over a period of 30 min at
Figure 1. Molecular structure of complex 1. Color code: Zn, green; O,
red; N, blue; C, gray; Cl, yellow; H, neon green.
3
7 °C. The pH of each dilution is provided in Table S3. No
precipitation was found in any of the ZnO-Cys·HCl or ZnO-Gly·
HCl dilutions because of the low pH. Figure S2 shows a
comparison of the percent transmission of each 1 dilution. As the
figure shows, there was an increase in precipitation as the
solution became more diluted up to 16-fold. Further dilution to
32-fold revealed a higher percent transmission due to the excess
amount of water with the precipitate. The 16-fold dilution
product of 1 was relatively stable in the first 15 min; however, a
rapid decrease in the percent transmission in the next 10 min
suggested that a large amount of precipitate was formed in a short
period of time.
selected crystallographic data and bond lengths and angles of 1
are summarized in Table 1 and Table S1. In this complex, the Zn
Table 1. Crystallographic Data for Compound 1
empirical formula
fw
C H N O Cl Zn
12 32 4 6 2
464.69
293(2)
0.71073
temperature (K)
wavelength (Å)
cryst syst
space group
unit cell dimens
a (Å)
monoclinic
P21
The white precipitate was isolated and collected by filtration.
The PXRD pattern of this precipitate, shown in Figure 2,
5.275(2)
17.040(5)
11.407(3)
1022.5(5)
2
b (Å)
c (Å)
volume (Å3)
Z
3
calcd density (g/cm )
abs coeff (mm−1)
1.509
1.496
θ range for data collection (deg)
data/restraints/param
GOF on F2
1.79−24.99
3468/1/230
1.180
final R indices [I > 2σ(I)]
R indices (all data)
R1 = 0.0318, wR2 = 0.0816
R1 = 0.0340, wR2 = 0.0830
ion is coordinated by two lysine ligands with two N atoms from
its amine groups and O atoms from the carboxylic groups, which
are in a trans position from each other in an equatorial plane. It is
interesting to discover that a Cl ligand is apically coordinated to
the Zn ion, which leads to the distorted square-pyramidal
geometry. This novel structure gives rise to a positive cation
Figure 2. Comparison of the PXRD pattern of the 1 precipitate (blue) to
that of the ZnO standard (red). The result clearly shows that the
precipitate formed from hydrolysis of 1 is ZnO.
−
moiety as expected, to which a second Cl ion acts as the
counterion to form an ionic salt. The powder X-ray diffraction
(
PXRD) pattern of the product powder agrees well with
simulated results from the single-crystal structure (Figure S1). In
addition, elemental analyses of C, H, N, and Zn ions from the
crystal match with the calculated values (see the Characterization
section in the Supporting Information). These instrumental
analyses indicate the high purity of the crystal. In contrast to the
insoluble zinc(II) arginine complex reported by Alagha et al.,
the water-soluble nature of complex 1 provides great
matches that of wurtzite ZnO very well. This finding strongly
indicates the formation of ZnO by the hydrolysis reaction of 1.
Additionally, the filtrate gave rise to colorless column-shaped
single crystals, which were analyzed crystallographically. A lysine
hydrochloride dihydrate crystal structure was confirmed using
single-crystal X-ray diffraction (Figure S4). When the findings
from the PXRD pattern of the precipitate and the X-ray
diffraction pattern of the crystal found in the filtrate are
30
opportunities for potential applications.
B
DOI: 10.1021/acs.inorgchem.6b01663
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
combined, the hydrolysis reaction of 1 can be described as
follows:
applied after the fingers were air-dried. The changing of the
indicator color from red-orange to blue-purple indicates the
detection of zinc. Figure S4 clearly demonstrates that both ZnO
[
Zn(lysine) Cl]Cl + H O ⇄ ZnO + 2lysine·HCl
2
2
and ZnCl were easily removed from the skin by simple rubbing
2
It is hypothesized that, in this reversible reaction, the aqua
ligands are competing with both lysinate and chloride in 1 to
form a zinc−aqua coordination compound, which, in turn,
converts to zinc hydroxide and further hydrolyzes to ZnO. The
equilibrium of the reaction depends on the amount of water in
the solution. As a result, a stock solution of 1 with 2.4% zinc
appeared transparent, indicating no formation of ZnO. However,
as the solution was diluted, aqua ligands became dominant and
were able to move the equilibrium to the right and trigger the
formation of ZnO. At 16-fold dilution, with about 0.13% zinc,
there was a sufficient amount of water to replace the coordinated
chloride and lysinate. As a result, optimal amounts of precipitate
at this dilution were observed. Further dilution completed the
hydrolysis reaction; however, a lower amount of ZnO was
detected because of excess dilution.
The unique process of forming ZnO in situ by diluting
precursor 1 in water provides a novel, facile, safe, and cost-
effective method to grow ZnOs coating on the surfaces of any
inorganic or biological substance. This process has inherent
advantages over previously published methods because it occurs
at neutral pH and ambient conditions. Instead of triggering the
deposition reaction by heat or pH change, simple dilution of the
precursor solution under ambient conditions leads to ZnO
deposition. As a result, 1 has the propensity for many potential
applications in pharmaceutical and cosmetic products; e.g., it can
readily deposit ZnO on human skin, hair, and teeth. A conceptual
explanation for the ZnO deposition mechanism is presented in
Scheme 1. It is proposed that (1) a positively charged 1 molecule
under running water because of the lack of blue color after
Zincon was sprayed. In contrast, the treatment with 1 showed a
firm deposition of zinc on the skin, as the prominent blue-purple
color confirmed.
In order to compare the zinc bioactivity of deposited zinc from
1 against regular ZnO, the antibacterial effects of the zinc activity
were also examined. An in vitro study was conducted on pig skin
33
using an Alamar Blue assay. The percentage of reduction was
defined based on nontreatment as the control. As shown in Table
2, the results suggest that, after using the 1 formula, the residue
a
Table 2. Effect of Deposited Zinc on S. aureus
concn of Zn
ions (%)
mean of % reduction vs
nontreatment control
standard
deviation (%)
1
1.0
1.0
0
48
29
8
6
ZnO
placebo
−9
21
a
The results are the mean of triplicate experiments.
left over on the pig skin inhibited Staphylococcus aureus better
than the ZnO formula and placebo. This directly shows that ZnO
deposited from a 1 solution possessed a much better antibacterial
efficacy than regular ZnO.
In summary, we illustrate the design, synthesis, and character-
ization of the first water-soluble cationic zinc lysine coordination
compound 1 as a precursor for ZnO deposition. This novel
coordination compound can be hydrolyzed to form ZnO when
the solution is simply diluted with water. The in situ generation
of ZnO takes place at neutral pH and under mild conditions,
which is highly compatible with biomaterials. This method paves
a new avenue to gently coat insoluble ZnO on the surfaces of
biomaterials and other arbitrary 3D materials in a facile, efficient,
and safe manner. Moreover, the reprecipitated zinc is more
effective on bacterial inhibition than the parent compound ZnO.
Further research will seek to understand the details of the
deposition process and the mechanism of the antibacterial effect
of a1 1-deposited ZnO film, as well as to develop 1 for consumer
products applications.
Scheme 1. Proposed Mechanism of In Situ ZnO Formation by
Hydrolysis of 1 on a Biomaterial Surface
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01663.
■
*
S
PXRD data and experimental details (PDF)
Crystallographic data (CCDC 1022208) (CIF)
AUTHOR INFORMATION
Corresponding Author
E-mail: longyuepan@yahoo.com.
Notes
■
*
first attaches to the negatively charged biomaterial surface by
electrostatic attraction, (2) becaue of dilution, 1 is surrounded by
large amounts of water molecules, which trigger the hydrolysis
reaction, and (3) ZnO is formed in situ on the surface of
biomaterials.
The authors declare no competing financial interest.
The deposition of zinc on human skin has been demonstrated
using an indicator test. Human fingers were simultaneously
ACKNOWLEDGMENTS
■
soaked in aqueous 1, a ZnCl solution, and a ZnO suspension
The authors acknowledge Dr. Laurence Du-Thumm and Dr.
Ravi Subramanyam for their support and many useful
discussions.
2
under the same conditions for 30 s. The treated fingers were then
rinsed under running water for 30 s. An indicator (Zincon) was
C
DOI: 10.1021/acs.inorgchem.6b01663
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(
21) Ellinger, C. R.; Nelson, S. F. Selective Area Spatial Atomic Layer
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DOI: 10.1021/acs.inorgchem.6b01663
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