Probing cations recognized by a crown ether with the 3D-RISM theory. II. 18-crown-6 ether
In the previous study, we investigated the cation recognition by 12-crown-4 ether with the 3D-RISM theory [Ikuta, Y. et al., Chem. Phys. Lett., 2007, 433, 403], and showed the applicability of the theory to the problem. The results show that a 12-crown-4 ether recognizes Li⁺ and Mg²⁺. 18-crown-6 eth...
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| Cite this: | Probing cations recognized by a crown ether with the 3D-RISM theory. II. 18-crown-6 ether / Y. Maruyama, M. Matsugami, Y. Ikuta // Condensed Matter Physics. — 2007. — Т. 10, № 3(51). — С. 315-322. — Бібліогр.: 28 назв. — англ. |
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Maruyama, Y. Matsugami, M. Ikuta, Y. 2017-05-31T03:51:59Z 2017-05-31T03:51:59Z 2007 Probing cations recognized by a crown ether with the 3D-RISM theory. II. 18-crown-6 ether / Y. Maruyama, M. Matsugami, Y. Ikuta // Condensed Matter Physics. — 2007. — Т. 10, № 3(51). — С. 315-322. — Бібліогр.: 28 назв. — англ. 1607-324X PACS: 02.30.Rz, 31.15.Bs, 61.25.Em DOI:10.5488/CMP.10.3.315 https://nasplib.isofts.kiev.ua/handle/123456789/118698 In the previous study, we investigated the cation recognition by 12-crown-4 ether with the 3D-RISM theory [Ikuta, Y. et al., Chem. Phys. Lett., 2007, 433, 403], and showed the applicability of the theory to the problem. The results show that a 12-crown-4 ether recognizes Li⁺ and Mg²⁺. 18-crown-6 ether is widely used in comparison with 12-crown-4 ether. In this study, we calculated three-dimensional distributions of water as well as cations (Li⁺, K⁺, Mg²⁺, Ca²⁺) inside and outside 18-crown-6 ether in aqueous solutions of electrolytes. A 18-crown-6 ether recognizes not only Li⁺ and Mg²⁺ but also K+ and Ca²⁺. K⁺ is dehydrated in the host cavity whereas other cations are hydrated. It is found that a hydration structure, as well as cation size, is important for the molecular recognition by crown ether. В попереднiх дослiдженнях ми вивчали в рамках теорiї ЗD-RISM катiонне впiзнавання 12–корона–4 ефiром [Ikuta, Y. et al., Chem. Phys. Lett., 2007, 433, 403] i показали застосованiсть цiєї теорiї. Результати показують, що 12-корона-4 ефiр впiзнає iони Li⁺ i Mg²⁺. 18-корона-6 ефiр ширше використовується порiвняно з 12-корона-4 ефiром. В данiй роботi розраховано тривимiрнi функцiї розподiлу води вiдносно iонiв Li⁺, K⁺, Mg²⁺, Ca²⁺ в серединi та зовнi 18-корона-6 ефiру у водних розчинах електролiту. Показано, що 18-корона-6 ефiр впiзнає не тiльки iони Li⁺ i Mg²⁺, але також iони K⁺ i Ca²⁺. K⁺ всерединi порожнини дегiдратується, тодi як iншi катiони є гiдратованими. Встановлено, що гiдратацiйна структура та катiоннi розмiри є важливими для молекулярного впiзнавання коронним ефiром. The authors are grateful to the support by the grant from the Next Generation Super Com- puting Project, Nanoscience Program, MEXT, Japan, and by the grant from Scientific Research on Priority Area of “Water and Biomolecules”. The authors thank to Dr. Yoshida for the fruitful discussions and useful information concerning their earlier work. en Інститут фізики конденсованих систем НАН України Condensed Matter Physics Probing cations recognized by a crown ether with the 3D-RISM theory. II. 18-crown-6 ether Дослiдження теорiєю ЗD-RISM катiонiв, впiзнаваних ефiрною короною: II 18–корона–6 ефiр Article published earlier |
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Probing cations recognized by a crown ether with the 3D-RISM theory. II. 18-crown-6 ether |
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Probing cations recognized by a crown ether with the 3D-RISM theory. II. 18-crown-6 ether Maruyama, Y. Matsugami, M. Ikuta, Y. |
| title_short |
Probing cations recognized by a crown ether with the 3D-RISM theory. II. 18-crown-6 ether |
| title_full |
Probing cations recognized by a crown ether with the 3D-RISM theory. II. 18-crown-6 ether |
| title_fullStr |
Probing cations recognized by a crown ether with the 3D-RISM theory. II. 18-crown-6 ether |
| title_full_unstemmed |
Probing cations recognized by a crown ether with the 3D-RISM theory. II. 18-crown-6 ether |
| title_sort |
probing cations recognized by a crown ether with the 3d-rism theory. ii. 18-crown-6 ether |
| author |
Maruyama, Y. Matsugami, M. Ikuta, Y. |
| author_facet |
Maruyama, Y. Matsugami, M. Ikuta, Y. |
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2007 |
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English |
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Condensed Matter Physics |
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Інститут фізики конденсованих систем НАН України |
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Article |
| title_alt |
Дослiдження теорiєю ЗD-RISM катiонiв, впiзнаваних ефiрною короною: II 18–корона–6 ефiр |
| description |
In the previous study, we investigated the cation recognition by 12-crown-4 ether with the 3D-RISM theory [Ikuta, Y. et al., Chem. Phys. Lett., 2007, 433, 403], and showed the applicability of the theory to the problem. The results show that a 12-crown-4 ether recognizes Li⁺ and Mg²⁺. 18-crown-6 ether is widely used in comparison with 12-crown-4 ether. In this study, we calculated three-dimensional distributions of water as well
as cations (Li⁺, K⁺, Mg²⁺, Ca²⁺) inside and outside 18-crown-6 ether in aqueous solutions of electrolytes. A 18-crown-6 ether recognizes not only Li⁺ and Mg²⁺ but also K+ and Ca²⁺. K⁺ is dehydrated in the host cavity whereas other cations are hydrated. It is found that a hydration structure, as well as cation size, is important for the molecular recognition by crown ether.
В попереднiх дослiдженнях ми вивчали в рамках теорiї ЗD-RISM катiонне впiзнавання 12–корона–4 ефiром [Ikuta, Y. et al., Chem. Phys. Lett., 2007, 433, 403] i показали застосованiсть цiєї теорiї. Результати показують, що 12-корона-4 ефiр впiзнає iони Li⁺ i Mg²⁺. 18-корона-6 ефiр ширше використовується порiвняно з 12-корона-4 ефiром. В данiй роботi розраховано тривимiрнi функцiї розподiлу води вiдносно iонiв Li⁺, K⁺, Mg²⁺, Ca²⁺ в серединi та зовнi 18-корона-6 ефiру у водних розчинах електролiту. Показано, що 18-корона-6 ефiр впiзнає не тiльки iони Li⁺ i Mg²⁺, але також iони K⁺ i Ca²⁺. K⁺ всерединi порожнини дегiдратується, тодi як iншi катiони є гiдратованими. Встановлено, що гiдратацiйна структура та катiоннi розмiри є важливими для молекулярного впiзнавання коронним ефiром.
|
| issn |
1607-324X |
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https://nasplib.isofts.kiev.ua/handle/123456789/118698 |
| citation_txt |
Probing cations recognized by a crown ether with the 3D-RISM theory. II. 18-crown-6 ether / Y. Maruyama, M. Matsugami, Y. Ikuta // Condensed Matter Physics. — 2007. — Т. 10, № 3(51). — С. 315-322. — Бібліогр.: 28 назв. — англ. |
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Condensed Matter Physics 2007, Vol. 10, No 3(51), pp. 315–322
Probing cations recognized by a crown ether with the
3D-RISM theory. II. 18-crown-6 ether
Y.Maruyama1, M.Matsugami2, Y.Ikuta1
1 Department of Theoretical Studies, Institute for Molecular Science, Okazaki, Aichi, 444–8585, Japan
2 Chemistry, General Education, Kumamoto National College of Technology, 2659–2, Suya, Koshi, Kumamoto,
861–1102, Japan
Received June 4, 2007, in final form July 18, 2007
In the previous study, we investigated the cation recognition by 12-crown-4 ether with the 3D-RISM theory
[Ikuta, Y. et al., Chem. Phys. Lett., 2007, 433, 403], and showed the applicability of the theory to the problem.
The results show that a 12-crown-4 ether recognizes Li+ and Mg2+. 18-crown-6 ether is widely used in
comparison with 12-crown-4 ether. In this study, we calculated three-dimensional distributions of water as well
as cations (Li+, K+, Mg2+, Ca2+) inside and outside 18-crown-6 ether in aqueous solutions of electrolytes.
A 18-crown-6 ether recognizes not only Li+ and Mg2+ but also K+ and Ca2+. K+ is dehydrated in the host
cavity whereas other cations are hydrated. It is found that a hydration structure, as well as cation size, is
important for the molecular recognition by crown ether.
Key words: 3D-RISM theory, molecular recognition, crown ether, cation
PACS: 02.30.Rz, 31.15.Bs, 61.25.Em
1. Introduction
Several types of molecular recognitions occurring in our body play an essential role in life
sustaining. For example, the hemoglobin which is contained in aemia carries the oxygen molecules
to the proper cells. Investigation of such a recognition process (mechanism) is an extremely essential
issue in understanding the functions of biomolecules in a living system. We have little knowledge
of such a process in detail, in spite of strenuous efforts on scrutinizing the recognition process.
The interaction between a host molecule and guest molecules is so complicated that it is not easy
for both experimental and theoretical studies to reach a straightforward solution with existing
methods. Here we propose that we reexamine small system, many aspects of which are well known
because some obscurity remains even in the origin of the recognition between crown ethers and
metal cations.
Since Pedersen has accidentally discovered the crown ethers, the supra-molecule has attracted
many chemists [1–5]. They have carried out numerous investigations of special nature known as one
of host-guest molecular recognitions. At present, crown ether is indispensable for the experiments
in organic and analytical chemistry, since it plays a role of removing unnecessary metal cations
through the experiments. In other words, the wonderful nature of the crown ether is its capabi-
lity of recognizing metal cations. For instance, 12-crown-4 recognizes a lithium ion, while it does
not recognize a potassium ion. Furthermore, it is empirically known that 18-crown-6 recognizes
potassium ion selectivity under lithium ion and potassium ion coexistence. 18-crown-6 is highly
flexible and can take many conformations in aqueous solution. In particular 18-crown-6 shows the
two typical conformations: known as Ci and D3d. It is clear that the Ci conformation, which forms
intramolecular hydrogen bonds, is not capable of trapping a cation, because the cavity of the ether
ring is too small to recognize the cation. On the other hand, the D3d conformation can trap a
metal cation since its cavity is large enough to recognize and to capture an ion: the shape of the
cavity is regarded as a hole. Taking into account the experimental results of this recognition that
several metal cations are trapped by the 18-crown-6, it is found that all we have to study is related
c© Y.Maruyama, M.Matsugami, Y.Ikuta 315
Y.Maruyama, M.Matsugami, Y.Ikuta
to the D3d conformation for our purpose [6]. We have developed a methodology based on the three
dimensional reference interaction site model (3D-RISM) theory, which is a statistical mechanics
theory of liquids [8]. The theory succeeded in probing water molecules trapped in a cavity of the
hen egg-white lysozyme [9]. It has also been successfully applied to the selective ion binding by the
mutants of human lysozyme to reproduce the experimental results [10]. Furthermore, we reported
that 3D distributions of water as well as cations (Li+,K+, Mg2+ and Ca2+) inside and outside
a crown ether (12-crown-6) in aqueous solutions of electrolytes were calculated by means of the
3D-RISM theory. The results showed conspicuous peak inside the host cavity in the case of Li+
and Mg2+, indicating that the cations are recognized by the 12-crown-6. In the case of Li+ and
Mg2+, the sizes of ions are small enough to fit into the cavity, where the effects of core repulsion
are insignificant. In the case of K+, the core repulsion between the cation and the cyclic ether is
simply dominant over the other factors, so that it cannot be accommodated in the host cavity. On
the contrary, the stabilization due to the chelation is largely compensated by the core repulsion in
the case of Ca2+, and it is not enough to pay for the cost of dehydration penalties [11].
In this study, we examined an 18-crown-6 ether in LiCl, KCl, MgCl2 and CaCl2 0.1M aqueous
solutions. There were earlier contributions concerning a crown ether in water using the ordinary
1D-RISM method [12,13]. One of the papers has actually dealt with 18-crown-6 [13]. Although the
system was not seriously affected by the notorious problem of the ergodic trap, they still had to
search around the free energy surface of the host-guest complex, using the combined RISM and
Monte Carlo procedure, which was proposed earlier by Kinoshita et al. [14]. However, the method
is somewhat time consuming, especially when more than two components, say water and ions, are
considered as ligands. On the contrary, for our purpose, it is enough to employ the present method
(see the next section), which provides a picture of host-guest complexation, in a manner similar to
an electron-density map obtained from the X-ray diffraction measurement.
2. Method
In our procedure, the solute-solvent correlation functions are obtained from the 3D-RISM theory
[8], which is an integral equation theory based on statistical mechanics to obtain the correlation
functions from the intermolecular potential functions and the thermodynamics conditions such
as temperature and density. The theoretical procedure consists of two steps. The first step is
to calculate the density pair correlation function in the electrolyte solutions based on the one-
dimensional RISM theory, which represent the microscopic structure of distribution of the solvent
molecules. In the second step, we immerse a crown ether (18-crown-6) into the solvent, and calculate
the 3D distribution of solvent atoms (O, H, cation and anion) around as well as inside the supra-
molecule by means of the 3D-RISM theory. In the following, we provide just an outline of the
3D-RISM method.
The 3D-RISM integral equation for the 3D solute-solvent site total and direct correlation func-
tions, huv
γ (r) and cuv
γ (r), is written as [15–18]
huv
γ (r) =
∑
γ′
cuv
γ′ (r) ∗
(
ωvv
γ′γ(r) + ρvhvv
γ′γ(r)
)
, (1)
where ωvv
γ′γ(r) = δ(r − lvv
γ′γ) is the intramolecular matrix of solvent molecules with site separations
lvv
γ′γ , γ and γ′ are interaction sites of solvent molecules denoted with superscripts “u” and“v”,
respectively, ρv is the solvent number density and * denotes convolution in direct space. The radial
site-site correlation functions of pure solvent, hvv
γ′γ(r), are obtained independently of the conven-
tional 1D-RISM theory modified with the dielectrically consistent bridge corrections of Perkyns
and Pettitt [19,20]. It provides a proper description of the dielectric properties of solution. In the
3D-RISM context, this ensures a proper macroscopic dielectric constant of solvent around the so-
lute [18,21]. The 3D solute-solvent site correlation functions are specified on a 3D linear grid in
a rectangular supercell, and the convolutions in (1) are handled by means of the 3D fast Fourier
transform.
316
Cations recognized by a 18-crown-6 ether
We have recently proposed and tested the new closure which will be referred to as the Kovalenko-
Hirata approximation [17,18,21,22]. In the 3D case, however, it should include corrections for the
supercell periodicity artifact to both the direct and total 3D site correlation functions, cuv
γ (r) and
huv
γ (r), which become essentially important for a solute carrying charges. The closure thus takes
the form [17,23]
guv
γ (r) =
{
exp(χ) + ∆Quv
γ for χ 6 0,
1 + χ + ∆Quv
γ for χ > 0,
(2)
χ = −βuuv
γ (r) + huv
γ (r) − cuv
γ (r) − ∆Quv
γ ,
where guv
γ (r) = huv
γ (r) + 1 is the 3D solute-solvent site distribution function, β = 1/(kBT ), the in-
teraction potential uuv
γ (r) between solvent site γ and the whole solute is calculated on the supercell
grid using the Ewald summation method, and
∆Quv
γ =
4πβ
Vcell
q0 lim
k→0
∑
γ′
qα
k2
(
ωvv
γ′γ(k) + ρvhvv
γ′γ(k)
)
(3)
is the shift in the distribution functions due to the supercell background for the solute with net
charge q0 = Σαqu
α comprising the partial site charges qu
α. The 3D solute-solvent site direct corre-
lation functions cuv
γ (r) calculated from the 3D-RISM equation (1) with the closure (2) must be
corrected by subtracting the long-range electrostatic asymptotic of the periodic potential uuv
γ (r)
synthesized by the Ewald summation, and adding back that of the single, non-periodic solute
simply tabulated on the 3D grid within the supercell [18,21].
For the pair potentials between interaction sites of every species, we employ the common model
which consists of the Lennard-Jones (LJ) and electrostatic interaction terms
uαγ(r) = 4εαγ
[
(σαγ
r
)12
−
(σαγ
r
)6
]
+
qαqγ
r
, (4)
where qα is the partial charge on site α, and εαγ and σαγ are the LJ energy and size parameters
for a pair of sites α and γ, respectively. For the crown ether and ions, we employ the parameters
from the OPLS force field [24,25]. We use the SPC/E model [26] with the hydrogen Lennard-Jones
term (σ = 0.4 Å and ε = 192.5 J/mol) for the water sites. Calculations are carried out for aqueous
solutions at 298.15 K. These parameters and the thermodynamic conditions of solvents are the
same as those in the previous study [11].
The molecular structures of the crown ether are optimized in vacuo at B3LYP/6-31G(d) level
using Gaussian 03 [7]. We used D3d conformation of 18-crown-6 in this study. The 3D-RISM/KH
equations were solved on a grid of 1283 points in a cubic supercell of size 64 Å3. The supercell is
sufficiently large to hold a crown ether with enough solvation space. The grid space of 0.5Å is fine
enough to avoid significant numerical errors. For better convergence of the equations, we used the
modified direct inversion in the iterative subspace (MDIIS) method [27].
3. Results and discussions
First, we examine whether an 18-crown-6 ether can recognize an ion or not by using the 3D-
RISM theory. The 3D distribution functions (3D-DFs), g(r) which are obtained by the 3D-RISM
theory, are very useful and appropriate for answering this question. If g(r) inside the host molecule
is greater than unity, which is the case in the bulk solution, it indicates that “the host molecule
recognizes the guest one”. Through the present article, we employ the isosurface representation to
show 3D distributions g(r) of water and/or the cations, calculated with the 3D-RISM theory: the
region where g(r) is greater than a threshold value is shown by color for each species of solvent.
Figure 1 shows the g(r) for (a) water oxygen colored with light blue and (b) water hydrogen
colored with white around 18-crown-6 in pure water. The threshold value of g(r) are 2, 4 and 8
from the left to the right, respectively. For example, g(r) = 2 means that the corresponding site
317
Y.Maruyama, M.Matsugami, Y.Ikuta
is distributed twice as probable as in the bulk phase. These results give us some information that
the spots inside the crown ether are not only small but also conspicuous. The right figures in (a)
and (b), of which thresholds are g(r) = 8, indicate that the supramolecule does recognize water
molecules: that is, the cavity is not empty when the host molecule exists even in pure water. This
result is not surprise if one compares the size of a water molecule (about 2.8 Å) with the cavity
diameter (about 5.8 Å).
Figure 1. Isosurface representation of the three-dimensional distribution function of water (a)
oxygen and (b) hydrogen around and inside 18-crown-6. The surfaces show the regions where
the distribution function, g(r), is larger than 2 (left), 4 (center) and 8 (right), respectively. The
structure of 18-crown-6 was optimized at B3LYP/6-31(d) level.
Figure 2. Site-site radial distribution functions between oxygen atom on 18-crown-6 ether and
water oxygen atom (solid line) as well as water hydrogen atom (broken line).
Figure 2 shows radial distribution functions (RDFs): one is the RDF between an oxygen atom
on the 18-crown-6 and the water oxygen as well as the hydrogen, respectively. From 1D-RDFs
we can obtain an additional information about the molecular recognition with regard to the dis-
tance between the host and the solvent molecules. The site-site distribution functions guv
αγ(r) are
318
Cations recognized by a 18-crown-6 ether
calculated by numerical averaging of the 3D site distributions guv
γ (r) over the solute orientation,
guv
αγ(r) =
1
4π
∫
dΩrg
uv
γ (rα + r), (5)
where rα is the position of solute site α. We calculated the integral over sphere in equation (5)
by employing the 700-point set of the repulsion scheme within the quadrature method of spherical
harmonic reductions or elimination by a weighted distribution (SHREWD), elaborated by Eden
and Levitte [28]. A well-defined peak located around 1.5 Å in the H(water)-O(18-crown-6) RDF
shows that a hydrogen bond is formed between the host molecule and the guest molecules. Although
the peak around 1.5 Å is contributed from water molecules both inside and outside the cavity (see
the left and the center pictures in figure 1), the contributions from the inside of the cavity are
much larger than from the outside: hydrogen distribution is hardly seen outside the cavity for the
rigid threshold value (g(r) = 8).
Figure 3. Isosurface representation of the
three-dimensional distribution function of cati-
ons around and inside 18-crown-6. The sur-
faces show the regions where the distribution
function, g(r), is larger than 2 (left), 4 (center)
and 8 (right), respectively. The structure of
18-crown-6 was optimized at B3LYP/6-31(d)
level. (a) Lithium cation. (b) Potassium cation.
(c) Magnesium cation. (d) Calcium cation.
The 3D distributions g(r) of each cation
around 18-crown-6 are shown in figure 3: the
threshold values are 2, 4 and 8 from the left to
the right, respectively. In figure 3(a), g(r) for Li+
around the ether, colored by green, shows Li+
distributes to form a ring-like shape inside the
cavity. This picture shows a shift of the position
of Li+ from the center of cavity. Note that the
size of Li+ is smaller than that of cavity. In gen-
eral, it is difficult to obtain an X-ray structure of
lithium due to its small atomic number. There-
fore, the accuracy of the calculation results in
comparison with the experimental data is still
unknown. Figure 3(b) shows g(r) of K+ around
18-crown-6, which is colored by purple. The hi-
ghest peak is seen in the center of the ring, al-
though g(r) of Li+ spreads like a ring shape and
is not seen in the center. This result indicates
that 18-crown-6 accommodates K+, whereas 12-
crown-6 does not recognize K+. K+ locates at
the center of the cavity because the size of K+ is
almost the same as that of cavity. The result is
consistent with the experimental one: K+ has a
higher affinity for the 18-crown-6 than any other
cation in alkali metals. g(r) of Mg2+ around 18-
crown-6 is painted in blue (figures 3(c)). It is
found that the blue surface outside the ether is
completely different from those of Li+ and K+,
while the isosurface inside the 18-crown-6 is very
similar to the g(r) of Li+. This indicates that the
electrostatic interaction between oxygen atoms
on the 18-crown-6 and Mg2+ is stronger than
the mono cation cases. In figure 3(d), the 3D
distribution function of Ca2+ around the crown
ether is indicated. Apart from 12-crown-4, the
calcium cation is found inside the cavity of 18-
crown-6. This result indicates that the size of
cavity is large enough to trap a cation with being
hydrated even after the entrapment when it ap-
proaches the binding-site. The cations are small
enough to be accommodated in the host cavity
319
Y.Maruyama, M.Matsugami, Y.Ikuta
and stabilized by the electrostatic potential produced by the surrounding oxygen atoms. Our heuri-
stic approach to the molecular recognition does not need any further calculations. This method
has been successfully applied to the molecular recognition between 18-crown-6 and the cations as
in our previous studies [9–11].
Figure 4. Stable positions of cations inside 18-crown-6. (a) Lithium cation. (b) Potassium cation.
(c) Magnesium cation. (d) Calcium cation.
Each cation is located at the most suitable position inside the host’s cavity, which is estimated
with taking into consideration 3D-DFs (figure 4). Figure 4(b) shows there seems to be no space to
hydrate K+ and the others in figure 4 show the cavity is not filled by each cation. It is, therefore,
necessary to examine whether these trial cations are hydrated or not when they are trapped by the
supra-molecule. Converting 3D-DFs to 1D-RDFs gives us hints as to this question. According to the
1D-RDFs around each trapped cation which is explicitly located in the cavity, it is found that there
are characteristic peaks of hydrogen bond, except K+ (figure 5). This result can be explained from
the point of whether the cation size is large enough to fill the cavity or not. Then the recognition pro-
cess between K+ and the host molecule requires the dehydration process before the capture. Once
K+ is trapped by the host, the host-guest formation would overcome the dehydration penalty. On
the other hand, other cations are trapped by 18-crown-6 without the dehydration process because
these naked cations are too small to keep their stability in the cavity. Mg2+ is the typical case of hy-
dration even after the entrapment because the distribution function between the cation and water
oxygen represents an intensive peak. Less pronounced peaks are, of course, shown in Li+ and Ca2+.
Then it is concluded that these cations except K+ and the supramolecule could form the host-guest
complex without dehydration process. In our previous study on 12-crown-4 ether, the core repulsion
between K+ and the host molecule was found to be a dominant factor to determine whether K+ is
recognized or not. Contrary to 12-crown-4, the K+ size matches that of the cavity 18-crown-6. The
stability of the complex between K+ and 18-crown-6 is more intensive than any other cation because
K+ is directly trapped by the host molecule without forming hydration structure. As shown in the
previous study, Ca2+ is not recognized by 12- crown-4 because Ca2+ keeps the hydration structure
through all the recognition process. Meanwhile, Ca2+ can be recognized by 18-crown-6 with being
hydrated because the cavity size of this is large enough to accommodate the hydration structure.
Whether solvated structures of cations are kept or not when forming the host-guest complex
is determined by the species of each cation. We shall point out that whether hydrated structure
is required or not affects the complex stability during such a recognition process. It is an essential
information to predict whether a combination of a host molecule and guest molecules is suitable
or not, especially when eliminating unnecessary or obstructive components in reaction systems.
320
Cations recognized by a 18-crown-6 ether
0
0.5
1
1.5
2
2.5
3
0
0.5
1
1.5
2
2.5
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 10
Ow
Hw
Ow
Hw
Ow
Hw
Ow
Hw
(a) (b)
(c) (d)
r
g
(r
)
Figure 5. Site-site radial distribution functions between ions and water oxygen atom (solid lines)
as well as water hydrogen atom (broken lines). (a) Lithium cation. (b) Potassium cation. (c)
Magnesium cation. (d) Calcium cation.
4. Conclusion
In this article, we studied the molecular recognition between 18-crown-6 and cations (Li+,
K+, Mg2+, Ca2+) by carrying out 3D-RISM calculation. Apart from 12-crown-4, it is difficult to
predict the discriminative recognition because the cavity size of the host ether is so large that the
hydrated cations are located at the crown center. The results show any cation which was examined
in the 3D-RISM study was recognized by an 18-crown-6 ether. Moreover, 1D-RDF between cations
of host-guest complex and solvent water shows that the hydration structure is maintained in
the molecular recognition process in the cace of a small size cation, Mg2+. On the other hand,
K+ is dehydrated in the cavity of an 18-crown-6 ether. Thus, not only cation size but also a
hydration/dehydration structure is important for the molecular recognition by crown ether. In this
study we carried out the calculations without the flexibility of 18-crown-6 because of the huge
computational cost for the 3DRISM-SCF calculation in electrolyte solution. In the near future, we
will report further investigations on this type of molecular recognition including RISM/3DRISM-
SCF results in progress in our laboratory.
Acknowledgement
The authors are grateful to the support by the grant from the Next Generation Super Com-
puting Project, Nanoscience Program, MEXT, Japan, and by the grant from Scientific Research
on Priority Area of “Water and Biomolecules”. The authors thank to Dr. Yoshida for the fruitful
discussions and useful information concerning their earlier work.
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Дослiдження теорiєю ЗD-RISM катiонiв, впiзнаваних ефiрною
короною: II 18–корона–6 ефiр
Ю.Маруяма1, М.Матсугамi2, Я.Iкута1
1 Факультет теоретичних дослiджень, Iнститут молекулярних наук, Оказакi,Аiчi, Японiя
2 Хiмiя, Загальна освiта, Нацiональний технологiчний коледж в Кумоното, Суя, Кошi, Кумоното, Японiя
Отримано 4 червня 2007 р., 18 липня 2007 р.
В попереднiх дослiдженнях ми вивчали в рамках теорiї ЗD-RISM катiонне впiзнавання 12–корона–4
ефiром [Ikuta, Y. et al., Chem. Phys. Lett., 2007, 433, 403] i показали застосованiсть цiєї теорiї. Резуль-
тати показують, що 12-корона-4 ефiр впiзнає iони Li+ i Mg2+. 18-корона-6 ефiр ширше використо-
вується порiвняно з 12-корона-4 ефiром. В данiй роботi розраховано тривимiрнi функцiї розподiлу
води вiдносно iонiв Li+, K+, Mg2+, Ca2+ в серединi та зовнi 18-корона-6 ефiру у водних розчинах
електролiту. Показано, що 18-корона-6 ефiр впiзнає не тiльки iони Li+ i Mg2+, але також iони K+ i
Ca2+. K+ всерединi порожнини дегiдратується, тодi як iншi катiони є гiдратованими. Встановлено,
що гiдратацiйна структура та катiоннi розмiри є важливими для молекулярного впiзнавання корон-
ним ефiром.
Ключовi слова: ЗD-RISM, теорiя, молекулярне впiзнавання, коронний ефiр, катiон
PACS: 02.30.Rz, 31.15.Bs, 61.25.Em
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