Crown Ethers and Binding of Metals
Lara Dakkak
Department of Chemistry, University of West Georgia
Summer Research under Professor Khan
29 June 2007
Abstract
The binding affinities of crown ethers, 12-crown-4, 15-crown-5 and 18-crown-6, with
alkali metal ions, Na+, K+ and Cs+, were measured by electrospray ionization mass
spectrometry. The intensities of the adducts in mass spectra reflect the solution binding
affinities, which can be correlated with the best fit between the radius of the cation and
the cavity size of the crown ether.
Introduction
The study of crown ethers was intended to assess their guest-host binding
selectivity by using electospray ionization mass spectrometry (ESI-MS) in order to
measure the intensities of the mixtures. The origin of the name, crown ether, stems from
the fact that when an ion enters the middle of the crown it bears a resemblance to a crown
placed on a head. Crown ethers are fundamentally chemical compounds that are repeating
units of ethylene oxide (C2H4O). The unique binding quality of crown ethers allows for
them to bind strongly with ions. The interior of the ring is the portion where Oxygen (in
most cases) binds with the metal, whilst the exterior remains hydrophobic. This creates a
polar substance that is soluble in a non-polar solvent.
The selectivity of the metals that bind with the ethers may be based upon the size
of the cavity as well as the atomic radii of the guest ion. Much study has revolved around
the composition of crown ethers and their guest-host binding selectivity as they play an
important role in biological systems as well as industrial processes. For example, they are
responsible for enzyme interactions, ion transport processes, and antibody-antigen
recognition.
Experimental Details / Methods Employed
The instrument used mostly in this experiment has been Electrospray Ionization
Mass Spectrometer as it has been successful in analyzing a wide array of non-covalent
host guest complexes. It has proven to be a very useful tool in analyzing these ether-metal
species, as it provides a fast and efficient method for probing their binding interactions.
This analytical method presents results in a rapid manner, does not require large samples,
is compatible with a wide range of samples, and is compatible with polar molecules
ranging in molecular mass from 100 Da to more than 1,000,000 Da. These characteristics
make the ESI-MS a well-suited candidate for studying crown.
The process of Electrospray Ionization begins with the pumping of a sample that
has been dissolved in a polar solvent
through a narrow, stainless steel capillary.
A high voltage is applied to the tip of the
capillary resulting in the emergence of the
sample as highly charged ions engulfed in
droplets of solvent. The spray of charged
droplets is directed with the aide of
Nitrogen toward the mass spectrometer. Next, a warm flow of drying gas (also Nitrogen)
results in the evaporation of the droplet, leaving behind highly charged ions. The charged
ions, now free from a solvent, pass through a sampling cone towards an intermediate
vacuum area, and finally enter the analyzer of the mass spectrometer, which is under a
high vacuum.
Data
Typically, several ions (guest substances) and
one crown ether (host complex) are included in a mixture and are passed through the ESIMS in order to analyze the mixture to see which guest complex best serves a specific host
substance. Depending on the resulting peaks, one can infer which complex is presented in
the largest quantity. The four guest complexes used in this experiment were 12 crown 4,
15 crown 5, 18 crown 6, and 24 crown8. The host metals used were sodium, potassium,
and cesium. The crown ethers were purchased and the metals used were those of chloride
salt. The process of creating a solution began with the mixing of all three metals in a
1.0x10-3 molar concentration with just one crown ether in a 4.5x10-4 molar concentration.
This resulted is four solutions in total. The sheath gas flow rate used was twelve, the aux
gas flow rate was eight, the spray voltage was 4.00 kV, the capillary temperature was
125ºC, the capillary voltage was 9.00 V, and the tube lens was 100.00 V.
The relative amounts and the binding selectivity of the ions to the crown ethers
were concluded using the relative intensities of the mass to charge ratio presented by the
mass spectrometer. The first crown complex analyzed was 12 crown 4. The spectrum
demonstrated three main peaks with relative intensities ranging from 40-96%. The
highest intensity peak, at 199.1 m/z, was a result of the binding of 12 crown 4 with
sodium. The second peak had a relative intensity of approximately 74% and was the
product of 12 crown 4 binding with potassium. The final peak was that of 12 crown 4
binding with cesium, having a relative intensity of 40 % and 309.1 m/z.
15-crown-5 + Na
15-crown-5 + K
15-crown-5 + Cs
A typical mass spectrum in ESI-MS. The adducts formed by the binding of alkali metal
ions to 15-crown-5 are shown.
The second aza crown ether analyzed was 15 crown 5, which demonstrated three
main peaks. The first peak was one of 100% intensity and represented the binding of
sodium with 15 crown 5 at 241 m/z. The second largest peak was the complex of 15
crown 5 and potassium which was at a relative intensity of 19% and 259 m/z. The final
was that of a 2% intensity and 350 m/z corresponding to 15 crown 5 and cesium.
Host 18 crown 6 was the third crown examined, also producing three main peaks.
The first peak was of a 99% intensity, a complex of 18 crown 6 and potassium having a
relative mass to charge of 302. The second, less intense peak was 18 crown 6 and
sodium with an intensity of just 3% and 287.21 m/z. The final peak was slightly less
intense, with a relative abundance of 2% and 397.15 m/z signifying the bond between 18
crown 6 and cesium.
The results are summarized in the table below.
Table 1. Dimensions of cavities in crown ethers and sizes of cations
CROWN
CAVITY SIZE BEST FIT
(Angstroms)
(Size, Angstroms)
12-crown-4
.6 - .75
Na+
1.02
15-crown-5
.86 - .92
Na+
1.02
18-crown-6
1.34 - 1.43
K+
1.38
The data indicate that that Na+ has the highest binding affinity for 12-crown-4 and 15crown-5, and K+ has the highest binding affinity for 18-crown-6. Cs+ was too large to fit
these crown ethers. These reflect the best match between the size of the cavity and the
size of the ion.
Conclusions and Future Directions
In future studies, the binding affinities will be extended to larger crown ethers and
families of ions that have a charge of +2 and 3. In addition, the binding of small
molecules such as peptides to crown ethers will also be explored.
References
1. Brodbelt, J.S.; Williams, S.M., A.P. Marchland, D.Cal, and K. Mlinaric-Majerski,
Anal. Chem. 2002, 74, 4423-4433.
2. Kiviniemi, Sari. “Complexation of N-HeteroAromatic Cations with Crown Ethers
and Tetraphenylborate.” Oulu: Oulu University Press: 2001.
3. Blair, S. M.; Kempen, E. C.; Brodbelt J.S. “Determination of Binding
Selectivities in Host-guest Complexation by Electrospray/Quadruple Ion Trap
Mass Spectrometry”. American Society for Mass Spectrometry. 1998, 1049-1059.
4. Oshima, T.; Matsuda, F.; Fukushima, K.; Tamura, H.; Matsubayashi, G.; Arakwa,
R. J. Chemistry Society. 1998, vol 2, 145-148