ISSN 1070-4272, Russian Journal of Applied Chemistry, 2015, Vol. 88, No. 11, pp. 1839−1847. © Pleiades Publishing, Ltd., 2015.
Original Russian Text © M.V. Suyasova, Yu.V. Kul’velis, V.T. Lebedev, V.P. Sedov, 2015, published in Zhurnal Prikladnoi Khimii, 2015, Vol. 88, No. 11, pp. 1626−1634.
Clustering of Gadolinium Endofullerenols in Aqueous Solutions
M. V. Suyasova, Yu. V. Kul’velis, V. T. Lebedev, and V. P. Sedov
Konstantinov Petersburg Nuclear Physics Institute, National Research Centre Kurchatov Institute,
Orlova Roshcha, Gatchina, Leningrad oblast, 188300 Russia
Received October 19, 2015
Abstract—Nature of structuring of aqueous solutions of paramagnetic endofullerenols Gd@C
determined in relation to the concentration and pH factor from small-angle neutron scattering, viscometric, and
conductometric data . Molecular fractal clusters of the type of branched polymers were found, with their correlation
radius being about 20 nm and aggregation numbers reaching a value of 10
. The clusterized systems obtained are
promising as a basis for MRI-contrasting agents and other biomedical preparations for diagnostics and therapy.
A multitude of various fullerene derivatives with wide
and diverse application areas, from superconductors to
radiopharmaceutical preparations, have been synthesized
. A special mention should go to endohedral metal-
fullerenes (EMFs) M@C
characterized by the presence
of one or several metal atoms inside a carbon cage.
Quantum-chemical studies conﬁ rmed by results of an
X-ray diffraction analysis enabled conclusions about the
structure of molecules of this kind [2, 3]. Encapsulated
metal atoms are shifted from the center of a fullerene
to its inner surface, with a considerable charge transfer
from the metal atom to the carbon cage. This speciﬁ c
feature is clearly traced for a group of EMFs with rare-
earth elements: compounds of this kind are characterized
by incorporation of a metal atom into a C
molecule; endofullerenes M@C
, etc. occur in lower yields [4–6].
Gadolinium promising as an agent for magnetic-
resonance tomography (MRI) stand out among a number
of rare-earth elements encapsulated in a carbon shell:
seven unpaired electrons on the 4f orbital and one
localized on the fullerene skeleton contribute to the
paramagnetic moment of a molecule of an endofullerene,
such as Gd@C
. It has been found that presence of water-
soluble fullerene derivatives of the Gd@C
(endofullerenols) in aqueous solutions accelerates the spin
relaxation of water protons. The gadolinium ion affects
via dipole forces the magnetic moments of protons in
surrounding water molecules bound by hydrogen bonds
to OH groups on the periphery of the carbon skeleton
[6, 7]. Due to these properties, gadolinium endofullerenol
introduced into a biological tissue enhances the contrast,
with this effect being an order of magnitude stronger
than that with the existing commercial preparations for
MRI, chelate complexes of Gd [7–9], which are unstable
compounds capable of losing the metal atom.
Signiﬁ cantly, the acceleration of the spin relaxation
of water protons depends on the spatial distribution of
molecules of the MRI agent.
It was shown in a study  concerned with the
clustering of EMFs and aimed to determine the extent
of the possible aggregation of Gd@C
(precursor of the
water-soluble MRI agent) in an ortho-xylene solution that
fullerenes and EMFs in ortho-xylene predominantly form
molecular pairs, which is close to a molecular solution.
As shown below, EMFs modified via addition
of hydroxy groups to EMFs and soluble in water
(fullerenols) demonstrate, compared with the above
objects, complex forms of structural organization in
solution, which is not only of academic interest, but is also
important for prospective diagnostic applications (MRI).
A study of supramolecular structures based on fullerene
clusters formed via interaction with surfactants and lipid