P. De Maria, M. Pierini et al.
FULL PAPER
Partition Experiments: Partition experiments between n-octanol
and water were carried out at 25.0 0.2 °C by the “shake-flask”
technique.[32] Substrate concentration in either phase was deter-
mined spectrophotometrically at the λmax of aggregated FD1–5
(260 nm) in water and at the λmax of monomeric FD1–5 (254 nm)
in n-octanol. FD1–5 solutions were centrifuged for 15 min at
3000 runs/min before the experiments and shaken for 24 h with a
tube-rotator apparatus at 33 runs/min. The water/n-octanol volume
ratio was 5:1 for FD1–5. The initial concentrations of the aqueous
solutions of FD1–5 were 5×10–6 m.
towards the hydrophilic side. However, the water solubilities
of the studied fullerene derivatives are still very low and
comparable to those of medium-sized PAHs.
In addition, a linear correlation between log kw and API2
values was observed. This linearity is linked to the intrinsic
similarity of the two processes we are observing: interaction
of the fullerene derivatives with the apolar stationary phase
(log kw) and autoassociation to form the aggregates (API2).
Clearly both processes are dominated by solvophobic inter-
actions.
UV/Visible Determinations: UV/Vis determinations were carried
out on a Cary 1E Varian or a V550 Jasco spectrophotometer.
As previously stated, log kw and log P for several systems
are linearly related. With the present work we show that a
a similar correlation also exists [Equation (12)] between
log kw and log Pm/a for the homologous series FD1–3.
Transmission Electron Microscopy: Drops of a dilute solution of
FD3 (5×10–5 m) were placed onto copper grids pre-coated with car-
bon and then evaporated at room temperature. The experiments
were carried out on a FP 505 Morgagni Series 268D electron micro-
scope (Philips) operating at 60 kV, equipped with a Megaview III
digital camera and a Soft Imaging System (Germany).
log kw = 1.47 log Pm/a + 5.56 (n = 3, r2 = 0.9987)
(12)
Molecular Mechanics Calculations: The superficial area of the
FD1–5 molecules was calculated with the Hyperchem 6.03 program
on an optimized conformation obtained with the MMFF Force
Field as implemented in the Titan 1.0.1 package.[33]
On the other hand, the strong deviation of FD4 from the
correlation of Equation (12) suggests that the hydrophilic
appendix of FD4 is involved in specific solute–solvent inter-
actions, upon transfer between n-octanol and water, which
have no parallel counterparts in the chromatographic pro-
cess. These interactions are probably due to the hydrogen-
bonding properties of n-octanol as compared to the hydro-
carbon-like properties of the apolar stationary phase.
Synthesis of FD1–FD5: The synthetic details and appropriate litera-
ture citations for the preparation and characterization of fullerop-
yrrolidines FD1–FD5 are available as Supporting Information (see
footnote on the first page of this article).
Acknowledgements
We thank MIUR for financial support (COFIN 2002: protocol
number 2002032171) and the CESI center of the University “G.
d’Annunzio” for making the transmission electron miscroscope
available. M.M. wish to thank the University of Padova (grant
CPDA012428).
Conclusions
The different approaches adopted in this paper clearly
show that useful descriptors can be obtained for the hydro-
philic/hydrophobic balance for the investigated FDs. Aggre-
gation measurements in aqueous/organic mixtures, par-
tition measurements between n-octanol and water and re-
tention factors in reverse-phase chromatography are par-
ticularly convenient for describing the reversible transition
from the monomeric to the aggregated forms of the FDs
when passing progressively from organic solvents to aque-
ous solutions. Some interesting linear relationships have
been found between the investigated parameters, which re-
fer directly to the aggregation equilibria. Differences in
morphologies and in size of the aggregates have been visual-
ized in the absence of solvent by TEM measurements. The
importance of solvophobic interactions in the aggregation
process has been highlighted.
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Experimental Section
Liquid Chromatography: HPLC analyses were performed with a
TSP chromatograph equipped with a P2000 pump, a UV6000 LP
diode array detector and a Reodyne 7125 injector. In RP-HPLC
experiments a Hypersil HyPURITY® C4 5 μm 250×4.0 mm ID
column or a Hypersil BDS C18 3 μm 100×4.6 mm ID column were
used. In GP-HPLC experiments TSK GEL PW 6000 column was
used and the fractionation range was tested towards blue destrane
(Sigma) and a dilute solution of FeSCN to evaluate the exclusion
and the maximum inclusion volume of the resin, respectively.
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[16] All spectroscopic and analytical data of FD2–5 were consistent
with the proposed molecular structure (see Supporting Infor-
mation)
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Eur. J. Org. Chem. 2005, 1884–1891