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Figure 3. Generation 4 PAMAM-H 2 O
Figure 4. Generation 4 PAMAM-PEO-H 2 O
In order to obtain configurations suitable for simulation, the systems were subjected to energy minimization after
application of conjugate gradient and steepest descend techniques. The systems were then simulated in room temperature by
the aid of the DL_POLY [5] molecular simulation package which was parallelized with MPI. The first part of the
simulations involved equilibration run in the isobaric-isothermic ensemble. The duration of simulations was long enough to
bring systems to equilibrium in terms of density, total energy and radius of gyration of dendrimer/linear chain. Production
runs of 4ns were then conducted in the microcanonical ensemble.
RESULTS
The radial distribution function of water in all studied systems was calculated. The peak which occurs at roughly 2.8Å which
is the well known average hydrogen bond length in water, exhibits a higher value in systems which contain linear chain
whereas in all systems that contain dendrimer molecule there is a higher peak with respect to the one in the pure water
system. Slight differences that appear between systems of different generations can be attributed to the different number of
water molecules which are adsorbed to the dendrimer surface or interior. For example, the highest value of the peak which
occurs in 4 th generation systems is consistent to the larger number of the dendrimer surface functional groups (amine groups)
which contribute to the formation of hydrogen bonds.
Dynamics of size fluctuations was probed via the autocorrelation function of the mean squared radius of gyration. This
dynamic quantity is relevant to the drug-release rate in controlled-release mechanisms. Such relaxation times indicate that
systems of 4 th generation relax in shorter time-scales compared to those of 3 rd generation. In addition, the presence of the
linear chain results modifies the relaxation time in complex systems. It therefore becomes possible to adjust the desired timescale
for size fluctuations by utilizing dendrimers of appropriate generation complexed or not with linear polyelectrolytes.
REFERENCES
[1] Gillies E.R., and Frechet J.M.J., DDT, 10 (2005) 1.
[2] Boas U., and Heegaard P.H.H., Chem.Soc.Rev., 33 (2004) 43.
[3] Jorgensen W.L., Chandrasekhar J., Madura J.D., Impey R.W., and Klein M.L., J.Chem.Phys. 79 (1983) 926.
[4] Gasteiger J., and Marsili M., Tetrahedron 36 (1980), 3219.
[5] Smith W., Forester T.R., Todorov I.T., and Leslie M., THE DL_POLY 2 USER MANUAL, CCLRC Daresbury
Labororatory.
ACKNOWLEDGEMENTS
This work is financially supported by the Hellenic General Secretariat for Research & Technology under the framework of
the PENED 2003 program, Grant No 03ΕΔ716. A large part of the simulations were carried out under the project HPC-
EUROPA (RII3-CT-2003-506079), with the support of the European Community - Research Infrastructure Action under the
FP6 “Structuring the European Research Area” program.
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