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Scientific Report 2007-2009
Condensed matter physics and biophysics
C19. Mesoscopic solutes in water solvent
Water solutions with mesoscopic solutes represent still
an open problem for what concerns both thermodynamic
and statistical mechanic. The hydrogen bounds network
in the solvent itself and, often, with the solute interface
represent the undefined quantity. Must of the thermodynamic
properties of these solutions depend on the extend
of solute-solvent interface and on the overall solvent
modification due to the presence of the solute (phase diagram,
solute-solute interaction, eventual solute flocculation
etc). To have an idea of the solute-solvent interfacial
contribution we can evaluate the extent of the surface of
1 cc of solution, about 5 cm 2 surface, and the total surface
of 10 −6 molar solution of spherical solutes having
50Å of radius, it results to be about 5 10 4 cm 2 , quite a
big number! My work has been developed in the last
5 years in collaboration with students graduated in my
laboratory: Marco Maccarini, an experimentalist expert
on SANS and SpinEcho neutron scattering now working
at ILL Grenoble France, Fabio Sterpone, now working at
the Dep. of Chemistry Ecole Normale Superiore, Paris
France, and with Simone Melchionna, PhD fellow in my
laboratory and now working at the Institute of Materials
Ecole Polytechnique, Losanne Switzerland, the last two
expert in Molecular Dynamic Simulation (MD).
P(S Max /N w )
P(S Max /N w )
P(S Max /N w )
15
10
5
0
15
10
5
0
15
10
5
0
0 0.2 0.4 0.6 0.8 1
S Max /N w
Figure 1: Maximum fully connected water cluster at different
temperatures for Meso, Thermo and Hyperthermo organisms
[2].
The first step of my study concerned the hydration
properties of the G-domain of an omnipresent protein in
all the living organisms by means of MD simulations in
collaboration with Simone Melchionna. In this work we
pointed out the fundamental role of water in the thermal
stability of such a protein [1]. Afterward, with the help of
Fabio Sterpone, we analyzed the same protein extracted
by three different organisms, one having its optimal living
condition (OLC) at 37 C a mesophile organism (M),
the second a thermophile organism (T) having its OLT
at 70 C, the third hyperthermophile (H) with OLT at 97
C, all of them present a high degree of sequence affinity.
The main result of this work concerns the identification
of a fully connected water network covering each of the
organisms that shows an increasing thermal resistance
H
T
M
going from M to H proteins (see Fig. 1). This result
reinforcs the initial idea that water has a fundamental
role in the protein thermal stability [2].
Figure 2: View of the (oil core)-water interface, the polymeric
chains are hidden [3].
The second system we analyzed, in collaboration with
Marco Maccarini, concerned solutions of nonionic surfactant
belonging to the family C 12 E j , constituted by a
tail of 12 hydrocarbon and j polyethylene units (E). This
surfactants presents a very complex phase diagram generally
associated to the interfacial degree of hydration.
Up to now the main results we have obtained concern
the effective exposure of the hydrophobic micellar core
to the solvent: the distribution of the hydrophilic terminations
is not uniform, thus living extended hydrophobic
portion of surface in contact with water. Therefore
the micellar equilibrium condition is characterized by a
competing contributions between water-micellar core repulsion
and the interfacial polymer-polymer attraction,
an aspect not taken into account previously [3].
Recently Marco Maccarini and me start to work
on gold nanoparticles (NP) activated with chemically
bounded polymer chains of 45 E units. Preliminary
results have shown that the NP is characterized by
three shells: the first containing only the gold core, the
second is polymer shell practically unhydrated, and the
external shell with about 50 % by weight of water. Md
simulation are now on going.
References
1. G. Briganti, et al., Langmuir 23, 1518 (2007).
2. F. Sterpone, J. Phys. Chem. B 113,131 (2009).
3. F. Sterpone, Lagmuir 25, 8960 (2009).
4. F. Sterpone, et al., Langmuir 24, 6067 (2008).
Authors
G. Briganti, S. Melchionna, F. Sterpone, M. Maccarini
Sapienza Università di Roma 72 Dipartimento di Fisica