Biomedical Engineering Reference
In-Depth Information
Figure 6.5
Atomic partial charges for phenylanine zwitterion
permittivity of a gas. Polarizabilities can be routinely calculated by quantum mechanical
techniques, but the calculations are compute-intensive and this route is of no help if your aim
in life is the high-throughput screening of very many potential pharmaceutical molecules.
An insight into the order of magnitude of polarizabilities can be obtained by considering
Figure 6.6, which represents a (mythical) atom. The nucleus of charge
Q
is at the coordinate
origin, and the nucleus is surrounded by an electron cloud of radius
a
. The electrons are
assumed to have a uniform density in space. The total electron charge is
−
Q
and so the
atom is electrically neutral.
Q
d
a
E
Figure 6.6
Construct needed to discuss polarizability
When an external uniform electric field is applied, the nucleus is displaced a relative
distance
d
in the direction of the field. At this point, the force on the nucleus
QE
is exactly
balanced by the force exerted on the nucleus by the electron cloud.
