534
11 Biomolecular Structure and Interactions
is referred: Dispersion force; Induced-dipole-induced-dipole force; London force;
London-van der Walls force; Quantum fluctuation force; and Charge-fluctuation
force. The term dispersion force, invented by London, is used because the same
frequency-dependent polarizabilities appearing in the force also affect the dispersion
of light in dielectric materials.
Molecular forces may be significantly reduced by the presence of screening
counter-ions surrounding macromolecules, forming molecular complexes, and leaving only fluctuation forces between the molecules.
For identical or similar large molecules, there will be a large number of electrons
taking part in the attraction of these molecules. A resonance occurs between
each fluctuating charge in one molecular group of atoms with its corresponding
fluctuating charge in the second molecule, since each has the same ‘zero-point
fluctuation frequency’. As a result, there is an enhancement of the attractive force
between the pair of molecules.
The London dispersion effect can account for the accumulation and clumping in
a solution of one kind of macromolecule. 16
The forces between uncharged molecules, permanent multipolar, induced polar,
and dispersion forces, are collectively called the van der Waals forces.
11.7.8 Depletion and Other Shadow Forces
A macromolecule in a medium containing particles moving in all directions will
experience diffusive effects when the media particles collide with it. The impact of
these collisions produces a ‘random walk’ (Brownian motion) of the molecule.
Now consider two molecules near to each other. If the gap between these
molecules is comparable to the size of the medium particles, then each of these
molecules will ‘shade’ the other from collisions with the medium particles. The
same shading will occur if a molecule comes close to a boundary. Because of such
shading, there will be a net force due to collisions with the medium particles from
the unshaded side toward the shaded region. Near a boundary, there will be a slight
increase in the density of molecules, and a slight increase in the pressure toward
the boundary. Nearby macromolecules will experience a net attractive force from
medium particle collisions. These forces are often referred to as ‘depletion forces’.
An additional shading effect will occur if the effective surface temperature of a
16 H. Jehle, W.C. Parke, and A. Salyers, Intermolecular Charge Fluctuation Interactions in
Molecular Biology, Electronic Aspects of Biochemistry (B. Pullman, ed.), 313, [Academic
Press, NY], (1964); H. Jehle, W.C. Parke, and A. Salyers, Charge fluctuation interactions
and molecular biophysics, Biophysics IX, 401–412, [Moscow] (1964), translation to English,
Biophysics 9:4 433–447, [Pergamon Press, Oxford] (1965); H. Jehle, W.C. Parke, M. Shirven,
D. Aein, Biological Significance of Charge Fluctuation Forces (London-van der Waals) between
Sterically Complementary and Sterically Similar Molecules, Biopolymers Symposia 13, 209–233
(1964).
11 Biomolecular Structure and Interactions
is referred: Dispersion force; Induced-dipole-induced-dipole force; London force;
London-van der Walls force; Quantum fluctuation force; and Charge-fluctuation
force. The term dispersion force, invented by London, is used because the same
frequency-dependent polarizabilities appearing in the force also affect the dispersion
of light in dielectric materials.
Molecular forces may be significantly reduced by the presence of screening
counter-ions surrounding macromolecules, forming molecular complexes, and leaving only fluctuation forces between the molecules.
For identical or similar large molecules, there will be a large number of electrons
taking part in the attraction of these molecules. A resonance occurs between
each fluctuating charge in one molecular group of atoms with its corresponding
fluctuating charge in the second molecule, since each has the same ‘zero-point
fluctuation frequency’. As a result, there is an enhancement of the attractive force
between the pair of molecules.
The London dispersion effect can account for the accumulation and clumping in
a solution of one kind of macromolecule. 16
The forces between uncharged molecules, permanent multipolar, induced polar,
and dispersion forces, are collectively called the van der Waals forces.
11.7.8 Depletion and Other Shadow Forces
A macromolecule in a medium containing particles moving in all directions will
experience diffusive effects when the media particles collide with it. The impact of
these collisions produces a ‘random walk’ (Brownian motion) of the molecule.
Now consider two molecules near to each other. If the gap between these
molecules is comparable to the size of the medium particles, then each of these
molecules will ‘shade’ the other from collisions with the medium particles. The
same shading will occur if a molecule comes close to a boundary. Because of such
shading, there will be a net force due to collisions with the medium particles from
the unshaded side toward the shaded region. Near a boundary, there will be a slight
increase in the density of molecules, and a slight increase in the pressure toward
the boundary. Nearby macromolecules will experience a net attractive force from
medium particle collisions. These forces are often referred to as ‘depletion forces’.
An additional shading effect will occur if the effective surface temperature of a
16 H. Jehle, W.C. Parke, and A. Salyers, Intermolecular Charge Fluctuation Interactions in
Molecular Biology, Electronic Aspects of Biochemistry (B. Pullman, ed.), 313, [Academic
Press, NY], (1964); H. Jehle, W.C. Parke, and A. Salyers, Charge fluctuation interactions
and molecular biophysics, Biophysics IX, 401–412, [Moscow] (1964), translation to English,
Biophysics 9:4 433–447, [Pergamon Press, Oxford] (1965); H. Jehle, W.C. Parke, M. Shirven,
D. Aein, Biological Significance of Charge Fluctuation Forces (London-van der Waals) between
Sterically Complementary and Sterically Similar Molecules, Biopolymers Symposia 13, 209–233
(1964).