  The structural diversity of the solute molecules involved in biomolecular
processes necessitates the characterization of the forces between charged
macromolecules beyond the point-ion description. From the field theoretic
partition function of an electrolyte confined between two anionic membranes, we
derive a contact-value identity valid for general intramolecular solute
structure and electrostatic coupling strength. In the electrostatic mean-field
(MF) regime, the inner charge spread of the solute particles is shown to induce
the twofold enhancement of the short-range Poisson-Boltzmann (PB)-level
membrane repulsion, and a longer-range depletion attraction. Our contact
theorem indicates that the twofold repulsion enhancement by solute size is
equally present in the opposite strong-coupling (SC) regime of linear and
spherical solute molecules. Upon the inclusion of the dielectric contrast
between the electrolyte and the interacting membranes, the emerging
polarization forces substantially amplify the solute specificity of the
macromolecular interactions. Namely, the finite size of the solute particles
composed of similar terminal charges such as putrescine molecules weaken the
intermembrane repulsion. However, the extended structure of the solute
molecules carrying opposite elementary charges such as ionized atoms and
zwitterionic molecules enhance the membrane repulsion by several factors. We
also show that these polarization forces can extend the range of the solute
structure effects up to intermembrane distances exceeding the solute size by an
order of magnitude. This radical alteration of the intermembrane interactions
by the salt structure identifies the solute specificity as a key ingredient of
the thermodynamic stability in colloidal systems.
