  This study is concerned with behaviour of fluid of monoclonal antibodies
(mAbs) when trapped in a confinement represented by rigid spherical obstacles
that attract proteins. The antibody molecule is depicted as an assembly of
seven hard spheres, organized to resemble Y shaped molecule. The antibody has
two Fab and one Fc domains located in the corners of letter Y. In this
calculation, only the Fab-Fab and Fab-Fc attractive pair interactions are
effective. The confinement is formed by the randomly distributed hard-spheres
fixed in space. The spherical obstacles, besides the size exclusion, also
interact by the Yukawa attractive interaction with with each bead of the
antibody molecule. We applied the combination of the scaled-particle theory,
Wertheim's thermodynamic perturbation theory and the Flory-Stockmayer theory to
calculate: (i) the liquid-liquid phase separation, and (ii) the percolation
threshold. All these quantities were calculated as functions of the strength of
the attraction between the monoclonal antibodies, and monoclonal antibodies and
obstacles. The conclusion is that while the hard-sphere obstacles decrease the
critical density as also, the critical temperature of the mAbs fluid, the
effect of the protein-obstacle attraction is more complex. Adding the
attraction to obstacle-mAbs interaction first increases the wideness of the
temperature-density envelope. However, with the further increase of the
obstacle-mAbs attraction intensity we observe reversal of the effect, the
temperature-density curves become narrower. At some point, depending on the
AC=BC interaction, the situation is observed where two different temperatures
have the same fluid density (reentry point ). In all the cases shown here the
critical point decreases below the value for the neat fluid, but the behaviour
with respect to increase of the strength of obstacle-mAbs attraction is not
monotonic.
