Topological and entropic repulsion in biopolymers

We estimate numerically the entropic repulsion felt by two or more approaching polymers with fixed topology. Our results complement field theory estimates valid for infinitely long chains. For two loops, the steric contributions to repulsion outweigh the topological one. When many loops come together, the entropy loss scales nonlinearly with their number. This could set a thermodynamic size for a 'rosette', a putative organizing motif of interphase chromosomes. (letter)