  The specificity of molecular recognition is important to molecular
self-organization. A prominent example is the biological cell where, within a
highly crowded molecular environment, a myriad of different molecular receptor
pairs recognize their binding partner with astonishing accuracy. In thermal
equilibrium it is usually admitted that the affinity of recognizer pairs only
depends on the nature of the two binding molecules. Accordingly, Boltzmann
factors of binding energy differences relate the molecular affinities among
different target molecules that compete for the same probe. Here, we consider
the molecular recognition of short DNA oligonucleotide single strands. We show
that a better matching oligonucleotide strand can prevail against a
disproportionally more concentrated competitor that exhibits reduced affinity
due to a mismatch. The magnitude of deviation from the simple picture above may
reach several orders of magnitude. In our experiments the effective molecular
affinity of a given strand remains elevated only as long as the better matching
competitor is not present. We interpret our observations based on an
energy-barrier of entropic origin that occurs if two competing oligonucleotide
strands occupy the same probe simultaneously. In this situation the relative
binding affinities are reduced asymmetrically, which leads to an expression of
the free energy landscape that represents a formal analogue of a Landau
description of phase transitions. Our mean field description reproduces the
observations in quantitative agreement. The advantage of improved molecular
recognition comes at no energetic cost other than the design of the molecular
ensemble, and the introduction of the competitor. It will be interesting to see
if mechanisms along similar lines as exposed here, contribute to the molecular
synergy that occurs in biological systems.
