  Local changes in pH are known to significantly alter the state and activity
of proteins and in particular enzymes. pH variations induced by pulses
propagating along soft interfaces (e.g. the lipid bilayer) would therefore
constitute an important pillar towards a new physical mechanism of biochemical
regulation and biological signaling. Here we investigate the pH-induced
physical perturbation of a lipid interface and the physiochemical nature of the
subsequent acoustic propagation. Pulses are stimulated by local acidification
of a lipid monolayer and propagate, in analogy to sound, at velocities
controlled by the two-dimensional compressibility of the interface. With
transient local pH changes of 0.6 units directly observed at the interface and
velocities up to 1.4 m/s this represents hitherto the fastest protonic
communication observed. Furthermore simultaneously propagating mechanical and
electrical changes in the lipid interface up to 8 mN/m and 100 mV are detected,
exposing the thermodynamic nature of these pulses. Finally, these pulses are
excitable only beyond a threshold for protonation, determined by the pKa of the
lipid head groups. When combined with an enzymatic pH-optimum, the proposed
communication can be very specific, thus providing a new physical basis for
intra- and intercellular signaling via two-dimensional sound waves at
interfaces.
