Propagation of 2D Pressure Pulses in Lipid Monolayers and Its Possible Implications for Biology

The existence and propagation of acoustic pressure pulses on lipid monolayers at the air-water interface are directly observed by simple mechanical detection. The pulses are excited by small amounts of solvents added to the monolayer. Controlling the state of the lipid interface, we show that the pulses propagate at velocities $c$ following the lateral compressibility $\kappa$. This is manifested by a pronounced minimum in $c$ ($\sim 0.3\mathrm{m}/\mathrm{s}$) within the transition regime. The role of interface density pulses in biology is discussed, in particular, in the context of communicating localized alterations in protein function (signaling) and nerve pulse propagation.

Figure 1

(a) Film balance used for monolayer-pulse analysis. The trough is equipped with two pressure sensors and an additional barrier, which separates the excitation site from the detection compartment. (b) Time course of the sensor readouts for a pulse traveling from sensor 2 towards sensor 1 on a DPPC monolayer ($30\mathrm{mN}/\mathrm{m}$, $24°\mathrm{C}$). The time $t=0$ was arbitrarily chosen to be 1 s before the first pressure rise in the response of sensor 2. Before the pulse signal arrived at sensor 2, it had to propagate through the small gap between the additional barrier and the trough walls. To assure that the pressure pulse indeed travels the anticipated path, the additional barrier has been moved to the left side of the trough, opening the small gap near the excitation point (right). In fact, the order in which the pressure sensors responded was exactly reversed.

Figure 2

Response of pressure sensor 1 for the excitation with four different solvents in the liquid expanded phase of a DPPC monolayer ($T=24°\mathrm{C}$). Again, the time $t=0$ was arbitrary chosen to be 1 s before the first pressure rise in the responses of sensor 2. Obviously, the measured pulse shapes depend on the solvent, whereas oscillations may be attributed to reflections of the wave within the propagation compartment (e.g., for ethanol). Pentane and chloroform, for example, remain longer in the monolayer ($\tau \sim 10\mathrm{s}$) before complete evaporation. Correspondingly, the recorded pulse extends over a longer period of time. A fast Fourier transform frequency analysis of the pulses suggests dominant frequencies up to ${\omega }_{\max }\sim 1\mathrm{Hz}$.

Figure 3

Propagation velocities of the pulses, determined by the run time differences between sensors 2 and 1 (distance $d_{21}\sim 14.5\mathrm{cm}$) for different lateral pressures of the DPPC monolayer ($T=24°\mathrm{C}$). Additionally, the model according to Eq. (4) and the isothermal compressibility ${\kappa }_T$ are plotted into the corresponding graphs for an average pulse frequency of $⟨\omega ⟩\sim 1\mathrm{Hz}$. Both curves coincide very well; even the typical $({\kappa }_T{)}^{-1}$ minima in the phase transition regime of the DPPC monolayer ($24°\mathrm{C}$) are well-reproduced.

Figure 4

Measured pulse heights as a function of lateral pressure in the lipid film (DPPC; $T=24°\mathrm{C}$). Similar to the propagation velocity, a distinct minimum evolves at the compressibility maximum, indicating the phase transition regime. Hence, both pulse velocities and heights depend on the phase state of the DPPC monolayer.