3. Effects of high pressure on model membranes
In the pioneering study by Bernsdorff et al., time-resolved fluorescence anisotropy measurement on model membranes, dipalmitoylphosphatidylcholine (DPPC) and palmitoyl-oleoyl phosphatidylcholine(POPC), was performed under high pressure using a high-pressure optical
chamber [21]. At temperatures where the membranes remain in the liquid-crystalline phase, the order parameter S for the TMA-DPH-labeled DPPC and POPC membrane shows a sharp increase at around 75 MPa at 58 °C and 95 MPa at 15 °C, respectively. These increases in S correspond
to the liquid-crystalline-to-gel phase transition. Addition of 30 mol% cholesterol to the membrane systems results in an increase in S and concomitant loss of the pressure-induced sharp phase transition in both DPPC and POPC membranes [21]. This behavior is attributable to the
formation of homogeneous liquid ordered phase in the systems. In our recent study, we also created a system that enabled highpressure time-resolved fluorescence anisotropy measurement, namely HP-TRFAM, under high pressure, aimed at understanding the effects of high pressure on the cell membrane in deep-sea piezophiles (referred to hereafter) [22]. The HP-TRFAM system comprises of a highpressure optical cell, a high-pressure pump, and a TCSPC device
(Fig. 1A). The strain birefringence of the quartz windows is simply corrected by determining the r0 values for DPH in mineral oil under various hydrostatic pressures. This is based on the fact that the r0 values do not change by applying pressures at the experimental pressure range
(b200 MPa). The measured r0 of DPH is almost unchanged at pressures up to 50 MPa, but the birefringence of the window becomes considerable at pressures greater than 50 MPa (Fig. 1B). Thus, measured r0 and r∞ values are multiplied by the correction coefficients to determine the
correct r0 and r∞ values at each pressure. Fig. 2 shows a typical example