pH-Induced Changes in the Surface Viscosity of Unsaturated Phospholipids Monitored Using Active Interfacial Microrheology

Abstract

Lipid membranes, a major component of cells, are subjected to significant changes in pH depending on their location in the cell: the outer leaflet of the cell membrane is exposed to a pH of 7.4 whereas lipid membranes that make up late endosomes and lysosomes are exposed to a pH of as low as 4.4. The purpose of this study is to evaluate how changes in the environmental pH within cells alter the fluidity of phospholipid membranes. Specifically, we studied pH-induced alterations in the surface arrangement of monounsaturated lipids with zwitterionic headgroups (phosphoethanolamine (PE) and phosphocholine (PC)) that are abundant in plasma membranes as well as anionic lipids (phosphatidylserine (PS) and phosphatidylglycerol (PG)) that are abundant in inner membranes using a combination of techniques including surface tension vs area measurements, interfacial microrheology, and fluorescence/atomic force microscopy. Using an active interfacial microrheology technique, we find that phospholipids with zwitterionic headgroups show a significant increase in their surface viscosity at acidic pH. This increase in surface viscosity is also found to depend on the size of the lipid headgroup, with a smaller headgroup showing a greater increase in viscosity. The observed pH-induced increase in viscosity is also accompanied by an increase in the cohesion pressure between zwitterionic molecules at acidic pH and a decrease in the average molecular area of the lipids, as measured by fitting the surface pressure isotherms to well-established equations of state. Because fluorescent images show no change in the phase of the lipids, we attribute this change in surface viscosity to the pH-induced reorientation of the P-N dipoles that form part of the polar lipid headgroup, resulting in increased lipid-lipid interactions. Anionic PG headgroups do not demonstrate this pH-induced change in viscosity, suggesting that the presence of a net negative charge on the headgroup causes electrostatic repulsion between the headgroups. Our results also show that active interfacial microrheology is a sensitive technique for detecting minute changes in the lipid headgroup orientation induced by changes in the local membrane environment, even in unsaturated phospholipids where the surface viscosity is close to the experimental detection limit.

Partition of Glutamic Acid-Based Single-Chain and Gemini Amphiphiles into Phospholipid Membranes

“…The addition of H + may move the N + end toward the water phase and also induce the P – –N + dipoles of the phospholipid headgroup to stack more closely and change the orientation of the phosphate segment. Therefore, the size of pure DOPC vesicles becomes larger at acidic pH . By the partitions of C 12 Glu and C 12 (Glu) 2 C 12 into the DOPC vesicles, the average diameters of the vesicles decrease to 124 and 135 nm (Figure c), respectively.…”

“…This understanding was suggested by Scherer and Seelig in the study about the interaction of phosphatidylcholine with other cationic and anionic amphiphile mixtures. 41,42 For the present system, the addition of amphiphiles has an opposite effect on the P − −N + dipoles of DOPC compared to the effect of the acidic pH, which engenders the reorientation of the phospholipid headgroup and the increase of membrane curvature, finally lessening the vesicles.…”

“…Therefore, the size of pure DOPC vesicles becomes larger at acidic pH. 42 By the partitions of C 12 Glu and C 12 (Glu) 2 C 12 into the DOPC vesicles, the average diameters of the vesicles decrease to 124 and 135 nm (Figure 4c), respectively. This is an interesting phenomenon that the partitions of the two amphiphiles induce a decrease in the DOPC vesicle size rather than an increase, and the DOPC vesicle after the partition of C 12 (Glu) 2 C 12 is larger than that after the partition of C 12 Glu.…”

Impact of Engineered Carbon Nanodiamonds on the Collapse Mechanism of Model Lung Surfactant Monolayers at the Air-Water Interface

“…Another theoretical model published by the same group describes the liquid expanded region of the Π-A isotherms of different amphiphilic molecules [34]. Recently, Ghazvini et al used the theoretical model proposed by Feinerman and Volhardt to understand the impact of pH on the packing of phospholipid membranes [35]. Furthermore, these equations are also capable of predicting the material loss.…”

Cell to Cell Signaling through Light in Artificial Cell Communities: Glowing Predator Lures Prey

“…27 First, light does not interfere with soluble signaling molecules. 28 Unlike some of the chemical signals such as pH, 29 oxidative species, and metal ions, 30 which alter the activity of many biomolecules, the response to light is rare and highly specific in molecules. Second, visible light passes across optically transparent barriers, such as membranes, and eliminates the need for transporters or pores.…”