Since GDGT lipids occur in thermoacidophilic archaea, we investigated the effect of acidic pH on the mesophase behavior of these lipids. The phosphate containing lipids P2 and P1Glc could not be dispersed in 50 mM Tris (pH 2.8) likely because of partial protonation of the phosphate groups. Interestingly, both the phosphoglycerol containing lipids PG2 and PG1Glc were found to form hexagonal phases over the temperature range (30-90 °C) surveyed (Figure ). For PG1Glc, the HII peaks were sharp and three HII peaks (ratios of 1:√3:√4) were detected at all temperatures. Similar to Mg 2+ -induced hexagonal phase formation, we reason that due to protonation of the PG groups (pKa ~ 3) they too become dehydrated and therefore decreases in volume. Notably, we did not observe hexagonal phase formation in PG2 and PG1Glc at pH 5.1, where the PG headgroups are expected to be fully deprotonated.