Which of the following phospholipids has a charge of -1 at pH 7?

Phosphatidylserine is unique among the listed phospholipids because it contains a serine residue as its head group, which has a carboxyl group that can donate a proton. At physiological pH, which is around 7, this carboxyl group is deprotonated, resulting in a net negative charge. This characteristic is significant for various biological functions, such as in cell signaling and apoptosis.

In contrast, the other phospholipids listed have neutral or positive charges at this pH. Phosphatidylcholine and phosphatidylethanolamine both have head groups that do not ionize to give a negative charge at physiological pH. Cardiolipin, while having two negative charges, is involved in mitochondrial membranes and is not -1 overall like phosphatidylserine under the same conditions. Therefore, phosphatidylserine stands out as having a net charge of -1 at pH 7.