Structure of ceramide-1-phosphate at the air-water solution interface in the absence and presence of Ca²⁺

Ceramide-1-phosphate, the phosphorylated form of ceramide, gained attention recently due to its diverse intracellular roles, in particular in inflammation mediated by cPLA₂α. However, surprisingly little is known about the physical chemical properties of this lipid and its potential impact on physiological function. For example, the presence of Ca²⁺ is indispensable for the interaction of Cer-1-P with the C2 domain of cPLA ₂α. We report on the structure and morphology of Cer-1-P in monomolecular layers at the air/water solution interface in the absence and presence of Ca²⁺ using diverse biophysical techniques, including synchrotron x-ray reflectivity and grazing angle diffraction, to gain insight into the role and function of Cer-1-P in biomembranes. We show that relatively small changes in pH and the presence of monovalent cations dramatically affect the behavior of Cer-1-P. On pure water Cer-1-P forms a solid monolayer despite the negative charge of the phosphomonoester headgroup. In contrast, pH 7.2 buffer yields a considerably less solid-like monolayer, indicating that charge-charge repulsion becomes important at higher pH. Calcium was found to bind strongly to the headgroup of Cer-1-P even in the presence of a 100-fold larger Na⁺ concentration. Analysis of the x-ray reflectivity data allowed us to estimate how much Ca²⁺ is bound to the headgroup, ∼0.5 Ca²⁺ and ∼1.0 Ca²⁺ ions per Cer-1-P molecule for the water and buffer subphase respectively. These results can be qualitatively understood based on the molecular structure of Cer-1-P and the electrostatic/hydrogen-bond interactions of its phosphomonoester headgroup. Biological implications of our results are also discussed.