Low resolution X-ray diffraction study of dipalmitoyl phosphatidyl choline aqueous dispersions (with application to the case of tryptophan containing Lβ, phase)

In conclusion to a recent paper describing the structural and thermodynamic modifications induced by the amino acid tryptophan on the lamellar phases of dipalmitoyl phosphatidyl choline in excess water (pH 7.4), we report a low resolution X-ray diffraction comparative study of the lamellar Lβ′, phase observed on the DPPC/water system with and without the amino acid. The “swelling” method is used together with a recently proposed pattern recognition approach, resulting in a rather original procedure to solve the crystallographic “phase problem” and to determine the electron density distribution of lipid bilayers also in excess water condition, i.e. when the amount of interlayer water is unknown. From the electron density maps the structural parameters could then be measured directly. In the case of the pure DPPC/water system, they are in good agreement with data reported in the literature and, for the ternary system, the previously proposed amino acid solubilization sites are confirmed. Moreover, the electron density maps clearly indicate a progressive penetration of the tryptophan into the hydrophobic core of the bilayers as its concentration increases.