Crystallographic characterization of N-oxide tripod amphiphiles

Abstract

Tripod amphiphiles are designed to promote the solubilization and stabilization of intrinsic membrane proteins in aqueous solution; facilitation of crystallization is a long-range goal. Membrane proteins are subjects of extensive interest because of their critical biological roles, but proteins of this type can be difficult to study because of their low solubility in water. The nonionic detergents that are typically used to achieve solubility can have the unintended effect of causing protein denaturation. Tripod amphiphiles differ from conventional detergents in that the lipophilic segment contains a branchpoint, and previous work has shown that this unusual amphiphilic architecture can be advantageous relative to traditional detergent structures. Here, we report the crystal structures of several tripod amphiphiles that contain an N-oxide hydrophilic group. The data suggest that tripods can adapt themselves to a nonpolar surface by altering the hydrophobic appendage that projects toward that surface and their overall orientation relative to that surface. Although it is not possible to draw firm conclusions regarding amphiphile association in solution from crystallographic data, trends observed among the packing patterns reported here suggest design strategies to be implemented in future studies.

Figure 1

Chemical structures of widely used detergents for membrane protein studies.

Figure 2

Chemical structures of amphiphiles.

Figure 3

Packing patterns of TDAO (top) and T-N-oxide (bottom) in the crystalline state. The parallelepipeds represent the unit cells. The red lines represent the borders between polar and nonpolar layers.

Figure 4

The lattices observed for tripod amphiphile 1 crystallized from water (top) or from organic solvents (bottom). The parallelepipeds represent the unit cells. In the upper image the red lines represent the borders between polar and nonpolar layers. In the lower image alternating red and green lines are used to represent the borders between polar and nonpolar layers, because in this structure there are two independent molecules of 1 in the lattice. Layer thicknesses given in Table 1 are based on separations between red lines, or separations between green lines.

Figure 5

Crystal structures of tripod amphiphiles 2 (top left), 3 (top right), 4 (middle left), 5 (middle right) and 6 (bottom right). The parallelepipeds represent the unit cells. The red lines represent the borders between polar and nonpolar layers.

Figure 6

Crystal structures of dipod amphiphiles 7 (top) and 8 (bottom). The parallelepipeds represent the unit cells. The red lines represent the borders between polar and nonpolar layers.