Cryo-TEM structural analysis of conjugated nonionic engineered-micelles

Conjugated engineered-micelles, i.e. micelles that are composed of nonionic detergents and hydrophobic chelators and subsequently conjugated in the presence of divalent metal ions, have been shown to be remarkably suited to the task of membrane protein purification, maintaining these proteins in their native state. They also efficiently solubilize highly hydrophobic antibiotics. To date, however, the morphological changes induced in the initially spherical or ellipsoidal micelles by conjugation have not been explored. In this study, the very rapid sample-vitrification protocol of cryogenic transmission electron microscopy (cryo-TEM) has been used to capture structural transformations that engineered-micelles undergo immediately following conjugation with the [(bathophenanthroline)₃:Fe²⁺] hydrophobic complex. We found that condensed thread-like aggregates are formed when the detergents used are: octyl β-d-glucopyranoside (OG), octyl β-d-thioglucopyranoside (OTG) or pentaethylene glycol monododecyl ether (C₁₂E₅). However, with β-d-maltoside (DM), n-dodecyl β-d-maltoside (DDM) or β-d-glucopyranoside (DDG), lamellar structures, some of which appear as stacked lamellae or multilamellar vesicles (MLV's), were observed. Such architectural changes occur under very mild conditions i.e. low detergent concentration, no temperature or pH alterations and without the presence of any precipitants such as PEG or ammonium sulfate.