His₁-tagged DM or DDM detergent micelles are reversibly conjugated by nickel ions

Specific conjugation of decyl β-D-maltoside (DM) or dodecyl β-D-maltoside (DDM) detergent micelles is accomplished between pH 7.0–8.5 in the presence of an amphiphilic analog of the amino acid histidine, bound to a 10-carbon hydrocarbon chain (His₁-C10) and Ni²⁺ ions. Following addition of 10–15 wt% PEG-6000 as precipitant, phase separation in the form of oil-rich globules (30–600 µm) is observed by light microscopy. Other divalent cations: Zn²⁺, Fe²⁺, Cu²⁺ lead to dark precipitates rather than colorless globules; while Mg²⁺, Ca²⁺ do not promote any phase separation at all. Even in the absence of precipitant, dynamic light scattering (DLS) measurements demonstrate that DM micelles (hydrodynamic size ~ 6 nm) or DDM micelles (8 nm) self-associate into larger particles (9 nm and 411 nm for DM; 10 nm and 982 nm for DDM) in the presence of His₁-C10 and nickel ions. Micellar conjugation is partially reversible in the presence of water soluble 50 mM EDTA, histidine or imidazole chelators. Cryo-transmission electron microscopy (cryo-TEM) imaging revealed the formation of non-uniformly dense detergent aggregates for both DM and DDM micelles in the presence of precipitant. The possible utility of such His₁-tagged DM or DDM micelles for promoting crystallization of integral membrane proteins is discussed.