The influence of poly(ethylene glycol) on the micelle formation of alkyl maltosides used in membrane protein crystallization

Abstract

With the aim of better understanding the phase behavior of alkyl maltosides (n-alkyl-β-D-maltosides, C_nG₂) under the conditions of membrane protein crystallization, we studied the influence of poly(ethylene glycol) (PEG) 2000, a commonly used precipitating agent, on the critical micelle concentration (CMC) of the alkyl maltosides by systematic variation of the number n of carbon atoms in the alkyl chain (n = 10, 11, and 12) and the concentration of PEG2000 (χ) in a buffer suitable for the crystallization of cyanobacterial photosystem II. CMC measurements were based on established fluorescence techniques using pyrene and 8-anilinonaphthalene-1-sulfonate (ANS). We found an increase of the CMC with increasing PEG concentration according to ln(CMC/CMC₀) = k_Pχ, where CMC₀ is the CMC in the absence of PEG and k_P is a constant that we termed the “polymer constant”. In parallel, we measured the influence of PEG2000 on the surface tension of detergent-free buffer solutions. At PEG concentrations χ > 1% w/v, the surface pressure π_s(χ) = γ(0) − γ(χ) was found to depend linearly on the PEG concentration according to π_s(χ) = κχ + π_s(0), where γ(0) is the surface tension in the absence of PEG. Based on a molecular thermodynamic modeling, CMC shifts and surface pressure due to PEG are related, and it is shown that k_P = κc(n) + η, where c(n) is a detergent-specific constant depending inter alia on the alkyl chain length n and η is a correction for molarity. Thus, knowledge of the surface pressure in the absence of a detergent allows for the prediction of the CMC shift. The PEG effect on the CMC is discussed concerning its molecular origin and its implications for membrane protein solubilization and crystallization.