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Reentrant Phase Behavior in Protein Solutions Induced by Multivalent Salts: Strong Effect of Anions Cl– Versus NO3–
journal contribution
posted on 2018-11-21, 00:00 authored by Michal
K. Braun, Andrea Sauter, Olga Matsarskaia, Marcell Wolf, Felix Roosen-Runge, Michael Sztucki, Roland Roth, Fajun Zhang, Frank SchreiberIn this work, the
effects of the two anions Cl– and NO3– on the phase behavior of bovine
serum albumin (BSA) in solution with trivalent salts are compared
systematically. In the presence of trivalent metal salts, negatively
charged proteins such as BSA in solution undergo a reentrant condensation
(RC) phase behavior, which has been established for several proteins
with chlorides of trivalent salts. Here, we show that replacing Cl– by NO3– leads to a marked
change in the phase behavior. The effect is investigated for the two
different cations Y3+ and La3+. The salts are
thus YCl3, Y(NO3)3, LaCl3, and La(NO3)3. The experimental phase behavior
shows that while the chloride salts induce both liquid–liquid
phase separation (LLPS) and RC, the nitrate salts also induce LLPS,
but RC becomes partial with La(NO3)3 and disappears
with Y(NO3)3. The observed phase behavior is
rationalized by effective protein–protein interactions which
are characterized using small-angle X-ray scattering. The results
based on the reduced second virial coefficients B2/B2HS and 1/I(q → 0) demonstrate that the NO3– salts induce
a stronger attraction than the Cl– salts. Overall,
the effective attraction, the width of the condensed regime in the
RC phase diagram, and the nature of LLPS follow the order LaCl3 < YCl3 < La(NO3)3 <
Y(NO3)3. Despite the decisive role of cations
in RC phase behavior, isothermal titration calorimetry measurements
indicate that replacing anions does not significantly influence the
cation binding to proteins. The experimental results observed are
discussed based on an “enhanced Hofmeister effect” including
electrostatic and hydrophobic interactions between protein–cation
complexes.