What makes the guanidinium cation so special?
Biophysics
Final Report Abstract
The guanidinium cation is a prominent “outlier” in the Hofmeister series. In contrast to atomic cations that tend to destabilize the structure of biomolecules in solution, the molecular guanidinium cation has a rather low surface charge density. Nevertheless, aqueous solutions of guanidinium salts are amongst the most efficient protein denaturants and are frequently used in biotechnology to reversibly unfold and fold proteins. The aim of this project was to unravel the underlying molecular mechanisms, which make the guanidinium cation such effective in destabilizing protein structure. To explore the relevant binding sites for guanidinium- protein interaction, we have studied the rotational mobility of model molecules or water in solution. Using time-resolved infrared spectroscopy, we determined the rotational dynamics of water molecules in the hydration shell of solute molecules and tested the effect of different salts on water in the hydration shells. With dielectric relaxation spectroscopy we studied the rotational mobility of dipolar molecules, which mimic protein fragments. The effect of different salts on the thus detected rotational mobility was used as a measure for the interaction of the salts with the model molecules. Using this methodology, we could find no evidence for guanidinium altering water molecules in the hydration shell of hydrophobic molecular fragments. Hence, our results render guanidinium induced protein unfolding via destabilization of hydrophobic interactions of proteins unlikely. Studies on the rotational mobility of molecules that mimic the proteins’ amide backbone showed that the guanidinium cation indeed binds to amide groups, however the binding strength cannot fully explain the extraordinarily high protein denaturation of guanidinium salts: Amongst the studied salts, guanidinium exhibited only intermediate binding to the amide. Remarkably, guanidinium was the only cation for which we found that the interaction of the anion and the cation with amide is non-additive. This finding may indicate that the guanidinium cation competes with anions for the amide binding sites at the amide group, as opposed to conventional cations. This hypotheses was further supported by our results on the interaction between salts and the amino-acid arginine, where we found guanidinium to be similar to denaturing anions, rather than denaturing cations. Finally, our studies on the interaction of different salts with an oligopeptide show that the salt-peptide interaction is further enhanced for guanidinium, which suggests that binding of guanidinium to the charged protein termini contributes to protein destabilization. In summary, our results suggest that binding to multiple protein sites is responsible for the extraordinarily high ability of guanidinium to destabilize proteins, rather than interaction with a single binding site. Remarkably, comparison to conventional salts led to the notion that guanidinium-protein interaction is more similar to anion-protein interaction as compared to the interaction of conventional cations with proteins.
Publications
- (2019) Specific Ion Effects on an Oligopeptide: Bidentate Binding Matters for the Guanidinium Cation. Angewandte Chemie (International ed. in English) 58 (1) 332–337
Balos, Vasileios; Marekha, Bogdan; Malm, Christian; Wagner, Manfred; Nagata, Yuki; Bonn, Mischa; Hunger, Johannes
(See online at https://doi.org/10.1002/anie.201811029) - Quantifying Transient Interactions between Amide Groups and the Guanidinium Cation, Phys. Chem. Chem. Phys. 17, 28539 (2015) & Erratum: Quantifying Transient Interactions between Amide Groups and the Guanidinium Cation, Phys. Chem. Chem. Phys. 18, 1346 (2016)
V. Balos, M. Bonn, J. Hunger
(See online at https://doi.org/10.1039/c5cp04619j) - Dissecting Hofmeister effects: Direct anion-amide interaction is weaker than cation-amide binding Angew. Chem. Int. Ed. Engl. 55, 8125 (2016)
V. Balos, H. Kim, M. Bonn, J. Hunger
(See online at https://doi.org/10.1002/anie.201602769) - Anionic and Cationic Hofmeister Effects Are Non-Additive for Guanidinium Salts Phys. Chem. Chem. Phys., 19, 9724 (2017)
V. Balos, M. Bonn, J. Hunger
(See online at https://doi.org/10.1039/c7cp00790f) - Specific ion effects on protein fragments, PhD thesis, University of Amsterdam, 2017 (ISBN 978-3-95638-852-1)
V. Balos
