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Conformation and Metal Cation Binding of Zwitterionic Alanine Tripeptide in Saline Solutions by Infrared Vibrational Spectroscopy and Molecular Dynamics Simulations.

Juan ZhaoTiantian DongPengyun YuJianping Wang
Published in: The journal of physical chemistry. B (2021)
In this work, linear infrared (IR) spectroscopy and molecular dynamics (MD) simulations were used to examine the interaction of different metal cations (Na + , Ca 2+ , Mg 2+ , and Zn 2+ ) with backbone (amide C═O) and C-terminal carboxylate (COO - ) groups in zwitterionic alanine tripeptide (Ala3) in aqueous solutions with varying saline concentrations. Circular dichroism spectra and MD results suggest that Ala3 is predominantly in polyproline-II (PP II ) conformation, whose amide-I and asymmetric carboxylate stretching IR vibration signatures are also supported by quantum-chemistry calculations. The zwitterionic form of Ala3 separates the two amide-I modes in frequency, which are weakly coupled modes, as revealed by two-dimensional IR measurement, and can be used to probe backbone-cation interactions at different scenarios (near charged or neutral chemical groups respectively). Cation concentration-dependent IR frequency red shifts in the amide-I mode are seen for both amide-I modes, whereas blue shifts are also seen in the amide-I mode far from the NH 3 + group. The observed spectral changes are discussed from the perspective of the salting-in and salting-out abilities of the cations. In addition, all the metal cations studied here (except Zn 2+ ) can specifically coordinate to the COO - group in bidentate and pseudo-bridging forms simultaneously. For Zn 2+ , only the pseudo-bridging form exists. Our results shed light on the macroscopic protein salting-in and salting-out phenomena from the perspective of key chemical bonds in peptides.
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