@article {45363, title = {Interconverting hydrogen-bonding and weak n -> pi* interactions in aqueous solution: A direct spectroscopic evidence}, journal = {Journal of Physical Chemistry Letters}, volume = {9}, year = {2018}, month = {SEP}, pages = {5425-5429}, type = {Article}, abstract = {

Molecular structure and function depend on myriad noncovalent interactions. However, the weak and transient nature of noncovalent interactions in solution makes them challenging to study. Information on weak interactions is typically derived from theory and indirect structural data. Solvent fluctuations, not revealed by structure analysis, further complicate the study of these interactions. Using 2D infrared spectroscopy, we show that the strong hydrogen bond and the weak n -\> pi* interaction coexist and interconvert in aqueous solution. We found that the kinetics of these interconverting interactions becomes faster with increasing water content. This experimental observation provides a new perspective on the existence of weak noncovalent interactions in aqueous solution.

}, author = {Deb, Pranab and Jin, Geun Young and Singh, Santosh K. and Moon, Juran and Kwon, Hyejin and Das, Aloke and Bagchi, Sayan and Kim, Yung Sam} } @article {46334, title = {Arresting an unusual amide tautomer using divalent cations}, journal = {Journal of Physical Chemistry B}, volume = {123}, year = {2019}, month = {OCT}, pages = {8419-8424}, type = {Article}, abstract = {

Ion-specific effects on peptides and proteins are key to biomolecular structure and stability. The subtle roles of the cations are far less understood, compared to the pronounced effects of the anions on proteins. Most importantly, divalent cations such as Ca2+ and Mg2+ are crucial to several biological functions. Herein, we demonstrate that an amide-iminolate equilibrium is triggered by the binding of the divalent cations to the amide oxygen in aqueous solution. The excellent agreement between the experimental and theoretical results confirms the arrest of an unusual amide tautomer by the divalent cations, which is a rarely known phenomenon that might open up an array of applications in chemistry and biology.

}, issn = {1520-6106}, doi = {10.1021/acs.jpcb.9b08463}, author = {Kashid, Somnath M. and Singh, Reman K. and Kwon, Hyejin and Kim, Yung Sam and Mukherjee, Arnab and Bagchi, Sayan} } @article {47453, title = {Reply to {\textquotedblleft}comment on {\textquoteleft}arresting an unusual amide tautomer using divalent cations{\textquoteright}{\textquotedblright}}, journal = {Journal of Physical Chemistry B}, volume = {125}, year = {2020}, month = {DEC}, pages = {479{\textendash}483}, type = {Article}, abstract = {

In our publication, we assigned the blue-shifted shoulder of\ N-methylacetamide (NMA) in the infrared (IR) spectrum (\~{}1645 cm{\textendash}1) to an iminolate tautomer stabilized in the presence of divalent metal cations (e.g., 5 M Ca2+).(1)\ Cremer and co-workers comment that this shoulder arises from the dehydration of the amide oxygen upon interaction with the metal cations.(2,3)\ Their assignment is based on three interesting experimental observations: (1) the IR spectrum of\ 15N-isotope-labeled NMA does not show any considerable red-shift compared to that of NMA; (2) a new peak appears for NMA at 1680 cm{\textendash}1\ in the presence of 1 M DCl, which shows a 17 cm{\textendash}1\ red-shift for\ 15N-labeled NMA; and (3) the IR spectrum of acetone C=O stretch also demonstrates a blue-shifted shoulder in the presence of a high concentration of CaCl2.

}, issn = {1520-6106}, doi = {10.1021/acs.jpcb.0c06005}, author = {Kashid, Somnath M. and Singh, Reman K. and Kwon, Hyejin and Seol, Jin Gyu and Kim, Yung Sam and Mukherjee, Arnab and Bagchi, Sayan} } @article {48379, title = {Association-dissociation dynamics of ionic electrolytes in low dielectric medium}, journal = {Journal of Physical Chemistry B}, volume = {126}, year = {2022}, month = {JAN}, pages = {239-248}, type = {Article}, abstract = {

Ionic electrolytes are known to form various complexes which exist in dynamic equilibrium in a low dielectric medium. However, structural characterization of these complexes has always posed a great challenge to the scientific community. An additional challenge is the estimation of the dynamic association-dissociation time scales (lifetime of the complexes), which are key to the fundamental understanding of ion transport. In this work, we have used a combination of infrared absorption spectroscopy, two-dimensional infrared spectroscopy, molecular dynamics simulations, and density functional theory calculations to characterize the various ion complexes formed by the thiocyanate-based ionic electrolytes as a function of different cations in a low dielectric medium. Our results demonstrate that thiocyanate is an excellent vibrational reporter of the heterogeneous ion complexes undergoing association-dissociation dynamics. We find that the ionic electrolytes exist as contact ion pairs, dimers, and clusters in a low dielectric medium. The relative ratios of the various ion complexes are sensitive to the cations. In addition to the interactions between the thiocyanate anion and the countercation, the solute-solvent interactions drive the dynamic equilibrium. We have estimated the association-dissociation dynamics time scales from two-dimensional infrared spectroscopy. The exchange time scale involving the cluster is faster than that between a dimer and an ion pair. Moreover, we find that the dynamic equilibrium between the cluster and another ion complex is correlated to the solvent fluctuations.

}, issn = {1520-6106}, doi = {10.1021/acs.jpcb.1c08613}, author = {Ghosh, Deborin and Sakpal, Sushil S. and Chatterjee, Srijan and Deshmukh, Samadhan H. and Kwon, Hyejin and Kim, Yung Sam and Bagchi, Sayan} }