@article { ISI:000366339700024, title = {Cosolvent effects on solute-solvent hydrogen-bond dynamics: ultrafast 2D IR investigations}, journal = {Journal of Physical Chemistry B}, volume = {119}, number = {49}, year = {2015}, month = {DEC}, pages = {15334-15343}, publisher = {AMER CHEMICAL SOC}, type = {Article}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

Cosolvents strongly influence the solute-solvent interactions of biomolecules in aqueous environments and have profound effects on the stability and activity of several proteins and enzymes. Experimental studies have previously reported on the hydrogen-bond dynamics of water molecules in the presence of a cosolvent, but understanding the effects from a solute{\textquoteright}s perspective could provide greater insight into protein stability. Because carbonyl groups are abundant in biomolecules, the current study used 2D IR spectroscopy and molecular dynamics simulations to compare the hydrogen-bond dynamics of the solute{\textquoteright}s carbonyl group in aqueous solution, with and without the presence of DMSO as a cosolvent. 2D IR spectroscopy was used to quantitatively estimate the time scales of the hydrogen-bond dynamics of the carbonyl group in neat water and 1:1 DMSO/water solution. The 2D IR results show spectral signatures of a chemical exchange process: The presence of the cosolvent was found to lower the hydrogen-bond exchange rate by a factor of 5. The measured exchange rates were 7.50 X 10(11) and 1.48 X 10(11) s(-1) in neat water and 1:1 DMSO/water, respectively. Molecular dynamics simulations predict a significantly shorter carbonyl hydrogen-bond lifetime in neat water than in 1:1 DMSO/water and provide molecular insights into the exchange mechanism. The binding of the cosolvent to the solute was found to be accompanied by the release of hydrogen-bonded water molecules to the bulk. The widely different hydrogen-bond lifetimes and exchange rates with and without DMSO indicate a significant change in the ultrafast hydrogen-bond dynamics in the presence of a cosolvent, which, in turn, might play an important role in the stability and activity of biomolecules.

}, issn = {1520-6106}, doi = {10.1021/acs.jpcb.5b08643}, author = {Kashid, Somnath M. and Jin, Geun Young and Bagchi, Sayan and Kim, Yung Sam} } @article {43736, title = {Two-dimensional infrared spectroscopy reveals cosolvent-composition-dependent crossover in intermolecular hydrogen bond dynamics}, journal = {Journal of Physical Chemistry Letters}, volume = {8}, year = {2017}, month = {APR}, pages = {1604-1609}, type = {Article}, abstract = {Cosolvents have versatile composition-dependent applications in chemistry and biology. The simultaneous presence of hydrophobic and hydrophilic groups in dimethyl sulfoxide (DMSO), an industrially important amphiphilic cosolvent, when combined with the unique properties of water, plays key roles in the diverse fields of pharmacology, cryoprotection, and cell biology. Moreover, molecules dissolved in aqueous DMSO exhibit an anomalous concentration-dependent nonmonotonic behavior in stability and activity near a critical DMSO mole fraction of 0.15. An experimental identification of the origin of this anomaly can lead to newer chemical and biological applications. We report a direct spectroscopic observation of the anomalous behavior using ultrafast twodimensional infrared spectroscopy experiments. Our results demonstrate the cosolventconcentration-dependent nonmonotonicity arises from nonidentical mechanisms in ultrafast hydrogen-bond-exchange dynamics of water above and below the critical cosolvent concentration. Comparison of experimental and theoretical results provides a molecular-level mechanistic understanding: a distinct difference in the stabilization of the solute through dynamic solute solvent interactions is the key to the anomalous behavior.}, doi = {10.1021/acs.jpclett.7b00270}, author = {Kashid, Somnath M. and Jin, Geun Young and Chakrabarty, Suman and Kim, Yung Sam and Bagchi, Sayan} } @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} }