ATR-FTIR spectroscopy was performed on a series of ZnCl2-ethylene glycol (EG) mixtures with wide-range compositions (1:1.5-1:14 in molar ratios), involving the stable ZnCl2-4EG deep-eutectic solvent (DES) composition, to explore the...
Cooperative behaviors of the hydrogen bonding networks in proteins have been discovered for a long time. The structural origin of this cooperativity, however, is still under debate. Here we report a new investigation combining excess infrared spectroscopy and density functional theory calculation on peptide analogs, represented by N-methylformamide (NMF) and N-methylacetamide (NMA). Interestingly, addition of the strong hydrogen bond acceptor, dimethyl sulfoxide, to the pure analogs caused opposite effects, namely red- and blue-shift of the N−H stretching infrared absorption in NMF and NMA, respectively. The contradiction can be reconciled by the marked lowering of the energy levels of the self-associates between NMA molecules due to a cooperative effect of the hydrogen bonds. On the contrary, NMF molecules cannot form long-chain cooperative hydrogen bonds because they tend to form dimers. Even more interestingly, we found excellent linear relationships between changes on bond orders of N−H/N−C/C = O and the hydrogen bond energy gains upon the formation of hydrogen bonding multimers in NMA, suggesting strongly that the cooperativity originates from resonance-assisted hydrogen bonds. Our findings provide insights on the structures of proteins and may also shed lights on the rational design of novel molecular recognition systems.
Deep-eutectic
solvents (DESs) are a new class of green solvents.
Here, we report the hydrogen
bonding and structural properties of the archetypal DES ethaline,
a mixture of choline chloride (ChCl) and ethylene glycol (EG) of a
1:2 molar ratio, and its pseudo-binary mixtures with acetonitrile.
The investigations were carried out employing Fourier-transform infrared
(FTIR) spectroscopy combined with quantum chemical calculations. Excess
and two-dimensional (2D)-correlation spectroscopies were used to identify
favorable species in the solutions and to explore the heterogeneity.
The results show that the mixing process is the transformation from
ethaline and CH3CN dimer to the complexes of ethaline–1CH3CN and ethaline–2CH3CN, together with the
increased percentages of the EG dimer, EG trimer, and CH3CN monomer with respect to their total amounts in the mixtures. Theoretical
calculations show that, for ChCl, the positive charge is located at
the methyl groups and methylenes, rendering their ability to form
hydrogen bonds. Adding CH3CN to ethaline can hardly break
apart the doubly ionic hydrogen bonds between Ch+ and Cl–. The cosolvent molecules mainly surround the core
structure of ethaline, forming noncovalent hydrogen bonds with hydroxyl
groups of EG/Ch+ but not Cl–. These in-depth
studies on the properties of ethaline and CH3CN/CD3CN mixed solvents may shed light on exploring their applications.
Deep-eutectic solvents (DESs) are regarded as alternative green solvents to ionic liquids. In this work we report the structural properties and hydrogen bonding (H-bonding) interactions of an aqueous DES system. The used DES, ethaline (ETH), is composed of choline chloride and ethylene glycol (EG) in 1 : 2 molar ratio. The investigations were carried out by FTIR spectroscopy combined with quantum chemical calculations. Excess spectroscopy and two-dimensional correlation spectroscopy (2D-COS) were used to explore the data in detail. The results showed that, upon mixing, ETH transforms to EG dimers and trimers and D 2 O clusters transform to various ETH-D 2 O complexes. Theoretical calculations show that water molecules insert between the anion and cation of ETH, break the strong doubly ionic Cl À … HÀ O Ch + H-bond, share charges of the ions and form H-bond with them, thus modulate the interaction properties of ETH. This study deepens our molecular-level understanding of the system and would shed light on its applications.
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