Arresting an Unusual Amide Tautomer Using Divalent Cations

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.

Hydrogen Bond Exchange and Ca2+ Binding of Aqueous N-Methylacetamide Revealed by 2DIR Spectroscopy

“…In addition, specific binding of particular ions, such as Ca 2+ or Mg 2+ , is crucial for function in a wide array of proteins, making a detailed understanding of the mechanism of stability of such interactions of critical importance. − While many studies have explored the macroscopic and thermodynamic effects of various ions on simple model peptides, ,− few methods can directly measure both structure and dynamics. In recent years, evidence from vibrational spectroscopy has led a number of investigators to conclude that some cations can directly bind to the carbonyl oxygen in the backbone of peptides. ,, The primary evidence has been the manner by which divalent cations can split the amide-I transition (which primarily reports on the backbone carbonyl stretch) of peptides, resulting in two distinct peaks at high concentrations of CaCl 2 and MgCl 2 , one near 1620 cm –1 and the other near 1645 cm –1 . Previous experimental and simulation studies on model peptide groups have assigned these two peaks to hydrated carbonyl (appearing at a similar frequency to the aqueous transition) and cation-bound carbonyl, respectively. , This blue shift runs counter to the expected shift due to the vibrational Stark effect. − This has been explained as the combined effect of a large blue shift due to complete dehydration of the carbonyl and a smaller red shift due to the proximity the cation .…”

Comment on “Arresting an Unusual Amide Tautomer Using Divalent Cations”

“…This is significant, because acetone cannot tautomerize. Curiously, Bagchi and co-workers provide spectra for acetone in the presence of CaCl 2 in Figure S2 of their paper . The authors claim that these spectra serve as controls to demonstrate that the blue-shifted shoulder is specific to amide-containing molecules.…”

“…B 2019, 123 (40), 8419−8424, by Bagchi and co-workers. 1 In their paper, the authors assign the appearance of a blue-shifted shoulder on the amide I vibration of N-methylacetamide (NMA), N,Ndimethylacetamide (DMA), and urea to the formation of a tautomer-like structure in the presence of divalent metal cations. In other words, the shoulder should originate from a CN + resonance that is stabilized by the presence of divalent metal cations.…”

“…Curiously, Bagchi and co-workers provide spectra for acetone in the presence of CaCl 2 in Figure S2 of their paper. 1 The authors claim that these spectra serve as controls to demonstrate that the blue-shifted shoulder is specific to amide-containing molecules. Nevertheless, it is straightforward to demonstrate that such shoulders can also occur with simple ketones such as acetone.…”

Reply to “Comment on ‘Arresting an Unusual Amide Tautomer Using Divalent Cations’”

“…I n our publication, we assigned the blue-shifted shoulder of Nmethylacetamide (NMA) in the infrared (IR) spectrum (∼1645 cm −1 ) to an iminolate tautomer stabilized in the presence of divalent metal cations (e.g., 5 M Ca 2+ ). 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 15 N-isotopelabeled NMA does not show any considerable red-shift compared to that of NMA; (2) a new peak appears for NMA at 1680 cm −1 in the presence of 1 M DCl, which shows a 17 cm −1 red-shift for 15 N-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 CaCl 2 .…”