Interactions of Protein-Denaturing Salts with Model Amides^*

Abstract

The interactions of N-methylacetamide (NMA) and N,N-dimethylacetamide (DMA) with denaturants were studied by viscometry, calorimetry, and crystallography. Lithium and calcium chlorides show strong interactions by all three criteria; these cations probably interact with peptide groups.Alkali thiocyanate, nitrate, and perchlorate salts show T here are two general mechanisms by which salts may denature proteins: (1) interaction with the protein, probably at peptide or other polar groups (possibly including the ir electrons of the aromatic side chains);(2) alteration of the solvent structure. We shall not consider here salts with large apolar chains which can be bound to apolar regions of the protein. Geschwind (1960) reported that aqueous mixtures of urea and lithium bromide showed high viscosities; similar viscosity increases were found by Bello and Bello (1961,1962) for aqueous solutions of lithium chloride or bromide with N-methylacetamide (NMA)' or N,N-dimethylacetamide (DMA). DMA was investigated as an analog of proline and hydroxyproline which contain no NH group when present in peptide linkage. These observations led to the suggestion that lithium halides interact with the amide groups, and that they denature proteins by interaction with peptide or other polar groups. Crystalline complexes of NMA with lithium chloride and bromide were obtained (Bello and Bello, 1962) from an aqueous environment and their structures have been determined by X-ray diffraction (Haas, 1964). In these complexes there are carbonyl-lithium interactions and "-halide hydrogen bonds. In this communication, we report viscometric, calorimetric, and crystallographic data on the interactions of NMA and DMA with a number of denaturing salts.

Complete neglect of differential overlap study of the binding of salts to N-methyl acetamide

“…The cations are initially assumed to approach the carbonyl oxygen of NMA along a path making an angle of 1800 with the C=O bond, and the anions are also assumed to initially approach the amide group hydrogen in a direction making an angle of 1800 with the N-H bond. The binding of the anions at the imide nitrogen and the cations at the carbonyl oxygen are consistent with earlier suggestions (5,7,16) as well as with crystal structure studies (6). The optimum distances of approach of the ions along the above paths are then obtained by varying these distances for each ion-NMA system until the energy minimum is located, assuming that the geometry of NMA remains as in the isolated molecule.…”

Water-Induced Relaxation of a Degenerate Vibration of Guanidinium Using 2D IR Echo Spectroscopy

“…Equations (3) – (5) are readily obtained from the Redfield Equation36 for R αβγδ . In the present situation since the degeneracies have small splittings we assume that ω 12 ≈ ω 21 ≈ 0.…”

Effects of maturation and storage on solubility, emulsion stability and gelation properties of isolated soy proteins

“…is added to a protein suspension, the Na + and C1-ions tend to screen the charged amino acid residues, preventing protein and water interactions (38). As a result, it is not possible for protein to interact with water to form a three-dimensional gel matrix in a high salt environment.…”