Excess enthalpies of N-acetylglycineamide and N-acetyl-L-leucineamide in concentrated aqueous solutions of tetramethylurea.

A comparison has been made among the values of the enthalpic pairwise interaction coefficients of two N-acetylamides of the amino acids glycine (NAGA) and leucine (NALA) in concentrated aqueous solutions of tetramethylurea (TMU) and urea, in pure water, and in pure liquid amides. The second virial coefficients of the excess enthalpies are found to be negative for both the model peptidic molecules in 4M TMU, as in liquid amides, dimethylformamide (DMF) and fused N-methylacetamide (NMA), substances assumed to mimic the core of globular proteins. This is the reverse of what was found for concentrated aqueous solutions of urea (U), where all the enthalpic second virial coefficients were positive. This suggests a completely different mechanism for the denaturation of protein in the presence of urea and TMU, due to different protein-denaturant interactions.