The enthalpies of interaction of some amides with urea in water at 25 °C

Abstract

The heterotactic interactions occurring in water at 25 °C between some simple amides [formamide (F), acetamide (A), N-methylformamide (NMF), N-methylacetamide (NMA), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), N,N-diethylformamide (DEF) and N,N-diethylacetamide (DEA)] and the protein denaturant urea, have been investigated using enthalpy changes on mixing. The homotactic interactions of DEF and DMA have also been obtained from enthalpies of dilution and a redetermination of the enthalpy of dilution of A has also been performed. The results are discussed using the Savage and Wood additivity-of-groups approach and although this works quite well if all amidic groups are taken as being equivalent, much better agreement between calculated and experimental results is obtained if primary and secondary amide groups are assumed to be equivalent and in one category, and tertiary amidic groups are in another category. The enthalpies of interaction of urea with the groups in both categories are negative (i.e. are thermochemically favourable) but the tertiary amide group interacts with urea in water more favourably than primary and secondary amide groups. The somewhat surprising finding is commented on and addressed using some exact theories of solutions and is also considered with regard to urea-induced protein denaturation.