Spectrochimica acta part a-molecular and biomolecular spectroscopy vol:48 issue:8 pages:1091-1099
Two model dipeptides, N-tertiobutoxycarbonylsarcosine N'-methylamide (BSMA) and N-tertiobutoxycarbonylsarcosine N',N'-dimethylamide (BSDA) are investigated by FT-IR spectrometry. The conformation of BSMA is very sensitive to the environment. In solvents of weak polarity (carbon tetrachloride, cyclohexane), BSMA accommodates the extended and seven-membered ring conformation, but in 1,2-dichloroethane, the C-7 conformers are greatly destabilized. Hydrogen bonding between BSMA or BSDA and phenols is studied in carbon tetrachloride. The thermodynamic data (equilibrium constants and enthalpies of complex formation) show that the BSMA complexes are stronger than the BSDA complexes. The spectroscopic data suggest that for BSMA, complex formation occurs at the O atom of the amide function while for BSDA about 50% of the complexes are formed on the O atom of the urethane group. The differences between the two sarcosine dipeptides are discussed in terms of cooperative and steric effects. It can be concluded that the global polarity of the medium exerts a greater influence on the conformation of the C-7 dipeptides than the specific interactions taking place on a given site of the molecule.