| Abstract: | A diamide, N-acetyl-L -proline-N,N-dimethylamide (AcProDMA), in water solution has optical rotatory dispersion (ORD) and circular dichroism (CD) spectra very similar to those of poly-L -proline II and the fibrous protein collagen. In contrast, AcProDMA in cyclohexane solution has optical activity resembling that of poly-L -proline I. Conformational analysis shows that AcProDMA is confined by steric constraints to either of two narrow regions of conformational space. The trans isomer of AcProDMA assumes conformations near those of polyproline II and collagen nearest neighbors, while cis-AcProDMA assumes conformations near that of polyproline I nearest neighbors. Nuclear magnetic resonance (NMR) experiments show that an equilibrium mixture of the cis and trans isomers of AcProDMA is present in solution. The trans isomer predominates in aqueous solution, but the equilibrium shifts to favor the cis isomer in nonpolar organic solvents such as cyclohexane. Analysis of the ORD spectra in terms of two basic spectra reveals a solvent dependent isomerization which parallels that observed by NMR. The optical activity of the pure isomers of AcProDMA can be derived from the ORD, CD and NMR data. A comparison of component cotton effects confirms the similarity in optical activity of trans-AcProDMA, polyproline II, and collagen on the one hand, and of cis-AcProDMA and Polyproline I on the other. |
