CONFORMATIONAL CHARACTERISTICS OF THE N-ACETYL-N‘-METHYLAMIDES OF THE FOUR (LYS, TYR) DIPEPTIDES

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Abstract
The conformational properties of the N-acetyl-N''-methylamides of the dipeptides Lys-Lys, Lys-Tyr, Tyr-Lys and Tyr-Tyr were studied by means of conformational energy calculations, NMR measurements in deuterated dimethylsulfoxide and by circular dichroism (CD) in water, methanol, dioxane-water and trifluoroethanol. Since these 4 dipeptides occur occasionally as bends in proteins, it was of interest to see whether short-range interactions, acting within the terminally blocked dipeptides, are sufficient to stabilize bend conformations significantly over other conformations. The 4 dipeptides exist as ensembles of conformation in solution. Therefore, it appears that longer-range interactions, such as those present in proteins, are required if bend conformations of these dipeptide sequences are to exist as stable conformations. Three of the dipeptides behave rather similarly. Both the CD and the NMR experiments and computations indicate that the 4th (Lys-Tyr) differs from the others. It has a preference for compact conformations that appear to be stabilized by strong favorable interactions, primarily H- bonds, between the tyrosyl and the lysyl side chains. The computations suggest that the presence of these interactions and hence the existence of preferred conformations, is strongly solvent-dependent and that these interactions are weakened in aqueous solution.

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