Head-to-head dimerization of parallel beta helices in a synthetic model of the linear gramicidins

In chloroform solution, the D ,L -alternating stereo-co-oligopeptide HCO-L -Phe-(D -Phe-L -Phe)₃-OMe (I) forms three major species, two of which are dimeric and one tetrameric. One of the two dimeric species gives a specific set of ¹H-nmr signals at 25°C; the other, together with the tetrameric species, gives another set of resonance signals. In a carbon tetrachloride or cyclohexane solution at 25°C, I forms virtually only the tetrameric species. From the nmr data, it can be shown that the dimeric and tetrameric species, that are in rapid equilibrium with each other in chloroform solutions, are a right-handed ↑↑β^5.6 helical dimer and the head-to-head (formyl-ends-to-formyl-ends) dimerization product of this dimer. It is suggested that the linear gramicidins may also form head-to-head dimers of parallel β helices, as observed for the model oligopeptide I.