Vibrational CD studies of interchain hydrogen-bonded tripodal peptides

The solution conformations of three trispeptides--L,L,L-1,3,5-C6H3[CH2NHCOCH(X)-NHBoc++ +]3, X = CH3 (Ala) or CH2CH(CH3)2 (Leu), and L,L,L-N(CH2CH2NHCOCH[CH2-CH(CH3)2]NHBoc)3--have been determined from their ir and vibrational CD (VCD) spectra in the NH stretching and carbonyl stretching regions in apolar solution. The compounds containing L-Leu are shown to occur primarily in a propeller conformation with C3 symmetry that is stabilized by interchain hydrogen bonds. Through application of the coupled oscillator model of VCD, a right-handed sense for the hydrogen-bonded chains in the propeller is deduced, in agreement with previous empirical force field calculations. The spectra also provide evidence for interchain association between two chains, resulting in a C10-ring. For chains not involved in interchain association, the spectra reveal the presence of C7-rings within a chain. The trispeptide containing L-Ala is found to occur primarily in a random form.