Design, construction and properties of peptide N-terminal cap templates devised to initiate alpha-helices. Part 3. Caps derived from N-[(2S)-2-chloropropionyl]-(2S)-Pro-(2R)-Ala-(2S,4S)-4-thioPro-OMe

The construction of a 12-membered macrocyclic template capable of entrainin g attached peptides in helical conformations from acyclic N-[(2S)-2-chlorop ropionyl]-(2S)-Pro-(2S)-Pro(2S,4S)-4-thioPro-OMe precursors has been severe ly hampered by the problem of simultaneously aligning carboxamide dipoles i n the transition state for cyclisation. Previously we provided a detailed c onformational analysis of the system and tested two methods for forcing the acyclic precursor into the macrocyclic conformation required for helix ini tiation. First, the destabilisation of unwanted conformations in the transi tion state for cyclisation, and second, the stabilisation of the favoured t ransition state structure through the introduction of a hydrogen-bonding in teraction. Both strategies were unsuccessful. A third strategy based upon r emoving the requirement fbr all of the carbonyl dipoles to align in the tra nsition state at the same time was also tested and the results are presente d here. The relaxation of the highly restrained C-alpha-N bond torsion for Pro(3) in the acyclic precursor, through its substitution for a (2R)-alanin e residue, effectively decouples the motion of the second carboxamide group from the C-alpha-N bond torsion and allows the second carboxamide group to rotate. This rotation allows a helical conformation to develop in the tran sition state to the macrocycle without the need to align all of the carboxa mide dipoles and results in successful cyclisation to give template structu res of the all trans (ttt) form. Derivatives of the template were prepared by extending the C-terminus and these were characterised by NMR spectroscop y and restrained simulated annealing;. In deuterochloroform solution at low temperature, separate sets of NMR signals were observed for two rapidly in terconverting helical conformational isomers of the thioether macrocycle ba sed on (2R)-N-propionyl-(2S)-Pro-(2R)-Ala-(2S)which possessed an appended t rialkylammonium ion. The free energy of activation for the transition (Delt a G(c)double dagger) was 48 kJ mol(-1). A similar time-averaged conformatio n was also observed in aqueous solution. At - 80 degrees C in dichlorometha ne the rate of conformational exchange was slowed sufficiently to obtain re sonance assignments and NOE data separately for each isomer. In the minor i somer (40%), the four carbonyl oxygen hydrogen-bond accepters of the templa te are aligned in an a-helical conformation and in the major conformer the Pro(2) carbonyl dipole was anti-aligned with the other three dipoles. Thus, the conformers differ in the orientation of one carbonyl group. Molecular modelling calculations showed that the minor isomer was stabilised by coulo mbic interactions between the trialkylammonium salt and the carbonyl group dipole moments.