PEPTIDE MODELS .3. CONFORMATIONAL POTENTIAL-ENERGY HYPERSURFACE OF FORMYL-L-VALINAMIDE

Out of the 27 legitimate minima of the 3D Ramachandran map, E = E(phi, psi,chi1), the existing 20 conformations of formyl-L-valinamide have b een determined by ab-initio SCF-MO computations. In the gauche side-ch ain conformations (chi1 = 60-degrees and chi1 = 300-degrees), the patt ern of minima on the backbone potential energy surface, i.e. on the 2D Ramachandran map, E = E(phi,psi), is equivalent to the backbone confo rmation of the corresponding L-alanine derivative, which shows the abs ence of the alpha(L) and epsilon(L) conformations. However, in the ant i conformation (chi1 = 180-degrees) an additional backbone conformatio n, the one labeled as delta(L), has disappeared. This implied that, at the chi1 = 180-degrees torsional angle, on the E = E(chi1) potential curve crosssection, the delta(L) conformation is destabilized to such a degree that the delta(L) minimum is replaced by a higher indexed cri tical point (lambda = 1) on the potential energy hypersurface of 3N-6 independent variables. The beta(L) backbone conformation is also desta bilized at chi1 = 180-degrees to a higher energy than either of the tw o nonequivalent gauche conformations; nevertheless, it remained a mini mum. In contrast to the above, three backbone conformations (gamma(L), gamma(D), and alpha(D)) are stabilized in the anti (chi1 = 180-degree s) side-chain conformation with respect to the two nonequivalent gauch e conformations. A new method has been developed for a unique energy p artitioning in order to quantify the magnitude of the side chain/backb one interaction. The numerical values for such side chain/backbone int eractions have been calculated for the (i)Pr group in the various back bone conformations of formyl-L-valinamide relative to that of hydrogen in the corresponding backbone conformations of formylglycinamide. The computations have clearly shown that even an apolar side chain was ab le to interact with the peptide backbone so drastically that it could annihilate one of the otherwise legitimate minima through an unfavorab le backbone and side-chain torsional angles combination.