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 been determined by ab-initio SCF-MO computations. In the gauche side-chain conformations (chi1 = 60-degrees and chi1 = 300-degrees), the pattern of minima on the backbone potential energy surface, i.e. on the 2D Ramachandran map, E = E(phi,psi), is equivalent to the backbone conformation of the corresponding L-alanine derivative, which shows the absence of the alpha(L) and epsilon(L) conformations. However, in the anti conformation (chi1 = 180-degrees) an additional backbone conformation, 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 critical point (lambda = 1) on the potential energy hypersurface of 3N-6 independent variables. The beta(L) backbone conformation is also destabilized at chi1 = 180-degrees to a higher energy than either of the two nonequivalent gauche conformations; nevertheless, it remained a minimum. In contrast to the above, three backbone conformations (gamma(L), gamma(D), and alpha(D)) are stabilized in the anti (chi1 = 180-degrees) side-chain conformation with respect to the two nonequivalent gauche conformations. A new method has been developed for a unique energy partitioning in order to quantify the magnitude of the side chain/backbone interaction. The numerical values for such side chain/backbone interactions have been calculated for the (i)Pr group in the various backbone 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 able to interact with the peptide backbone so drastically that it could annihilate one of the otherwise legitimate minima through an unfavorable backbone and side-chain torsional angles combination.