Comprehensive and Accurate Ab Initio Energy Surface of Simple Alanine Peptides

Secondary structures for AcAlanNH2 (n = 2-4) peptides were analyzed by ab initio MP2, CCSD(T), and DFT-B3PW91 methods using large basis sets and including implicit hydration effects and thermal corrections. In addn. to the classical pure right-handed 310- and α-helixes, left-handed polyproline II, inverse γ-turn, and fully extended conformations, a large no. of mixed structures obtained by combining characteristic φ/ψ angles of each residue in every way were found. All mixed structures are energetically accessible and can be present at significant levels in aq. soln. Also, they represent the way through which hierarchical unfolding of 310- and α-helixes or nonhierarchical polyproline II to fully extended transitions occur. Computational results are in qual. agreement with the large amt. of exptl. data for simpler polyalanines and provide significant insight into their thermodn. properties and how these can be modulated by interactions with solvent.