Residue types, structural states adopted

In the statistical method of Chou and Fasman (Chou and Fasman, 1978), the propensities for a residue type to adopt three structural states a-helix (

), p-sheet () and turn () conformations are calculated for all of the 20 amino acids according to... 

The left-hand side of this expression is the energy of the sequence S adopting the conformation C, relative to some reference state. The right-hand side is the sum of the database-derived probabilities multiplied by the quantity /. In the purely statistical frameworks / = 1, and when statistical mechanics are used / = —kT. P (cj) is the probability that a specific sequence component s (individual residues, residue pairs or triplets) adopts a specific structural state c (defined by values of backbone angles, or spatial distances between atoms or residues). The indices i and j indicate the position or positions along the polypeptide chain of the sequence components and the structural states, respectively. P(Cj) represents the probability that any sequence component adopts a structural state c at position j along the polypeptide chain, and is compiled by considering all amino acid types as equivalent. This sequence-nonspecific probability is a normalization factor, applied in order to factor out many-body effects. 

A crystal structure of astin B (285) has been reported (Fig. 37) [133]. The solid state conformation exhibits one cis peptide bond between Abu and Pro . This structural feature is the main difference between the solid state structures of the astins, cyclochlorotine [142] and islanditoxin. ) The latter two compounds are toxic metabolites of yellow rice mold, Penicillium islandicum Sopp., whose occurrence on a variety of foodstuffs constitutes a human health hazard. In cyclochlorotine, only the astin Abu residue is replaced with serine, yet the molecule adopts a stable type I P-turn conformation with a trans proline amide bond and a transannular hydrogen bond [142]. 

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