Geometry isoleucine

An apparently similar, antiperiplanar elimination with loss of the isoleucine 4-pro-S proton, to generate a double-bond of opposite geometry in alkaloidal necic acid fragments, has been observed13 (cf. Vol. 11, p. 2). 

Gould, R. O., Gray, A. M., Taylor, P., and Walkinshaw, M. D. Crystal environments and geometries of leucines, isoleucine, valine, and phenylalanine provide estimates of minimum nonbonded contact and preferred van der Waals interaction. J. Amer. Chem. Soc. 107, 5921-5927 (1985). 

Isoleucine has the same geometry as valine and is one carbon larger. Likewise, heptanal has the same geometry as octanal and is one carbon smaller. It is plausible that these two features may compensate at the active site of the mouse receptor. The extra carbon on Ile-206 could therefore interfere with octanal binding but still nicely accommodate the shorter heptanal. 

The presence of a D-amino acid in the sequence of these peptides or the alternation of L and D residues (cf. malformin) results in preferred conformations (reverse turns) that facilitate ring closure. Geometries conducive to cy-clization are generated also by the presence of a proline residue in the sequence, while glycine, by the absence of an impeding side chain can be similarly helpful. The cyclization promoting effect of bulky side chains, particularly valine and isoleucine with branching at the j -carbon atom, can not be excluded. 

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