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Cover Illustration: Poly (2S-proline) forms a left-handed helix that is the most stable structural element attainable with a natural polypeptide. The stability of this polyproline type II helix is enhanced by installing an electron-withdrawing substituent in the R configuration at C y of the proline residues, as in this model of [(2S,4R)-4-fluoroproline]10. Electron-withdrawing substituents in the S configuration diminish helical stability. Thus, stereoelectronic effects can have a substantial impact on polyproline conformation and provide a rational means to stabilize a polyproline type II helix. (See Horng and Raines, pp.74-83. Cover illustration by H. Adam Steinberg.)