Unexpected stability of the urea cis-trans isomer in urea-containing modelpseudopeptides

[GRAPHICS] In contrast to the situation observed in the crystal state, the urea moiety in N-Boc-N'-carbamoyl-gem-diaminoalkyl derivatives (single-residue ureidop eptides) 1-4 exclusively assumes a cis-trans conformation in solution. When R-3 = H, the resulting structure can be further stabilized by an intramole cular hydrogen bond that closes an eight-membered pseudocycle. The root-mea n-square deviation calculated for heavy atoms between a peptide gamma -turn and the folded conformation that we propose to call urea turn is 0.60 Angs trom.