Browsing by Author "E. Subramanian"

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Coexistence of Folded and Extended Conformations of a Tripeptide Containing α,α-Di-n-propylglycine in Crystals
(1994) S. Prasad; S. Mitra; E. Subramanian; D. Velmurugan; R.B. Rao; P. Balaram
The crystal structure of the tripeptide Boc-Leu-Dpg-Val-OMe (Dpg, α, α-di-n-propylglycine) reveals the coexistence of two distinct backbone conformations. In molecule A the Dpg residue adopts a fully extended conformation (φ = 76.0°, ψ =180.0°) while in molecule B a left handed helical conformation (φ = 62.8°, ψ = 39.6°) is observed. Molecule B adopts a folded structure corresponding to a highly distorted Type II β-turn conformation, which lacks an intramolecular 4 → 1 hydrogen bond. In contrast, molecule A has an open, extended conformation. The results demonstrate that both fully extended and helical conformations are energetically accessible to the Dpg residue. © 1994 Academic Press. All rights reserved.
Restricted Inversion of Pyramidal Nitrogen through π-Electronic Interaction in an Acyclic System
(1994) Alaka Srivastava; Vandana Srivastava; Shiva M. Verma; E. Subramanian
An sp3 noninverting geometry of nitrogen in N-(isopropylylideneamino)imide stabilized by the π-electronic system has been demonstrated through 1HNMR and X-rayy crystallography. One of the carbonyls and the —C═N— group of N-(isopropylideneamino)-3,4-(9′,10′-dihydroanthracene-9′,10′-diyl)succinimide (4) are reduced to —CHOH and —CHNH— (8) with an excess of sodium borohydride in methanol. Shielding parameters of isopropyl methyls and VT NMR studies of 8 have indicated a stable pyramidal geometry of exocyclic nitrogen in solution. X-rayy crystallographic studies have demonstrated the sp3 geometry of nitrogen (