Peptide Models. 18. Hydroxymethyl Side-Chain Induced Backbone Conformational Shifts of <scp>l</scp>-Serine Amide. All ab Initio Conformers of For-<scp>l</scp>-Ser-NH₂

Abstract

Using ab initio conformational energy mapping (HF/3-21G) a maximum of nine characteristic backbone conformation clusters (αl, αd, βl, γl, γd, δl, δd, εl, and εd) were previously established for different amino acid diamides (e.g., For-l-Ala-NH2, For-l-Val-NH2, and For-l-Phe-NH2). Most of the above nine backbone conformers have been located in the [φ,ψ] space for various side-chain conformers. The present conformation analysis derives structural parameters of For-l-Ser-NH2 molecule based on a systematic investigation of the side-chain conformational energy maps {E = E(χ1,χ2)} associated with characteristic backbone structures. The systematic mapping of the E = E(φ,ψ,χ1,χ2) four-dimensional Ramachandran-type map has revealed 44 minima. This finding thus established the complete conformational set for For-l-Ser-NH2. Specific intramolecular hydrogen bonds of the 44 geometry optimized structures were analyzed. These ab initio structures can now be used with greater confidence during force field parameterizations, NMR, and X-ray structure elucidations or even for the characterization of protein backbone structures.