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15 N‐nmr spectroscopy. 33. Assignments of isotactic and syndiotactic sequences in poly( D , L ‐amino acids)
Author(s) -
Kricheldorf Hans R.,
Hull William E.
Publication year - 1982
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360210210
Subject(s) - tacticity , chemistry , nmr spectra database , enantiomer , nuclear magnetic resonance spectroscopy , stereoselectivity , copolymer , solvent , spectroscopy , stereochemistry , ternary operation , amino acid , crystallography , spectral line , organic chemistry , polymerization , polymer , physics , catalysis , quantum mechanics , astronomy , computer science , biochemistry , programming language
15 N‐enriched ( D , L ‐Leu) n , (γ‐OMe‐ D , L ‐Glu) n , ( D , L ‐Val) n , and ( D , L ‐Phe) n were prepared, 40.55‐MHz 15 N‐nmr spectra were measured in various solvents. The signal patterns depend strongly on the nature of the solvent, yet in most cases at least four signals are resolved, representing the four enantiomeric pairs of triads L ‐ L ‐ L ( D ‐ D ‐ D ), L ‐ D ‐ L ( D ‐ L ‐ D ), L ‐ L ‐ D ( D ‐ D ‐ L ), and D ‐ L ‐ L ( L ‐ D ‐ D ). Numerous copolypeptides of the general structure ( A ) n ‐ B *‐( A ) m (the asterisk denotes 40–50% 15 N enrichment) were synthesized and measured as models for syndiotactic sequences in the spectra of poly( D , L ‐amino acids). In this way unambiguous assignments for both isotactic and syndiotactic trials were obtained. A spectroscopic rule was established: “isotactic sequences absorb downfield of syndiotactic ones.” Furthermore, the spectra of various types of stereocopolypeptides such as ( L ‐Leu/ L ‐Val) n and ( L ‐Leu/ D ‐Val) n were investigated, including the ternary systems ( L ‐Leu/ L ‐Ala/ D ‐Ala) n ( L ‐Leu/ L ‐Ala/Gly) n , ( L ‐Leu/ D ‐Ala/Gly) n , ( L ‐Val/ L ‐Ala/Gly) n , and ( L ‐Val/ D ‐Ala/Gly) n . All copolymerization of D ‐ and L ‐amino acid NCAs investigated in this work showed a low degree of stereoselectivity.