Relationship between proton–proton nmr coupling constants and substituent electronegativities. III. Conformational analysis of proline rings in solution using a generalized Karplus equation

Abstract

AbstractThe relationship between published vicinal proton–proton coupling constants and the pseudorotation properties of the pyrrolidine ring in L‐proline, 4‐hydroxy‐L‐proline, 4‐fluoro‐L‐proline, and several linear and cyclic model proline peptides is investigated. Compared to earlier studies, several important improvements are incorporated: (1) a new empirical generalization of the classical Karplus equation is utilized, which allows a valid correction for the effects of electronegativity and orientation of substitutents on 3JHH; (2) an empirical correlation between proton–proton torsion angles and the pseudorotational parameters P and τm is derived; and (3) the best fit of the conformational parameters to the experimental coupling constants is obtained by means of a computerized iterative least‐squares procedure. Two pseudorotation ranges were considered, classified as type N (χ2 positive sign) and type S (χ2 negative sign). The conformational equilibrium is fully described in terms of four geometrical parameters (PN, τN, PS, τS) and the equilibrium constant K. The present results indicate that, in general, the geometrical properties found in x‐ray studies of proline and hydroxyproline residues are well preserved in solution. Several novel features are encountered, however. It is demonstrated that the proline ring occurs in a practically 1:1 conformational equilibrium between well‐defined N‐ and S‐type forms. Introduction of an amide group at the C‐terminal end has no observable effect on this equilibrium, but the formation of a peptide bond at the imino nitrogen site results in a pronounced, but not exclusive, preference for an S‐type form which is roughly 1.1 kcal/mol more stable than its N‐type counterpart. The hydroxyproline ring system in neutral or acidic medium displays a pure N‐type state, but N‐acetylation results in the appearance of a minor (S‐type) conformation. Cyclic proline dipeptides similarly exist in a biased conformational equilibrium. The major form (77–88%) corresponds to the N‐type conformer observed in the solid state; the minor S‐form has not been observed before. In contrast, cyclic hydroxyproline dipeptides display complete conformational purity. Ranges of endocyclic torsion angles deduced for the various classes of pyrrolidine derivatives in solution are presented. Each torsion appears confined to a surprisingly narrow range, comprising about 4°–8° in most cases. In all, the proline ring is far less “floppy” than hitherto assumed.