A dihedral angle‐vicinal proton coupling constant correlation for the α–β bond of amino acid residues

Abstract

SynopsisA coupling constant-dihedral angle correlation for the H-C,-C@-H system of amino acid residues in peptides has been derived from a set of model compounds covering the full range of dihedral angles. The expression obtained, J = 11.0 cosz 0 -1.4 cos 0 + 1.6 sin%, is close to those already used in pmr studies of peptide conformation, and provides a firmer foundation for them. A factor limiting the precision of this and other "Karplus relations" is illustrated.Coupling constant-dihedral angle correlations for the vicinal protonproton coupling of the H-C,-N-H system of amino acid residues's2 have been important to the use of nuclear magnetic resonance data for estimation of peptide backbone conformation in solution, particularly for cyclic oligopeptides. (Examples are cited in Refs. 3-6.) A natural extension would be the use of similar relations for the H-C,-C@-H unit, to obtain information about the side-chain angle x1. However, it should be conceded at the outset that the rotational freedom of most peptide side chains may limit the information present and available.Suggested relations between dihedral angle and H,-H@ coupling constant already exist, of course. Pachler' combined observed average a-@ coupling constants of amino acids with the results of an earlier analysis of the spectra of hydroxyprolines,8 and suggested that J1S0o = 13.6 Hz and JW = 2.6 Hz. These values have been used for describing side-chain conformation in terms of a distribution among the XI = 60, 180, and 300" r o t a m e r~.~ While it may not be accurate to describe side-chain conformation in terms of a distribution restricted to these rotamers, a Karplus equation for the c.-@ system ought to be useful for the more rigid cyclic amino acid residues and also for peptides sufficiently complex that non-bonded interactions cause one value of x1 t o be dominant. The expressions J = 8.5 cos2 e + 1.4 J = 10.5 C O S~ e + 1.4 oo < e < 90" 90" < e < 1800(1) based on the general experience for H -G G H systems,'O has been used for cyclotriprolyl, and yields results in agreement with the X-ray crystal-

The C-peptide Helix from Ribonuclease A Considered as an Autonomous Folding Unit

“…The rotamer populations for the Phe-3 aCH--/3,/3'CH z bond were calculated from measurements of the coupling constants and equations given by Kopple et al (1973) and by Pachler (1964) and Feeney (1976), with similar results. The fraction of rotamer III (gauche-gauche) is small and independent of pH between 1 and 4.5 (f[III] = 0.08-0.09 according to .12 according to Pachler and Feeney), whereas in this pH range f(II) falls from 0.36 to 0.26 (Kopple et al 1973) or to 0.27 (Pachler 1964 andFeeney 1976), and f(I) rises accordingly. The a-COOH group is farthest from the aromatic ring of Phe-3 in rotamer I.…”

A Method to Explore Protein Side Chain Conformational Variability Using Experimental Data

“…The different types of may account for experimental data. While the fact that the majority of the residues are limited to uni-or bimodal distributions of angle values might seem to suggest that the common practice of modeling side chain dynamics using a two-or three-site jump model [1,16,19,20,26,32,33,[44][45][46][47][48][49][50] is appropriate, the maxima of the distributions seen here do not necessarily correspond to the canonical rotamer positions of AE 608 and 1808. Moreover, even the narrowest peaks encompass a range of angle values spanning~608, indicating that the concept of discrete rotamer positions does not comprise an adequate description.…”

“…3) A two-or three-site jump model is used. [1,16,19,20,26,32,33,[44][45][46][47][48][49][50] This method allows for multiple rotamers; however, each of these is defined as an exact solution rather than a distribution of angle values. The angle values may be determined as part of the fitting procedure, but this increases the number of unknown values to five, thus the angle values are more often chosen a priori, [51] reducing the number of adjustable parameters to two.…”

Ring conformational aspects of cis‐ and trans‐n‐acetyl‐4‐carbomethoxy‐1, 3‐thiazolidine‐1‐oxide

“…t h e carbomethoxy group i s "sandwiched" between b o t h H5-protons ( 8 ) . N o t i c e a b l e i s t h e l a r g e long-range c o u p l i n g 4J(BA,6A) t h a t amounts t o 2 .…”