The proton chemical shifts of α‐helical poly‐L‐alanine

Abstract

The proton chemical shifts of NH, r-CH, and CH3 groups in x-helical poly-L-alanine (PLA) were calculated by taking account of diamagnetic shielding effect, electric-field effect, and magnetic anisotropy effect. The first and second terms were obtained from the electron density distributions in the model compounds of PLA hydrogen-bonded with formamide, and the last term was evaluated from the magnetic anisotropy effect due to the C=O, (O=)C-N, and r-CH-N bonds, where the two kinds of bond anisotropy of the C-N bond were estimated semi-empirically on the basis of the experimental data about six kinds of arnides. The calculated chemical shifts agreed fairly well with the observed values.

Chemical shift tensor – The heart of NMR: Insights into biological aspects of proteins

“…11 [118]. Semi-empirical quantum chemical calculations have been made in attempts to evaluate the contribution of the magnetic anisotropy and electric field effects besides the diamagnetic shielding to 1 H chemical shifts of α-helical polypeptide (initially for (Ala) n ) and the calculated chemical shifts agree fairly well with the observed values [150]. An empirical analysis of proton chemical shifts referenced to the chemical shift values in random coil protein from a variety of proteins of known structure [151–155] was made to decompose the last term in Eq.…”

Complexation-Induced Changes in1H NMR Chemical Shift for Supramolecular Structure Determination

“…We used values of d of 1.10 for the amide CO bond and 1.13 for the amide CÀN bond. [14] b) Parameters for calculation of electric field effects, illustrated for an amide carbonyl group. c) Parameters for calculation of ring current effects.…”

“…The values of Dc 1 and Dc 2 used were À 25.7 Â 10 À30 cm 3 and À 13.5 Â 10 À30 cm 3 for the CO bond and À 20.6 Â 10 À30 cm 3 and À 13.2 Â 10 À30 cm 3 for the CÀN bond. [13,14] The other parameters are defined in Figure 1a. The electric field effect of the amide group was calculated according to Equation (3).…”

“…[14] The charges, q, were À 1.980 for the amide oxygen, 1.365 for the carbon, À 1.133 for the nitrogen and 0.988 for the hydrogen. [15] The other parameters are defined in Figure 1b.…”

The relationship between amide proton chemical shifts and secondary structure in proteins

“…For example, the upfield shift of NH protons located in the peptide plane has been predicted by such a chemical shift calculation and this was found to be very important in interpreting chemical shifts in a-helices. However, in this previous study the parameters for the proton chemical shift calculation were determined from the chemical shift values of simple amide compounds because of limited experimental data (Asakura et al, 1977a). When we used the value of the C=O bond anisotropy reported by Zfircher (1967), the HN chemical shifts of BPTI were reproduced well and could be used for the refinement of the side-chain conformation (Asakura et al, ,1992b.…”