1H-2H Exchange Research Articles

A conformational study of methyl β-cellobioside-d8 by 13C nuclear magnetic resonance spectroscopy: dihedral angle dependence of 3JC–H in 13C—O—C—1H arrays

Methyl β-cellobioside (2) has been deuterated at positions C-2, -3, -3′, -4, -6, and -6′ by 1H–2H exchange with deuterium oxide in the presence of Raney nickel catalyst. The resulting simplification in the 13C nmr spectrum of this isotopically-labelled compound has facilitated the measurement of interresidue 13C–1H coupling, as a means of determining the orientation of the glycosidic linkage in 2. Three-bond coupling across the linkage between 13C-4′ and H-1 is found to be 4.3 Hz and that between 13C-4′ and H-1, 4.2 Hz. By reference to other 13C–1H coupling data, which exhibit a Karplus type of relationship between the magnitude of 3JC—H and dihedral angle in 13C—O—C—1H arrays of bonded atoms, it appears that torsion angles [Formula: see text] and ψ of the disaccharide in aqueous solution are in the region of 25–30°, i.e., close to the angles of crystalline 2. Corresponding data for the hepta-O-acetyl derivative indicate that ψ is smaller by ∼10° and [Formula: see text] is unchanged. An analysis of the 1H-coupled 13C spectrum of cyclohexaamylose-2,3,6,6′-d4 is presented in relation to these findings. Computer simulations of the observed spectra have been used to determine coupling constants and to assess the importance of second-order effects on the measurement of JC—H.

Conformation of corticotropin. An infrared spectrometry study of hydrogen exchange kinetics

1H 2H exchange kinetics of the peptide hydrogens in corticotropin have been examined in 2H 2O and CF 3C 2H 2O 2H solutions by means of infrared absorption measurements. In aqueous solution, around pH 3, the experimental data suggest a partially ordered structure, since in the two corticotropins 1–24 and 1–32 about 6 slowly exchanging peptide protons are numbered. These might belong to the N-terminal part of the molecule. The C-terminal 25–32 octapeptide segment appears to be unordered and slightly destabilizes the overall hormone conformation. For corticotropin 1–24 in CF 3C 2H 2O 2H, the qualitative interpretation of infrared spectra and the quantitative analysis of exchange data give evidence of a strong stabilization: a predominantly α-helical structure is induced by trifluoroethanol.

The hydrophobic heart of rhodopsin revealed by an infrared 1H 2H exchange study

The hydrophobic heart of rhodopsin revealed by an infrared 1H 2H exchange study

Nuclear-magnetic-resonance study of the histidine residues of S-peptide and S-protein and kinetics of 1H-2H exchange of ribonuclease A.

1H NMR spectroscopy at 100 MHz was used to determine the first-order rate constants for the 1H-2H exchange of the H-2 histidine resonances of RNase-A in 2H2O at 35 degrees C and pH meter readings of 7, 9, 10 and 10.5. Prolonged exposure in 2H2O at 35 degrees C and pH meter reading 11 caused irreversible denaturation of RN-ase-A. The rate constants at pH 7 and 9 agreed reasonably well with those obtained in 1H-3H exchange experiments by Ohe, J., Matsuo, H., Sakiyama, F. and Narita, K. [J. Biochem, (Tokyo) 75, 1197-1200 (1974)]. The rate data obtained by various authors is summarised and the reasons for the poor agreement between the data is discussed. The first-order rate constant for the exchange of His-48 increases rapidly from near zero at pH 9 (due to its inaccessibility to solvent) with increase of pH to 10.5 The corresponding values for His-119 show a decrease and those for His-12 a small increase over the same pH range. These changes are attributed to a conformational change in the hinge region of RNase-A (probably due to the titration of Tyr-25) which allows His-48 to become accessible to solvent. 1H NMR spectra of S-protein and S-peptide, and of material partially deuterated at the C-2 positions of the histidine residues confirm the reassignment of the histidine resonances of RNase-A [Bradbury, J. H. & Teh, J. S. (1975) Chem. Commun., 936-937]. The chemical shifts of the C-2 and C-4 protons of histidine-12 of S-peptide are followed as a function of pH and a pK' value of 6.75 is obtained. The reassignment of the three C-2 histidine resonances of S-protein is confirmed by partial deuteration studies. The pK' values obtained from titration of the H-2 resonances of His-48, His-105 and His-119 are 5.3, 6.5 and 6.0, respectively. The S-protein is less stable to acid than RNase-A since the former, but not the latter, shows evidence of reversible denaturation at pH 3 and 26 degrees C. His-48 in S-protein titrates normally and has a lower pK than in RN-ase-A probably because of the absence of Asp-14, which in RN-ase-A forms a a hydrogen bond with His-48 and causes it to be inaccessible to solvent, at pH values below 9.

The structure of two alanine containing ferrichromes: sequence determination by proton magnetic resonance.

Metal coordination confers an extraordinary structural stability to the ferrichromes which, independent of their variable amino acid composition, results in a basically unperturbed conformation for all the homologous peptides in the series. The proton magnetic resonance (pmr) characteristics for Al3+ analogues (alumichromes) reflect this conformational isomorphism in usual solvents so that single site substitutions are clearly recognized in the pmr spectra. Thus, the substitution of glycine by L-alanine or L-serine introduce new resonances characteristic of the sidechains and alter the pattern of the amide NH pmr region in that doublets substitute for glycyl triplets at the same site. Since for glycine- and L-serine containing alumichromes the resonances have already been identified, it is possible to unequivocally establish the primary structure of the two L-alanyl homologues ferrichrome C (see article) and sake colorant A (see article) on the basis of the comparative pmr spectra of their Al3+ analogues, namely, alumichrome C and alumisake. The resonance assignment, and hence the site occupancy, is substantiated by the temperature coefficients of the NH chemical shifts, rates of 1H-2H exchange and homonuclear proton spin decoupling experiments centered on the NH spectral region. Occupancy of site 1 by a glycine residue is observed for all known ferrichromes, which serves to conserve a "hairpin" turn. This method of obtaining sequence information should prove of general use for other systems of homologous polypeptides, provided their conformations are not affected by the residue substitutions.

A study by the hydrogen-exchange method of the complex formed between the basic pancreatic trypsin inhibitor and trypsin.

The kinetics of the 1H–2H exchange of the peptide groups is studied in 2H2O solutions of the basic pancreatic trypsin inhibitor, of trypsin, and of the inhibitor-trypsin complex. In average the exchange rates in solutions of the complex are slower than the exchange rates in solutions of the constituent proteins. The experimental results are expressed in terms of the probability ϱ of exposure to the solvent of the peptide groups of the proteins, and in the changes in standard free energy ΔG° of the conformational transitions by which the exposure is effected. The distribution of values of these parameters on the peptide groups of the proteins is discussed.

The interaction of platinumIIchloride with a carbon–carbon σ-bond

In 1H–2H exchange by PtII, cyclobutane shows a remarkably large multiple exchange factor, attributed to interaction between PtII and the C–C bond; tetraphenylcyclobutane is cleaved by PtII to give trans-stilbene while norbornane shows preferential exo exchange.