Calcium binding by troponin-C. A proton magnetic resonance study.

The conformational changes induced by the binding of Ca(II) to rabbit skeletal muscle troponin-C (TNC) have been followed by proton magnetic resonance spectroscopy. Ca(II)-free TNC (apo-TNC) contains definite ordered regions. Ca(II) titration of the high affinity sites (cf. Potter , Gergely, 1975) causes a large folding of the backbone, some of which involves refolding of an ordered region(s) and changes in several side-chains e.g. Glu, Asp and Phe. Titration of the low affinity sites does not alter the backbone but leads to changes among hydrophobic side-chains (one or more Val, Leu, Ile; two or more Phe, Glu and Asp) that define an ordered region(s) of apo-TNC. The rate constants for the conformation changes of the low and high affinity sites are approximately 10 s^?1 and < 20 s^?1, respectively. Final stages of the titration include a downfield shifted methyl group (likely Ile) and a Phe residue. The thermal stabilities of apo-TNC, TNC · Ca₂(II) and native TNC were compared. It was concluded that Ca(II) binding by the two high affinity sites both directs and stabilizes much of the structure. The role of the changes of the low affinity sites, which are thought to activate contraction, are briefly discussed.     

A proton nuclear magnetic resonance study of sulfmyoglobin cyanide

The proton nuclear magnetic resonance spectrum of sulfmyoglobin cyanide was studied at 400 MHz. The position of a methyl-group resonance at low field is consistent with a chlorin-like structure for the prosthetic group. The proton NMR spectrum of the cyanide derivative of the purified prosthetic group which decomposes upon extraction from the protein was found to be the same as that of the cyanide derivative of the prosthetic group extracted from myoglobin and a sample prepared from hemin-Cl.     

Nuclear magnetic resonance study of ring conformations of cyclic tetradepsipeptide AM-toxins and related peptides

Cyclic tetradepsipeptides, AM-toxin I and II, are the host-specific phytotoxins of Alternaria mali. In order to elucidate conformation-toxicity relationships, we analyzed the 270-MHz proton nmr spectra of AM-toxins and hydrogenated analogs, (D -Ala²)AM-toxin I (toxic) and (L -Ala²)AM-toxin I (not toxic), in (C²H₃)₂SO. These cyclic tetradepsipeptides do not contain N-substituted amino acid residues, and all the peptide and ester groups have been found to be transoid. Two conformers with very unequal populations have been found for AM-toxin I and II; the C^β?C^α? C?O conformations of the Dha² residues are nonplanar S-trans in the major conformer and nonplanar S-cis in the minor conformer. Only one ring conformation has been found for each of (L -Ala²) and (D -Ala²)AM-toxin I. (L -Ala²)AM-toxin I takes a C₄-type ring conformation; all the C?O groups and C^α-H bonds are oriented to the same side of the ring. (D -Ala²)AM-toxin I takes a new ring conformation; the side chain and C?O group of the L -Amp¹ residue are oriented to the same side of the ring. This new conformation is also found for the major conformers of AM-toxin I and II and thus appears to be required for the toxicity. The ring conformations of Tyr(OCH₃)¹-bearing analog tetradepsipeptides have been found to be much the same as those of Amp¹-bearing depsipeptides. Furthermore, on the basis of the two distinct conformations of (D -Ala²) and (L -Ala²)AM-toxin I, an empirical rule is proposed for the stable ring conformations of cyclic tetra-D ,L -peptides, not containing N-substituted amino acid residues.     

A proton magnetic resonance study of water in human stratum corneum

Proton magnetic resonance spectroscopy has been used to study the nature of water in human stratum corneum. For a single planar sheet of stratum corneum mounted at a specific orientation to the applied magnetic field, three distinct absorptions may be seen having different chemical shifts and spin-lattice relaxation times (T₁). All T₁ values for these resonances are smaller than that for normal liquid water. One absorption is unusual in that the resonance position is dependent upon the orientation of the sample within the field.     

Varied magnetic field, multiple-pulse, and magic-angle spinning proton nuclear magnetic resonance study of muscle water.

The nuclear magnetic resonance linewidth of 1H in water of frog muscle was studied as a function of magnetic field strength and angle of orientation. The results suggest that the observed spectra are dominated by demagnetization field anisotropy and dispersion, but a small static dipolar interaction of the order of a few hertz man be present. Data from line-narrowing, multiple-pulse experiments also indicate the presence of a small dipolar broadening.     

A proton nuclear magnetic resonance study on the solution structure of crotamine

Proton nuclear magnetic resonance (NMR) spectra of crotamine, a myotoxic protein from a Brazilian rattlesnake (Crotalus durissus terrificus), have been analyzed. All the aromatic proton resonances have been assigned to amino acid types, and those from Tyr-1, Phe-12, and Phe-25 to the individual residues. ThepH dependence of the chemical shifts of the aromatic proton resonances indicates that Tyr-1 and one of the two histidines (His-5 or His-10) are in close proximity. A conformational transition takes place at acidicpH, together with immobilization of Met-28 and His-5 or His-10. Two sets of proton resonances have been observed for He-17 and His-5 or His-10, which suggests the presence of two structural states for the crotamine molecule in solution.     

A proton magnetic resonance study of the conformation of methionine-enkephalin as a function of pH.

It is found that methionine-enkephalin has at least two different conformations in aqueous solution, one at low and one at high pH. From inspection of titration curves and coupling constant values, it seems reasonable to conclude that these conformations are characterized by a folding so as to bring the two ends of the molecule in close proximity. This behavior parallels that found recently in (CD3)2SO as the solvent. It follows that the Phe-Met region of the molecule constitutes a relatively rigid region, but that the chain possesses flexibility around the first Gly residue. Possible implications of this behavior with respect to the receptor site are discussed.     

A proton nuclear magnetic resonance study of the quaternary structure of human homoglobins in water.

Proton nuclear magnetic resonance spectra of human hemoglobins in water reveal several exchangeable protons which are indicators of the quaternary structures of both the liganded and unliganded molecules. A comparison of the spectra of normal human adult hemoglobin with those of mutant hemoglobins Chesapeake (FG4alpha92 Arg yields Leu), Titusville (G1alpha94 Asp yields Asn), M Milwaukee (E11beta67 Val yields Glu), Malmo (FG4beta97 His yields Gln), Kempsey (G1beta99 Asp yields Asn), Yakima (G1beta99 Asp yields His), and New York (G15beta113 Val yields Glu), as well as with those of chemically modified hemoglobins Des-Arg(alpha141), Des-His(beta146), NES (on Cys-beta93)-Des-Arg(alpha141), and spin-labeled hemoglobin [Cys-beta93 reacted with N-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)iodoacetamide], suggests that the proton in the important hydrogen bond between the tyrosine at C7alpha42 and the aspartic acid at G1beta99, which anchors the alpha1beta2 subunits of deoxyhemoglobin (a characteristic feature of the deoxy quaternary structure), is responsible for the resonance at -9.4 ppm from water at 27 degrees. Another exchangeable proton resonance which occurs at -6.4 ppm from H2O is a spectroscopic indicator of the deoxy structure. A resonance at -5.8 ppm from H2O, which is an indicator of the oxy conformation, is believed to originate from the hydrogen bond between the aspartic acid at G1alpha94 and the asparagine at G4beta102 in the alpha1beta2 subunit interface (a characteristic feature of the oxy quaternary structure). In the spectrum of methemoglobin at pH 6.2 both the -6.4- and the -5.8ppm resonances are present but not the -9.4-ppm resonance. Upon the addition of inositol hexaphosphate to methemoglobin at pH 6.2, the usual resonance at -9.4 ppm is shifted to -10 ppm and the resonance at 6.4 ppm is not observed. In the spectrum of methemoglobin at pH greater than or equal to 7.6 with or without inositol hexaphosphate, the resonance at -5.8 ppm is present, but not those at -10 and -6.4 ppm, suggesting that methemoglobin at high pH has an oxy-like structure. Two resonances (at -8.2 and -7.3 ppm) which remain invariant in the two quaternary structures could come from exchangeable protons in the alpha1beta1 subunit interface and/or other exchangeable protons in the hemoglobin molecule which undergo no conformational changes during the oxygenation process. These exchangeable proton resonances serve as excellent spectroscopic probes of the quaternary structures of the subunit interfaces in studies of the molecular mechanism of cooperative ligand binding to hemoglobin.     

Broadline proton magnetic resonance study of cellulose,pectin, and bean cell walls

We have used broadline proton magnetic resonance to study molecular motion in cellulose, a sodium pectate solution, a calcium pectate gel, and isolated bean cell walls. All samples were prepared in D₂O to minimize the contribution of water to the observed signals. For each sample, a free induction decay was obtained, and the second moment, spin-lattice relaxation, and dipolar relaxation were measured. Our results show that the large majority of protons in cellulose are immobile. Rigid and mobile domains were also observed in the pectate samples. We have shown that gelation induces large-scale changes in the free induction decay, the second moment, and the relaxation behavior of the pectate. As with the other samples, rigid and more mobile domains were found in bean cell walls. The fraction in the rigid domains is much larger than the fraction of cellulose in the sample, suggesting that the noncellulosic wall components are also organized into rigid and mobile domains.