Interaction of alpha-L-aspartyl-D-phenylalanine methyl ester with the receptor site of the bitter taste

The analysis of the structure-activity relationship previously performed on the sweet L,L isomer of Asp-Phe-OMe was extended to the bitter L,D diastereoisomer. A combination of NMR measurements and of an original method of PEM calculations gives a description of the conformational state of the L,D isomer that can be effectively used to study the interaction with a previously proposed model of the bitter taste receptor site.

Interaction of S-carboxymethylated uteroglobin with progesterone

S-Carboxymethylated uteroglobin, a small progesterone-binding globular protein, was studied by means of high-field proton magnetic resonance spectroscopy. Conformational changes induced by the steroid have been observed and indicate a well-defined rearrangement of the structure of the protein. The unusual stoichiometry of one progesterone to two uteroglobin dimers is confirmed by 1H NMR. His-8 plays a central role in the mechanism of interaction of uteroglobin with progesterone. The pH dependence of affinity constant for the complexation of the steroid parallels the titration of this histidine. Although it is not a part of the active site, it influences a crucial conformational transition of the protein through the charge carried by its imidazole ring. Formation of a tetramer of uteroglobin subunits able to bind progesterone is critically compared to a kinetic scheme involving only dimers.

Carbon magnetic resonance studies of the self-aggregation of calf thymus histones

Self-aggregation of calf thymus histones H2A and H4 was studied by means of 13C magnetic resonance. Analyses of the changes in the intensities of several side-chain resonances, induced by added NaCl, confirm that carbon magnetic resonance can be used to monitor aggregation phenomena more accurately than other physicochemical methods. In particular the validity of an original computer-based method to treat 13C intensities is confirmed. The details of these aggregation phenomena are critically discussed in the light of a simple experiment on a 13C-enriched sample of H2B.

Three-dimensional mapping of the sweet taste receptor site

The active sites of the receptors for sweet and bitter tastants are shown to be related by a simple symmetry operation. This relationship, in turn, allows the identification of the critical geometrical features of both receptor sites. The model proposed for the sweet site is shown to be consistent with a large number of (conformationally rigid) sweet molecules.

The interaction of histone H3 with histone H4 and with other histones studied by 19F nuclear magnetic resonance

The behaviour, upon variations in ionic strength, pH and temperature of 19F nuclear nuclear magnetic resonance signals of the trifluoroacetonylated derivative of histone H3 is compared with those of the H3-H4 complex and of the Hv fraction (an equimolar mixture of H2A, H2B, H3 and h4). The line width of the 19F-labelled histone H3 signals increases with ionic strength or pH, an effect consistent with aggregation of the protein. In the case of H3-H4 complex or Hv the line width decreases at intermediate ionic strengths (0.1-0.25 M NaCl). This effect is interpreted as the consequence of the formation of a well defined structure with ionic strength. At high salt concentrations the line width increases as a consequence of the final rigid quaternary structure or of the formation of higher aggregates.

A study of calf-thymus histone H2B using 13C magnetic resonance

Calf thymus histone H2B has been studied by means of 13c magnetic resonance. Most sidechain resonances have been assigned. Analysis of 13c peak intensities by a simple computer-aided method proves that 13c magnetic resonance spectroscopy can be used to monitor aggregation phenomena more accurately than 1H magnetic resonance or other physico-chemical methods. Re-examination of 1H spectral simulations shows substantial agreement with 13C results. It is concluded that in 0.1M added NaCL the H2b aggregates include the chain segment from 50 (+/-5) to 108 (+/-5).

Structural characterization of thermal prebiotic polypeptides

Thermal polycondensation of amino-acids as a possible prebiotic path of chemical evolution of life has been critically examined. The polymeric materials studied by nmr methods have scarce resemblance to natural peptidic material because beta, gamma and eplision peptide bonds largely predominate over a-peptide bonds.

NMR studies of prebiotic polypertides

Several polypeptides prepared by means of pyrocondensation have been the subject of structural investigations. Attention has been focused on the constitutional characterization of homo-and co-polymers containing Asp and Glu residues, whose role is essential for the formation of the so-called proteinoids. Contrary to the literature data based on chemical degradation, nmr studies show conclusively that in thermal poly-aspartic acid only beta-peptide linkages are present. This result casts serious doubt on the role thermal condensation might have played in prebiotic polypeptide syntheses.

Conformational transition in oligopeptides: an NMR spectroscopic study

We examined the 220-MHz NMR spectra for a series of oligopeptides derived from gamma-ethyl L-glutamate, an octapeptide based on beta-methyl L-aspartate, and a low molecular weight L-glutamate polymer ([unk]DP = 20) in deuterochloroform-trichloroacetic acid. A definite transition from folded to nonhelical forms was established for the glutamate systems. The aspartate behavior is explained by a progressive solvation of a peptide chain in a disordered conformation. In all the solvent systems studied, separate peaks for each N-H are observed for the glutamate and aspartate oligomers. Thus, end effects and polydispersity are important in interpreting NMR spectra of partially helical polypeptides. Slow exchange between NH groups on the same chain does not appear to play a significant role in the helix-coil transition.