Structure and dynamics of high molecular weight glutenin subunits of durum wheat (Triticum durum) in water and alcohol solutions studied by electron paramagnetic resonance and circular dichroism spectroscopies.

Complementary information on the structure and dynamics of high molecular weight glutenin subunits (HMW-GS) of durum wheat (Triticum durum) was obtained by means of two spectroscopic techniques. Electron paramagnetic resonance was used to investigate the dynamics of the HMW-GS hydrated with two 2-propanol/water mixtures at temperatures between 268 and 308 K by specific spin labeling of their cysteine residues. Spectra were of a composite type, resulting from two populations of spin labels differing in molecular mobility, both undergoing isotropic rotational diffusion. Diffusional coefficients and populations of the fast- and slow-moving spin labels, determined by an accurate spectral line shape analysis, are discussed as a function of temperature and water content in the solvent systems. Far-UV circular dichroism was employed to provide information on the secondary structure of the HMW-GS in three different solvents [aqueous 50% (v/v) 2-propanol, aqueous 0.1% (v/v) trifluoroacetic acid, and trifluoroethanol]. For the first one, the influence of temperature on HMW-GS structure was also investigated.

Front-surface absorbance spectra of wheat flour: determination of carotenoids.

Front-surface absorbance spectra of wheat flours in the 250-650 nm region can be obtained by measuring reflectance spectra with a conventional spectrofluorometer suitably set to detect light scattered from powder samples. The spectra recorded on flour samples, obtained from seeds of four bread and five durum wheats, show high-intensity absorption bands due to aromatic amino acids of wheat proteins and low-intensity bands due to chromophores bound to low-molecular-weight compounds. The intensity of these last bands is proportional to the concentration of the corresponding chromophores present in the flour; thus, it can be used to measure the content of the compounds containing the chromophore(s). In particular, a quantitative determination of the carotenoids actually present in the flours is made, obtaining information on the original content of the seeds. This determination is important, as, for example, xanthophylls are well-known antioxidants and free-radical scavengers involved in aging processes of seeds. Reflectance measurements on powder samples are far more economic in terms of time and materials consumption than methods such as extraction and HPLC analysis of extracts and, in addition, give an evaluation of the overall content of carotenoids with absorption bands in the spectral range 450-500 nm. Application of the technique to other food powders with low-intensity absorption bands in the near-UV and vis region is possible.

Determination of the photochemical efficiency of o-quinodimethane ring closure in room-temperature solutions by using time-delayed, two-color photolysis technique

Photochemical efficiency of o-quinodimethane (3) ring closure at room temperature was determined by using a time-delayed, two-color photolysis technique. o-Quinodimethane (3) was generated by the photolysis of 1,2-bis[(phenylseleno)methyl]benzene (1) by a KrF (248 nm) laser pulse and thus-generated 3 was photolyzed by a subsequent XeCl (308 nm)/XeF (351 nm) laser pulse with varying delay time of 0 to 3 s. The time profile of 3 was monitored by the chemical analyses of benzocyclobutene (5) (a photochemical product of 3), which was formed by a one-photon process, and the spiro dimer of 3 (4) (a thermal product of 3) in the two-color photolysis experiments. The time profile of 3 followed a second-order decay kinetics. The photochemical efficiency was obtained by the analysis of the delay-time dependence of the product yields; those of the consumption of 3 and the conversion 3-->5 by a single pulse of the excimer laser were 81% and 5.7% for the XeCl laser, and 73% and 2.3% for the XeF laser. This difference was attributed to the different excited states involved in the photolysis. In contrast to the photolysis of 3 in argon or rigid organic matrixes, it was revealed that photochemical conversion 3-->5 was not the main path in the solutions, and intermolecular reactions predominated.

Degradation of gluten by proteases from dry and germinating wheat (Triticum durum) seeds: An in vitro approach to storage protein mobilization.

Vital gluten was used as an ideal substrate to investigate the role of some proteases in storage protein degradation. Aspartic proteinase and carboxypeptidase were identified as endogenous enzymes adsorbed on gluten and their optimum pH values determined. SDS-PAGE of soluble products released by gluten digestion revealed that the activity of these proteases plays a minor role in protein mobilization, whereas cysteine proteinase, purified from wheat seeds at the fourth day of germination, is extremely effective, producing a remarkable protein degradation in short times. Synergistic effects of aspartic and cysteine proteinase were not observed. Spin labeling of the sulfhydryl groups of gluten proteins enabled a comparative EPR investigation of the consequences of proteolytic degradation on gluten elasticity. It was found that storage protein mobilization brings a loss of elasticity to the polymeric network of gluten, which is particularly marked when the hydrolysis is performed by cysteine proteinase.

Fluorescence of cereal flours.

Front-surface fluorescence spectra of cereal flours are easily measured with a commercial spectrofluorometer and a simple cell. The emission spectra are intense in the range 290-600 nm in which three emission bands are active. The most intense is due to the aromatic amino acid residues present in the proteins of flours. At least two other fluorophores are active in the above spectral range and compete for light with the chromophores present in the flours. Four absorption bands are revealed in the same spectral region by reflectance spectra, the most intense being due to the amino acidic fluorophores. Thus, the measured emission is the result of absorption, scattering, emission, and reabsorption processes. Information on the microenvironment of some fluorophores can be obtained. The possibility to recognize different species of cereals, even of different cultivars, is shown.

Aging, free radicals, and antioxidants in wheat seeds.

Free radical oxidative attack is considered a major cause of disruption and deteriorative changes observed in aged seeds. Antioxidant defense mechanisms may remove potentially damaging molecular species, and carotenoids may act as radical scavengers. The content of lutein, the major carotenoid in wheat seeds, was determined in the flours. It showed a rapid decrease during seed aging. In addition, the content of free radicals in glutens made from flours of wheat seeds after long-term storage was studied. The concentration of radicals appeared to be age dependent, because the highest content of radicals was detected between 13 and 15 years of aging over 36 years of storage. Specific spin labeling of the sulfhydryl groups of gluten proteins enabled comparative EPR studies of the rigidity of the protein chains. A progressive stiffening of polymeric gluten with seed storage was found.

CD and fluorescence studies of the human granulocyte-macrophage colony-stimulating factor and related peptide conformations in aqueous solution.

The absorption, CD, and fluorescence emission spectra, and the fluorescence emission and depolarization lifetimes of the human granulocyte-macrophage colony-stimulating factor (hGM-CSF) and related peptides previously tested for their immunological activity, were measured in water at various pHs and temperatures to obtain information on their conformation in solution. The aim was to correlate the amino acid sequences, and the chain conformations and dynamics of the peptides, with their immunological properties. The CD spectrum of hGM-CSF revealed, as expected, a structure in solution similar to that in the crystalline state, but the fluorescence data suggest that the Trp 122 residue is more accessible to the solvent than the x-ray data would lead one to expect. They also suggest that some flexibility exists between the protein's two domains, one made up of the alpha-helices A and C and the other of the alpha-helices B and D plus the two beta-strands. In aqueous solution, none of the tested peptide CD spectra could be linked to a recognizable ordered conformation, i.e., an alpha-helix or a beta-sheet. The fluorescence of the peptide 11-24 suggests that the Trp 13 residue may appear in two types of situations: (a) in aqueous solution and (b) within a globular structure. Its CD spectra show that the tryptophan residue exists in both cases in a highly asymmetric environment independent of the pH.

Circular dichroism of ketoprofen complexed to serum albumins: conformational selection by the protein: a novel optical purity determination technique.

Complexation of 2-(3'-benzoylphenyl)propionic acid (ketoprofen), 1, to bovine serum albumin (BSA) results in an intense negative circular dichroism in the ketonic n-->pi* band of the benzoylphenyl moiety. This high CD contrasts with the weak CD of 1-enantiomers dissolved in common solvents. Furthermore, a number of chiral and achiral molecules containing the benzophenone moiety are easily complexed to BSA: all these complexes show an intense CD at the same transition. To account for the observed CD intensities of the above molecules, it appears that BSA complexation markedly shifts the equilibrium between strongly asymmetric, antipodic conformers. Dissymmetry of these conformers is connected to the instability of a structure with phenyl rings coplanar to the carbonyl chromophore, as also indicated by molecular mechanics calculations. The magnification of the Cotton effects of the 1-antipodes, due to the protein, can be used to measure the optical purity of 1-samples with excellent precision. In contrast with BSA, human SA is unable to recognize the chirality of 1-antipodes; oleic acid cocomplexation modifies this fact as well as other features of the binding.

Thiol-dependent metal-catalyzed oxidation of bovine lens aldose reductase. II. Proteolytic susceptibility of the modified enzyme form.

Bovine lens aldose reductase (alditol: NADP+ oxidoreductase, EC 1.1.1.21) undergoes a modification induced by 2-mercaptoethanol in the presence of the redox system Fe(II)/Fe(III). The modified form (ARa) exhibits an increased hydrophobicity and tendency to aggregate. Moreover, while the native enzyme form is rather insensitive to proteolytic breakdown, the modified form is susceptible to limited proteolysis by trypsin and chymotrypsin. With both proteases, the degradation correlated with a loss of enzyme activity and results in the appearance of one molecular species of 26 KDa (for chymotrypsin) and two molecular species of 24 and 17 KDa (for trypsin). The decline in solubility and the increase in susceptibility to proteolysis of ARa suggests that the thiol-dependent metal-catalyzed modification is comparable to other oxidative systems that mark proteins for degradation.

Bovine lens aldose reductase: tight binding of the pyridine coenzyme.

Analysis by HPLC of the protein-free supernatant obtained after denaturation of aldose reductase shows that the native form of the enzyme (ARb) contains a tightly bound NADP+, which is absent in the oxidatively modified form (ARa). The absorption, fluorescence, and circular dichroism spectra of ARb and ARa are consistent with the presence of the cofactor only in the native form of aldose reductase. On the other hand, the modified enzyme, in appropriate thiol reducing conditions, can tightly bind NADP+. This indicates a potential reversibility of the modification of aldose reductase, at least in terms of retention of the cofactor.

Chiral recognition by biological macromolecules. Partial resolution of racemic enones by albumin.

A novel approach to the optical resolution of racemic enones has been introduced by using the binding properties of the transport protein albumin, which chemically binds preferentially one antipode of some alpha, beta-unsaturated cyclic enones in a reversible manner. Simple separation of the macromolecules by ultrafiltration leads to partial resolution of racemates.

A mechanical model for the half-of-the-sites reactivity of oligomeric enzymes.

We developed a model which is able to provide a rationalization of the half-of-the-sites reactivity of oligomeric enzymes. According to this model, a dimeric enzyme is considered as a system of two coupled oscilators, which are able to transmit energy (information) to each other via a weak elastic coupling. In such a system, the energy may fluctuate between the two coupled elements so as to accumulate in one of the two at a time, i.e., at one time a certain energy state will be present in only one of the two elements, if this energy state (protomer conformation) is relevant for the chemical reaction, the conditions of half-of-the-sites reactivity may be fulfilled. The limits, and possible generalization, of this model are discussed.