Effect of molecular aggregation on circular dichroism and optical rotatory dispersion of helical poly-L-glutamic acid in solution

Beyond Stereoisomeric Effects: Exploring the Importance of Intermolecular Electron Spin Interactions in Biorecognition

This work shows that electron spin polarization and stereoisomeric effects make comparable contributions to the enantioselective binding of amino acids. Magneto-electrochemical quartz crystal microbalance methods are used to study the adsorption of chiral amino acids onto a monolayer film of chiral molecules that are spin polarized by an underlying ferromagnetic substrate. The direction of the electron spin polarization affects both the kinetics and thermodynamics of the enantiospecific adsorption of the amino acids. Comparison of these data with the circular dichroism (CD) spectra of the amino acid adsorbates shows that the CD spectrum of the interacting group provides a good figure-of-merit for predicting the contributions of electron spin to the intermolecular interaction. These findings demonstrate the importance of electron spin in enantioselective intermolecular interactions between chiral amino acids and represent a paradigm shift for how selectivity should be viewed in biorecognition.

Biomolecular dynamics studied with IR-spectroscopy using quantum cascade lasers combined with nanosecond perturbation techniques

Early events of protein folding can be studied with fast perturbation techniques triggering non-equilibrium relaxation dynamics. A nanosecond laser-excited pH-jump or temperature-jump (T-jump) was applied to initiate helix folding or unfolding of poly-l-glutamic acid (PGA). PGA is a homopolypeptide with titratable carboxyl side-chains whose protonation degree determines the PGA conformation. A pH-jump was realized by the photochemical release of protons and induces PGA folding due to protonation of the side-chains. Otherwise, the helical conformation can be unfolded by a T-jump. We operated under conditions where PGA does not aggregate and temperature and pH are the regulatory properties of its conformation. The experiments were performed in such a manner that the folding/unfolding jump proceeded to the same PGA conformation. We quantified the increase/decrease in helicity induced by the pH-/T-jump and demonstrated that the T-jump results in a relatively small change in helical content in contrast to the pH-jump. This is caused by the strong pH-dependence of the PGA conformation. The conformational changes were detected by time-resolved single wavelength IR-spectroscopy using quantum cascade lasers (QCL). We could independently observe the kinetics for α-helix folding and unfolding in PGA by using different perturbation techniques and demonstrate the high sensitivity of time-resolved IR-spectroscopy to study protein folding mechanisms.

Protonation and chloroplast membrane structure

Light changes the structure of chloroplasts. This effect was investigated by high resolution electron microscopy, photometric methods, and chemical modification. (a) A reversible contraction of chloroplast membrane occurs upon illumination, dark titration with H⁺, or increasing osmolarity. These gross structural changes arise from a flattening of the thylakoids, with a corresponding decrease in the spacing between membranes. Microdensitometry showed that illumination or dark addition of H⁺ resulted in a 13–23% decrease in membrane thickness. Osmotically contracted chloroplasts do not show this effect. (b) Rapid glutaraldehyde fixation during actual experiments revealed that transmission changes are closely correlated with the spacing changes and therefore reflect an osmotic mechanism, whereas the light scattering changes have kinetics most similar to changes in membrane thickness or conformation. (c) Kinetic analysis of light scattering and transmission changes with the changes in fluorescence of anilinonaphthalene sulfonic acid bound to membranes revealed that fluorescence preceded light scattering or transmission changes. (d) It is concluded that the temporal sequence of events following illumination probably are protonation, changes in the environment within the membrane, change in membrane thickness, change in internal osmolarity accompanying ion movements with consequent collapse and flattening of thylakoid, change in the gross morphology of the inner chloroplast membrane system, and change in the gross morphology of whole chloroplasts.

Construction and characterization of human salivary histatin-5 multimers

Human salivary histatin-5 (Hsn-5), a 24-amino acid polypeptide, is a potent candidacidal molecule. In this study, we have explored the following two hypotheses: More potent Hsn molecules may be achieved by duplication of the functional domain of Hsn-5 (C16, residues 9-24 of Hsn-5), and Hsn may act like other cationic peptides which aggregate and form channels across the target membrane. A PCR-based gene splicing by overlap extension (SOE) method was used to construct the DNA fragments encoding the following fusion molecules: Hsn-5--Hsn-5, Hsn-5--C16, and C16--C16. These constructs were expressed in E. coli, the proteins produced were purified, and their anticandidal activities as well as secondary structures were determined. Contrary to our hypotheses, results showed that none of the multimers possessed increased candidacidal activity. Specifically, C16--C16 and Hsn-5--C16 displayed candidacidal activity comparable with that of Hsn-5, while Hsn-5--Hsn-5 possessed significantly decreased candidacidal activity, yet all molecules retained an alpha-helical structure in a hydrophobic environment. Additionally, the circular dichroism data showed that Hsn-5 in an alpha-helical conformation does not aggregate in a hydrophobic environment, not even at 14- to 18-fold its physiological concentration. Our results suggest that the development of enhanced Hsn-5 molecules may not be achieved by duplication of its functional domain, and that Hsns may not act like other antimicrobial cationic peptides which aggregate and form channels across the target membrane.

Underlying assumptions in the estimation of secondary structure content in proteins by circular dichroism spectroscopy--a critical review

Recombinant DNA technology has made possible the large-scale production of proteins for pharmaceutical applications. As a result, there has been a renaissance in methodology which can provide information on the structural stability and character of these materials. Circular dichroism (CD) spectroscopy, with its sensitivity to the secondary structure adopted by the polypeptide chain, is a powerful tool in this regard. Quantitative analysis of the CD spectra of proteins is now wide-spread, aided by the availability of such algorithms on commercial instrumentation. However, there are basic assumptions made when conducting these calculations, many of which have not been addressed or summarized. Some of these assumptions are independent of the selection of basis spectra and the algorithm employed. These assumptions are listed and the available data concerning their validity is presented and discussed.

Conformational transitions of alpha-elastin

The alpha-elastin, the soluble product obtained from elastin by oxalic acid hydrolysis, has been studied from a conformational point of view in different conditions using circular dichroism spectroscopy. The spectral variations of the alpha-elastin dissolved in water and in different organic solvents/water mixtures have been examined in order to establish the effect of the solvents on the structural properties of the protein. The effect of calcium ions has also been studied on alpha-elastin dissolved either in water or in organic solvents. Finally, the effect of pH and temperature on the conformational properties of the alpha-elastin has been investigated.

Effect of periodate oxidation on the structure and properties of glucose oxidase

In order to elucidate the molecular structure of glucose oxidase (beta-D-glucose: oxygen 1-oxidoreductase, EC 1.1.3.4) and the roles of its carbohydrate moiety, chemical, physiochemical and immunological experiments were performed with enzyme samples before and after periodate oxidation. Hydrodynamic parameters indicated that the native enzyme was a globular protein with values of 1.21 for the frictional ratio and 43 A for the Stokes radius. The enzyme contained about 12% carbohydrate by weight, of which the main component was mannose. The periodate treatment decreased the carbohydrate content to about 40% of its original value. Slight modifications were detected in the absorbance spectrum and the content of arginyl residue. However, no significant alteration was brought about by this treatment in the catalytic parameters, immunological reactivities of the gross structure, not in the secondary and quaternary structures of the protein moity. Thermal denaturation temperature (about 72.5 degrees C) and the enthalpy of denaturation (about 450 kcal/mol) were common to the native and the periodate-oxodozed enzymes. The native was found to be quite resistant to sodium dodecyl sulfate and fairly stable to urea and heating. The periodate-oxidized enzyme was also stable to heat treatment, but it showed a diminished stability when denaturing agents were present. Kinetic analyses of the thermal inactivation processes showed that the entropy of activation was greatly decreased by the denaturing agents, especially in the case of the periodate-oxidized enzyme. It is concluded that the carbohydrate moiety of the enzyme plays a role in increasing the stability of the protein moiety, but does not directly participate in the catalytic activity, the immunological reactivity, or in maintaining the conformation of the enzyme protein.

Comparison of fungal glucose oxidases. Chemical, physicochemical and immunological studies

Glucose oxidases (beta-D-glucose:oxygen 1-oxidoreductase, EC 1.1.3.4) from two fungal genera (Aspergillus and Penicillium) were studied chemically, physicochemically and immunologically to elucidate the similarities and dissimilarities between these enzymes. Investigation of circular dichroism spectra revealed that these enzymes proteins possess essentially identical conformations. However, differences found in thermal inactivation parameters, catalytic parameters and quantitative immunological reactivities indicate that these enzymes must have some minor but distinct variations in their structures. Interestingly, it was observed that the Penicillium enzyme cross-reacted with the antiserum against the Aspergillus enzyme with an association constant of two orders of magnitude lower than that of the Aspergillus enzyme, and that the precipitin one of the Penicillium enzyme fused together with that of the Aspergillus enzyme in the immunodouble diffusion test. These results lead to the conclusion that these enzymes are closely related but not completely identical, and suggest that they might have evolved from a common ancestral precursor.

Purification and characterization of Aspergillus niger exo-1,4-glucosidase

A specific exo-1,4-glucosidase (1,4-alpha-D-glucan glucohydrooase, EC 3.2.1.3) from Aspergillus niger has been partially purified and subsequently characterized by biochemical, physico-chemical and optical methods. Molecular sieve chromatography yields an enzyme with maximal activity at pH 4.2-4.5 close to its isoelectric point. Reduction and carboxymethylation leads to complete loss of activity and O-acetylation of 3 of the 13 tyrosine residues results in loss of 20 % of the activity. Sodium dodecylsulfate-polyacrylamide gel electrophoresis indicates that the native enzyme consists of two major components of molecular weights 63 000 and 57 500, respectively. Small amounts of dissociated material of molecular weight 28 000 and 16 000 as well as aggregates of the order of 100 000 are also present to the extent of 2-5% of the total potein. Following reduction and carboxymethylation under forcing conditions, the bands around 60 000 diminish and the 28 000-30 000, 16 000 and aggregate bands are dominant...