Nisin is a 3.4-kDa antimicrobial peptide that, as a result of posttranslational modifications, contains unsaturated amino acids and lanthionine residues. It is applied as a preservative in various food products. The solubility and stability of nisin and nisin mutants have been studied. It is demonstrated that nisin mutants can be produced with improved functional properties. The solubility of nisin A is highest at low pH values and gradually decreases by almost 2 orders of magnitude when the pH of the solution exceeds a value of 7. At low pH, nisin Z exhibits a decreased solubility relative to that of nisin A; at neutral and higher pH values, the solubilities of both variants are comparable. Two mutants of nisin Z, which contain lysyl residues at positions 27 and 31, respectively, instead of Asn-27 and His-31, were produced with the aim of reaching higher solubility at neutral pH. Both mutants were purified to homogeneity, and their structures were confirmed by one-and two-dimensional 1 H nuclear magnetic resonance. Their antimicrobial activities were found to be similar to that of nisin Z, whereas their solubilities at pH 7 increased by factors of 4 and 7, respectively. The chemical stability of nisin A was studied in the pH range of 2 to 8 and at 20, 37, and 75؇C. Optimal stability was observed at pH 3.0. Nisin Z showed a behavior similar to that of nisin A. A mutant containing dehydrobutyrine at position 5 instead of dehydroalanine had lower activity but was significantly more resistant to acid-catalyzed chemical degradation than wild-type nisin Z.

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(Received 21 May112 August 19Y6) -EJB 96 074213Nisin, a 34-residue peptide bacteriocin, contains the less common amino acids lanthionine, p-methyllanthionine, dehydroalanine (Dha), and dehydrobutyrine (Dhb). Several chemically modified nisin A species were purified by reverse-phase HPLC and characterized by two-dimensional NMR and electrospray mass spectrometry. Five constituents, [2-hydroxy-AlaS]nisin, [Tle4-amide,pyruvyl-Leu6Jdes-Dha5-nisin, [Met(0)2l]nisin, [Ser33]nisin, and nisin-(I -32)-peptide amide, were found in a commercial nisin sample. A further species, [2-hydroxy-AIaS]nisin-( 1 -32)-peptide amide, was obtained by freeze drying an acidic nisin solution. These compounds are formed by chemical modification of nisin: the addition of a water molecule to the dehydroalanine residues, which can lead to the cleavage of the polypeptide chain, or the oxidation of methionine residues.The 2-hydroxyalanine-containing products have a limited stability; they are spontaneously converted into the corresponding des-dehydroalanine derivatives. The growth-inhibiting activity of the modified nisins towards different bacteria was determined. The 2-hydroxyalanine-containing species and the desdehydroalanine derivative show a strong reduction in biological activity as compared to native nisin.[Met(0)2l]nisin and [Ser33]nisin show moderate or no reduction in biological activity.

Much research into the fundamentals of membrane formation and separation has been performed in order to improve the efficiency of the manufacture of ultrafiltration membranes. Determination of the membrane characteristics is a key problem in these investigations. In this paper, we report on a study of membrane morphology by fractional rejection measurements, using low molecular weight saccharides as the test solute, and by electron microscopy. Using a simple model for solute/solvent transport through cylindrical pores, a "characteristic pore size" was derived from saccharide rejection data. This pore size of a hypothetical isoporous membrane, interpreting the measured separation characteristics, provides a promising means of describing differences between membranes with respect to pore size and pore size changes caused by solute adsorption. From high resolution electron micrographs, information was obtained on the skin layer morphologies and, for some membranes the sizes of the larger pores could be estimated.

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Hydrodynamic radii from inelastic light-scattering experiments and radii of gyration from Zimm plots give an indication of the change of average casein micelle size when the pH is changed. Combination of the results of both types of measurements gives information on changes in the micelle protein matrix, i.e. changes in the voluminosity.The voluminosity was also determined by the pellet method and by electron microscopy which also provided comparative data on size parameters.Fermentation of milk to obtain a variety of consumable acid gels and fluids is an important process in the dairy industry. To understand the changes in milk that bring about the eventual physical properties of the products, detailed knowledge on a molecular level is needed of what happens to casein micelles when milk is acidified. This study combines the results of an intrinsically number-average technique, electron microscopy, with weight and Z-average techniques offered by light scattering. In this way the whole casein micelle size distribution can be studied for acidified milk samples.Data were obtained for a slightly pasteurized milk which is used extensively in cheese-making in the Netherlands. MATERIALS AND METHODS VoluminositySkim milk was cooled to 4 °C and the pH adjusted to the desired value; the milk was stored overnight at 4 °C. In the morning the milk was heated to 30 °C and kept at that temperature. The pH was checked and readjusted if necessary. After 1 h the pH was checked again. The samples were spun at 29000 rev/min (88000 g) for 2 h in a Beckman preparative ultracentrifuge at 30 °C using the 30 rotor, which had been preheated to 30 °C. The pellet was weighed before (w^) and after (w 2 ) freeze drying. The freeze-dried pellet was ground in a mortar and the nitrogen content of the powder (N) was determined. The voluminosity was calculated from: _ ( K -w 2 ) + w 2 [0-55 + (0-74-0-55) Nx 6-35/100]} V~ iv 2 Nx 6-35/100

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