Tryptophan Levels during Grape Ripening: Effects of Cultural Practices.

Some cultural practices that are carried out during the grape ripening period are associated with vine stress, including leaf removal, grape bunch removal, and vegetable cover crops. Additionally, several nitrogen and sulfur supplements have also been used directly on leaves during the last stage of the ripening period. In the work described here, five different cultural practices and the reference were applied in three replicates in the same vineyard. The evolution of tryptophan levels was evaluated from just after grape veraison until the harvest date. In some cases, certain specific treatments were also evaluated after the regular harvest date. The cultural techniques that involved the application of nitrogen led to higher levels of tryptophan at the harvest day when compared to other cultural techniques. It was also found that the application of nitrogen without sulfur had a faster effect on the level of tryptophan. It was established that a period of around 20 days is needed for the grapes to show clear differences in tryptophan levels after the application of nitrogen.

Effects of Different Freezing Treatments during the Winemaking of a Varietal White Wine with Regard to Its Phenolic Components.

In white wine production, the technique consisting of freezing whole or crushed grapes usually increases the levels of aroma-related compounds in the final wine products. However, this technique may affect phenolic compounds, among other chemical compounds. Phenolic compounds are crucial to white wines because of their susceptibility to oxidation and their role with regard to color stability. In this study, white wines made from Muscat of Alexandria grapes were subjected to two different freezing techniques: whole-bunch freezing and crushed-grape freezing. In addition, a pre-fermentative maceration was applied to each experiment in order to determine if the effects of freezing were comparable to those of maceration. The phenolic compounds studied were gallic acid, protocatechuic acid, caffeic acid, trans-coutaric acid, and epicatechin, which are the key compounds from the point of view of wine stability. The freezing of crushed grapes enhanced the extraction of phenolic compounds in comparison to the freezing of whole bunches of grapes without pre-fermentative maceration. On the other hand, the effect of pre-fermentative maceration was comparable to that resulting from freezing crushed grapes. This step made the must from whole frozen grapes having even larger levels of phenolic compounds. Without pre-fermentative maceration, freezing whole bunches of grapes only allowed a moderate extraction of phenolic compounds and produced wines with lower individual phenolic contents than those obtained through traditional winemaking procedures.

Minibands in twisted bilayer graphene probed by magnetic focusing.

Magnetic fields force ballistic electrons injected from a narrow contact to move along skipping orbits and form caustics. This leads to pronounced resistance peaks at nearby voltage probes as electrons are effectively focused inside them, a phenomenon known as magnetic focusing. This can be used not only for the demonstration of ballistic transport but also to study the electronic structure of metals. Here, we use magnetic focusing to probe narrowbands in graphene bilayers twisted at ~2°. Their minibands are found to support long-range ballistic transport limited at low temperatures by intrinsic electron-electron scattering. A voltage bias between the layers causes strong minivalley splitting and allows selective focusing for different minivalleys, which is of interest for using this degree of freedom in frequently discussed valleytronics.

Optimisation of an ultrasound-assisted extraction method for the simultaneous determination of phenolics in rice grains.

Fifteen phenolic compounds were determined in rice grains by ultrasound assisted extraction and ultra-performance liquid chromatography with photodiode array detection. Primarily, an UAE method has been developed and validated for the extraction of phenolics from rice grains. For the optimization, a Box-Behnken Design based on six factors including extraction temperature (10-70 °C), solvent composition (0-50% methanol in water), cycle (0.2-0.7 s-1), ultrasound amplitude (30-70%), and solvent to sample ratio (2.5:1 to 5:1) was employed. Multi-response optimization (MRO) was performed to develop a simultaneous extraction method of 15 phenolics. The optimal UAE conditions were: extraction temperature 45 °C, time 25 min, cycle 0.4 s-1, ultrasound amplitude 47%, solvent composition 80% methanol in water at pH 4.25, and sample to solvent ratio 1:5. Linearity, LODs, LOQs, precision and recovery were determined. In addition, the method was successfully applied to analyse a number of rice samples with different varieties.

Multiresponse optimization of a UPLC method for the simultaneous determination of tryptophan and 15 tryptophan-derived compounds using a Box-Behnken design with a desirability function.

A Box-Behnken design was used in conjunction with multiresponse optimization based on the desirability function to carry out the simultaneous separation of tryptophan and 15 derivatives by Ultra Performance Liquid Chromatography. The gradient composition of the mobile phase and the flow rate were optimized with respect to the resolution of severely overlapping chromatographic peaks and the total run time. Two different stationary phases were evaluated (hybrid silica and a solid-core-based C18 column). The methods were validated and a suitable sensitivity was found for all compounds in the concentration range 1-100µgL-1 (R2>0.999). High levels of repeatability and intermediate precision (CV less than 0.25% and 1.7% on average for the retention time and the signal area, respectively) were obtained. The new method was applied to the determination tryptophan and its derivatives in black pigmented glutinous and non-glutinous rice grain samples.

ESTUDIO POR ESPECTROFOTOMETRÍA UV-Vis DE LA REACCIÓN ENTRE LOS IONES CIANURO Y PICRATO. UN EJEMPLO PRÁCTICO DE APLICACIONES ANALÍTICAS Y ESTUDIOS CINÉTICOS / Uv-vs spectrophootometry study of the reaction between cianide ions and picrate. a practical example of analytical applications and kinetic studies / Estudo por espectrofotometria uv-is da reação entre os ions cianeto e picrato. uum exemplo prático de aplicações analíticas e estudos cinéticos

La cinética de la reacción entre los iones cianuro y picrato se estudió por espectro-fotometría UV-Vis. Se observó una reacción de orden uno para ambos reactivos, así como un desplazamiento batocrómico de la señal de máxima absorbancia, incrementando la concentración de cualquiera de los reactivos, asociada a la formación de complejos de diferente estequiometría y resultando en los consiguientes cambios en los parámetros espectrométricos. Estos resultados sustentan una interacción de tipo π para el complejo resultante, resultado que se pudo generalizar utilizando otras moléculas donadoras que muestran las posibles interferencias de algunos compuestos químicos en las determinaciones cualitativas y cuantitativas de la frecuencia cianogénica y la determinación cuantitativa de glucósidos cianogénicos en plantas.

The kinetics of the cianide-picrate reaction was studied on the basis of UV-Vis spectrophotometry. A first order reaction was found for each reagent. It was also observed a batocromic displacement of the maximum absorbance wavelength, by increasing the concentration of any of the reagents, this result is associated to the formation of different stoichiometry complexes and the subsequent spectrometric parameters change. These results support the existence of an interaction of the π kind for the resulting complex, and a parallel result was obtained by using other π electron donor molecules. This displays the evidence of plausible interferences of some chemicals in qualitative and quantitative analytical applications of this reaction, specially of those used in the cianogenic frequence and quantitaive cianogenic glycosides determination in plants.

A cinética da reação entre o íon cianeto e o picrato de sódio foi acompanhada por espectrofotometria no UV/Vis. Foi observada uma reação de primeira ordem para ambos reagentes, bem como um deslocamento batocrômico no Dmax, aumentando-se a concentração de qualquer um dos reagentes, associa-se a ela, a formação de complexos de diferentes estequiometrias e, consequentemente, mudanças nos parâmetros espectrométricos. Estes resultados sustentam a possibilidade de uma interação do tipo π para o complexo resultante e que pode ser aplicada, de forma generalizada, para outras moléculas doadoras, evidenciando possíveis interferências de alguns compostos químicos com as determinações qualitativa e quantitativa de derivados cianogênicos, bem como a determinação quantitativa de derivados glicosilados cianogênicos em plantas.