Rapid Kinetic Interactions of Sugar and Sugar Alcohol with Sweet Taste Receptors on Live Cells Using Stopped-Flow Spectroscopy.

The subjective measurement of the dynamic perception of sweetness is a problem in food science. Herein, the rapid interactions of sugars and sugar alcohols with sweet taste receptors on living cells on a millisecond timescale were studied via stopped-flow fluorescence spectroscopy. According to the rapid-kinetic parameters, sweeteners were divided into two groups. Sweeteners in group I disrupted the hydrogen bond network structure of water, and the apparent rate constant (kobs) was in the range of 0.45-0.6 s-1. Sweeteners in group II promoted the hydrogen bond formation of water, and the kobs was mostly in the range of 0.6-0.75 s-1. For most sweeteners, the kobs of cell responses was negatively correlated with the apparent specific volume of sweeteners. The differences in the cellular responses may be attributed to the disturbance in the water structure. Experimental results showed that the kinetic parameters of sweet cell responses reflected the dynamic perception of sweetness. Rapid kinetics, solution thermodynamic analysis, and water structure analysis enriched the physicochemical study of the sweetness mechanism and can be used to objectively evaluate the dynamic perception of sweetness.

Effect of Ionic and Non-Ionic Surfactants on the Pasting Characteristics and Digestive Properties of Regular and Frozen Starch for Oral Delivery.

Starch is an ideal wall material for controlled release in oral delivery systems due to its non-allergic properties, availability, and cheap price. However, because of its poor mechanical behavior and high water permeability, it is necessary to modify the amphiphilic nature of starch. Surfactants are essential components to emulsify the lyophobic food ingredients. However, the interaction of starch with emulsifiers and how they affect the pasting behavior and digestion of starch are not well understood. In this paper, surfactants, such as non-ionic Tween (TW) and ionic sodium fatty acid (NaFA), with varying hydrophobic carbon chain lengths, were selected as model amphiphiles to investigate the structural, pasting, rheological properties and in vitro digestibility of regular and frozen starch samples. The results showed that, in most cases, the addition of TW reduced the viscosity of starch. However, saturated medium-chain NaFA increased the starch viscosity and rheological modulus greatly. Both surfactants inhibited starch digestion. This paper presents a comparative investigation on the effect of ionic and non-ionic surfactant on the structure and properties of corn starch, and therefore the information is useful for structural-based formulation with starch for developing colloidal delivery systems. It is also helpful for developing functional food with controllable digestion properties.

l-Arginine inhibits the activity of α-amylase: Rapid kinetics, interaction and functional implications.

In this work, the rapid unfolding kinetics of pancreas α-amylase (PPA) induced by l-arginine and the interaction mechanism were investigated. The unfolding followed a first-level reaction kinetics equation, without intermediates. l-arginine interacted with PPA though diffusion-controlled process rather than complexion. The interaction between l-arginine and PPA resulted in a pronounced decrease in ß-sheet and a significant increase in random coil, and thereby the enzyme activity decreased. However, the unfolding of PPA could be compensated and the second structure change could be recovered to some extent by the macromolecular crowded medium of Pluronics. Further insight into the mechanism disclosed that the broken H-bond network of water may contribute to PPA unfolding. This work provides a new perspective on the interaction of l-arginine with digestive enzyme. The unfolding mechanism of enzymes by may help to understand the effects of other structurally similar drugs, which is of concern in food-drug interactions.

Rapid unfolding of pig pancreas α-amylase: Kinetics, activity and structure evolution.

α-Amylase plays an important role in food processing and in-vivo digestion. Many biological functions of α-amylase are affected by unfolding. The pre-steady-state rapid unfolding kinetics of α-amylase remains unknown. In this study, the rapid unfolding kinetics of porcine pancreatic α-amylase (PPA) with guanidine hydrochloride (GdmHCl) were investigated by stopped-flow spectroscopy. Structural characterization of PPA by fluorescence spectroscopy, and molecular dynamics simulation showed that the unfolding process of PPA might start from the internal active center, where the ß-sheet structure was destroyed, followed by the exposure of hydrophobic amino acid residues. Further research revealed that GdmHCl denaturized PPA not by complexing with PPA. The surrounding H-bond network of water was changed by GdmHCl. This research improves our understanding of the unfolding kinetics of the PPA on the microsecond scale. It also provides the evidence experimentally of the surrounding water contribution to protein denaturization.