The conformational change in the selectivity filter of KcsA as a function of ambient potassium concentration is studied with solid state NMR. This highly conserved region of the protein is known to chelate potassium ions selectively. We report solid-state NMR chemical shift fingerprints of two distinct conformations of the selectivity filter; significant changes are observed in the chemical shifts of key residues in the filter as the buffer potassium ion concentration is changed from 50 mM to 1 µM. Potassium ion titration studies reveal that the site-specific K d for K + binding at the key pore residue Val 76, is on the order of ∼7 µM and that relatively high sample hydration is necessary to observe the low K + conformer. Simultaneous detection of both conformers at low ambient potassium concentration suggests that the high K + and low K + states are in slow exchange on the NMR timescale (k ex < 500 s −1 ). The slow rate and tight binding for evacuating both inner sites simultaneously, differ from prior observations in detergents in solution, but agree well with measurements by electrophysiology, and appear to result from our use of a hydrated bilayer environment. These characteristics rule out participation of the low K + state on the timescale of ion transmission, which has been assumed to involve interchange of states where one of the inner binding sites is always occupied. On the other hand, these kinetic and thermodynamic characteristics of evacuation of the inner sites certainly could be compatible with participation in a control mechanism at low ion concentration, such as C-type inactivation, a process that is coupled to activation and involves closing of the outer mouth of the channel.

The effectiveness of using protease and combinations of protease and high-intensity ultrasound for high-purity, high-yield tamarind seed polysaccharide (TSP) production was investigated. Tamarind kernel powder (TKP) suspension was treated with protease alone at 0.16, 0.48, and 0.80 units/mL and with protease-ultrasound combinations over 3 different orders of sequence (before, simultaneous with, and after protease digestion) using combinations of 0.48 units/mL protease and high-intensity ultrasound at 25% and 50% amplitude for 15 and 30 min. The long protease digestion time could produce high-purity isolated TSP, but the polysaccharide yields were lower. The polysaccharide purity and yield were highly improved, even at a shorter protease digestion time, when the protease treatment was combined with high-intensity ultrasound. The increased amplitude level and sonication time decreased the average molecular weight of the polysaccharide. The rheological properties of the TKP and the isolated TSP, from nondestructive oscillatory measurements, demonstrated that the latter present a viscoelastic solution. The decreasing of protein content resulted in better elasticity of the solution. The power law model could be used to fit the down curve between shear rate and shear stress data. The consistency coefficient (K) increased while the flow behavior index decreased with the increased purity of the polysaccharide as a result of increasing increased digestion time, enzyme concentration, sonication power, and sonication time.

In this paper, we study how occupation (or industry) tradability shapes local labor‐market adjustment to immigration. Theoretically, we derive a simple condition under which the arrival of foreign‐born labor into a region crowds native‐born workers out of (or into) immigrant‐intensive jobs, thus lowering (or raising) relative wages in these occupations, and we explain why this process differs within tradable versus within nontradable activities. Using data for U.S. commuting zones over the period 1980–2012, we find—consistent with our theory—that a local influx of immigrants crowds out employment of native‐born workers in more relative to less immigrant‐intensive nontradable jobs, but has no such effect across tradable occupations. Further analysis of occupation labor payments is consistent with adjustment to immigration within tradables occurring more through changes in output (versus changes in prices) when compared to adjustment within nontradables, thereby confirming our model's theoretical mechanism. We then use the model to explore the quantitative consequences of counterfactual changes in U.S. immigration on real wages at the occupation and region level.

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