Unsteady solute dispersion in the presence of reversible and irreversible reactions

In an unsteady pulsatile non-Newtonian fluid past a tube with a thin wall layer, the dispersion of a narrow uniform slug of injected solute over a cross-section is examined. At the interface between the mobile fluid phase and the immobile wall phase, both irreversible and reversible reactions have been adopted. The Carreau-Yasuda model is used to describe the fluid's rheology. The impacts of fluid rheology and reaction parameters on the concentration profiles in the fluid- and wall-phases and the three transport coefficients, viz, the depletion coefficient (K-0), the convection coefficient (K-1), the dispersion coefficient (K-2) in the fluid phase are predicted numerically. A considerable shift in the behaviour of K-1 and K(2 )with a higher reaction rate may be observed in the transient stage. The axial dispersion of mobile-phase concentration in the unsteady Carreau-Yasuda II fluid model is significantly larger than in Poiseuille and steady Carreau-Yasuda II fluid models, and flow pulsatility on the immobile-phase concentration is prominent upstream at a longer time. In addition, the peak value of the mobile-phase section-mean concentration is consistently lower than in other fluid models. This study could help researchers to understand the drug delivery in blood vessels and pulmonary mechanical ventilation. (C) 2022 The Author(s) Published by the Royal Society. All rights reserved

Unsteady solute dispersion in the presence of reversible and irreversible reactions

In an unsteady pulsatile non-Newtonian fluid past a tube with a thin wall layer, the dispersion of a narrow uniform slug of injected solute over a cross-section is examined. At the interface between the mobile fluid phase and the immobile wall phase, both irreversible and reversible reactions have been adopted. The Carreau-Yasuda model is used to describe the fluid's rheology. The impacts of fluid rheology and reaction parameters on the concentration profiles in the fluid-and wall-phases and the three transport coefficients, viz, the depletion coefficient (K0), the convection coefficient (K1), the dispersion coefficient (K2) in the fluid phase are predicted numerically. A considerable shift in the behaviour of K1 and K2 with a higher reaction rate may be observed in the transient stage. The axial dispersion of mobile-phase concentration in the unsteady Carreau-Yasuda II fluid model is significantly larger than in Poiseuille and steady Carreau-Yasuda II fluid models, and flow pulsatility on the immobile-phase concentration is prominent upstream at a longer time. In addition, the peak value of the mobile-phase section-mean concentration is consistently lower than in other fluid models. This study could help researchers to understand the drug delivery in blood vessels and pulmonary mechanical ventilation. © 2022 The Author(s)

Enzymatic separation of epimeric 4-C-hydroxymethylated furanosugars: Synthesis of bicyclic nucleosides

Conversion of D-glucose to 4-C-hydroxymethyl-1,2-O-isopropylidene-α-D-ribofuranose, which is a key precursor for the synthesis of different types of bicyclic/spiro nucleosides, led to the formation of an inseparable 1:1 mixture of the desired product and 4-C-hydroxymethyl-1,2-O-isopropylidene-α-D-xylofuranose. A convenient environment friendly Novozyme®-435 catalyzed selective acetylation methodology has been developed for the separation of an epimeric mixture of ribo- and xylotrihydroxyfuranosides in quantitative yields. The structure of both the monoacetylated epimers, i.e., 5-O-acetyl-4-C-hydroxymethyl-1,2-O-isopropylidene-α-D-ribo- and xylofuranose obtained by enzymatic acetylation, has been confirmed by an X-ray study on their corresponding 4-C-p-toluenesulfonyloxymethyl derivatives. Furthermore, the two separated epimers were used for the convergent synthesis of two different types of bicyclic nucleosides, which confirms their synthetic utility

Synthesis of novel 3′-azido-3′-deoxy-α-L-<i>ribo</i> configured nucleosides: A comparative study between chemical and chemo-enzymatic methodologies

<p></p> <p>Syntheses of novel 3′-azido-3′-deoxy-2′-<i>O</i>,4′-<i>C</i>-methylene-<i>α-</i>L-<i>ribo</i>furanosyl nucleosides have been carried out from 3′-azido-3′-deoxy-4′-<i>C</i>-hydroxymethyl-β-D-<i>xylo</i>furanosyl nucleosides following both chemical and chemo-enzymatic methodologies. The precursor nucleoside in turn was synthesized from a common glycosyl donor 4-<i>C</i>-acetoxymethyl-1,2,5-tri-<i>O</i>-acetyl-3-azido-3-deoxy-<i>α,β</i>-D-<i>xylo</i>furanose, which was obtained by the acetolysis of 4-<i>C</i>-acetoxymethyl-5-<i>O</i>-acetyl-3-azido-3-deoxy-1,2-<i>O</i>-isopropylidene-α-D-<i>xylo</i>furanose in 96% yield. It has been observed that a chemo-enzymatic pathway for the synthesis of targeted nucleosides is much more efficient than a chemical pathway, leading to the improvement in yield for the synthesis of 3′-azido-3′-deoxy-<i>α-</i>L-<i>ribo</i>furanosyl thymine and uracil from 49 to 89% and 55 to 93%, respectively.</p