Biomedical subjects

P L Irwin

Publications and source records attributed to P L Irwin.

3 recordsLinked to original sources

PubMed1996

Polymerized cyclomaltoheptaose (beta-cyclodextrin, beta-CDn) inclusion complex formation with chlorogenic acid: solvent effects on thermochemistry and enthalpy-entropy compensation.

The inclusion of chlorogenic acid (CA) by epichlorohydrin-polymerized cyclomaltoheptaose (beta-cyclodextrin, beta-CDn) was studied with regard to temperature, solvent, and water activity aH2O approximately mole fraction = XH2O = 0.8-1 using MeOH as the diluent; 0.1 M sodium phosphate buffer). We discovered that the extreme convex curvature in K (the apparent stability constant) as a function of temperature was nearly eliminated at the lowest XH2O. The latter finding argues that this unusual CD behavior in aqueous media was due to perturbations in beta-CD's spatial organization in the polymeric matrix with temperature. Related to this we found, from the dependence of K on XH2O (K = K'XzH2O), that the beta-CDn.CA complex's stoichiometric coefficient, z, for water, varied between 5 and 8, depending on the temperature of the solution (K' = 400-800 M-1; T approximately 295-315 K). Our determinations of z were similar to those reported previously for beta-CD.(+)-limonene (z approximately 7), soluble beta-CD.CA (z approximately 6) or obtained by molecular dynamics calculations for beta-CD.CA reported herein (z approximately 5). However, beta-CDn.CA's z values did show a significant positive correlation with temperature not evident in equivalent solution experiments. Calculations of delta H and delta S at various XH2O values show linear enthalpy-entropy compensation (delta H plotted against delta S) but with a slope (Tc = theta delta H/theta delta S approximately 228 K) significantly less than Tc values determined from either standard aqueous thermodynamic experiments (Tc approximately 305 K on either beta-CD or beta-CDn) or variable XH2O (Tc approximately 272 K) experiments. To the best of our knowledge, this is the smallest Tc value detected in a multitude of CD.guest studies. This evident solvent effect on Tc strongly argues that the chemical part process of inclusion complex formation involves changes in the solvation of the beta-CDn's binding site.

Carbohydrate Conformation↗

PubMed1994

Binding geometry, stoichiometry, and thermodynamics of cyclomalto-oligosaccharide (cyclodextrin) inclusion complex formation with chlorogenic acid, the major substrate of apple polyphenol oxidase.

The inclusion complexes of cyclomaltohexaose (alpha-CD), cyclomaltoheptaose (beta-CD), cyclomaltooctaose (gamma-CD), and polymerized beta-CD (beta-CDn) with chlorogenic acid (CA), the major substrate of apple fruit polyphenol oxidase (PPO), were studied with regard to pH, ionic strength, and temperature in model buffer systems and apple juice. The thermodynamics of CD.CA inclusion complex formation, which were studied in solution using UV spectrophotometry, displayed enthalpy-entropy compensation typical of processes driven by solvation phenomena. We also found that the apparent association constants (K) of the CD.CA equilibrium were relatively insensitive to pH for beta-CD, compared to alpha- and gamma-CDs, but were subject to substantial enhancement at low ionic strengths. The beta-CD.CA inclusion complex was also characterized for binding geometry and stoichiometry at 9.4 T and 25 degrees C in 0.05 M Na phosphate buffer by 1H NMR spectroscopy. A 1:1 stoichiometric ratio for the complex was found using the method of continuous variations. 1H Spin-lattice relaxation and chemical-shift data indicate that the phenolic ring of CA docks within the cavity of beta-CD. The Ks for beta-, alpha-, and gamma-CD determined in apple juice, which contains a mixture of PPO substrates, were found to correlate with PPO activity-related data. Apple juice, treated with beta-CDn, did not brown until CA was added back. These latter findings strongly argue that the mechanism for inhibition of juice browning with cyclodextrins was mainly due to the binding of PPO substrates and not some other means such as enzyme inactivation via sequestration of Cu2+ by CDs.

Binding Sites↗

PubMed1992

Assay of reducing end-groups in oligosaccharide homologues with 2,2'-bicinchoninate.

For the first time, reducing values of homologous series of oligosaccharides have been determined by the 2,2'-bicinchoninate assay. The extent of Cu2+ reduction was monitored by spectrophotometric measurement of Cu+/2,2'-bicinchoninate complexes. Conditions of the assay were optimized so that relatively uniform reducing values were obtained with oligosaccharides derived from starch, polygalacturonic acid, and chitin, regardless of degree of polymerization. The uniform values resulted because members of each oligosaccharide series were oxidized to similar levels beyond the aldonic acid stage, while oxidation was limited to the reducing-terminal residue. This conclusion was based on direct measurements of quantitative loss of paramagnetic copper (Cu2+) by electron paramagnetic resonance spectroscopy.

Chitin↗