NMR proton relaxation measurements of water associated with high methoxy and low methoxy pectins

Nuclear magnetic resonance was used to study water proton relaxation in high (HMP) and low methoxy pectins (LMP). In solution, on the order of 40% of the water molecules may be affected by interactions with pectin molecules, dependent upon pectin type and concentration. Transverse relaxation rates were greater for HMP than LMP, and were attributed to decreased mobility of water protons and the lesser number of methoxyl groups in LMP. Plots of relaxation rates (R-2 = 1/T-2) versus the 90-180 degrees pulse spacing were used to calculate the proton exchange rate (k(b)) HMP and LMP has k(b) values of 128 and 135 s(-1) at pH 2.5. For LMP k(b) increased to 280 s(-1) at pH 6.5, but decreased for HMP above pH 6.5. Dry samples were equilibrated to water activity values between 0.11 and 0.75. Proton relaxation was quite rapid in these systems, suggesting limited water mobility. At higher moisture levels, R-2 values were progressively lower and more evenly spread over a wider pulse spacing range, indicating additional populations of more mobile water. Studies on LMP which were gelled with Ca2+, showed a substantial increase in relaxation rates and little dispersion behavior, and this was attributed to the decreased mobility of pectin biopolymers. (C) 2000 Elsevier Science Ltd. All rights reserved.