Changes in the structure and protein binding ability of condensed tannins during decomposition of fresh needles and leaves

Alterations of the chemical structure of condensed tannins (CT) during decomposition of Norway spruce (Picea abies) needles and white willow (Salix alba) leaves were investigated by gel permeation chromatography (GPC), ¹³C nuclear magnetic resonance (NMR) spectroscopy, and matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry (MS). The effect of these alterations on their protein binding capacity was determined by radial diffusion assay on an agarose plate. For these studies an incubation experiment was performed with spruce needles and willow leaves. From the fresh foliage and its decomposed materials obtained after 4-days, and 1-, 2-, 4-, and 8-weeks of incubation, CT were extracted and analyzed. Dynamics of CT alterations during decomposition of the fresh foliage were different for the two plant species, although the amount of extractable CT for both decreased soon after incubation and only slight amounts of CT were extractable after 8 weeks of incubation. The decrease was faster for the willow leaves than for the spruce needles. Solution ¹³C NMR revealed alterations of CT isolated from both degrading plant materials, but only to a small extent. However, considerable changes in chemical composition and chain length of the CT were detected by MALDI-TOF MS. Changes in the chemical composition of CT are expected to decrease the protein binding capacity: Applying the radial diffusion assay, this assumption was confirmed for spruce CT, but not for willow CT. This may be explained by (1) higher reactivity of prodelphinidin (PD) than procyanidin (PC), the former is contained in spruce CT but not in willow CT and (2) the slower decomposition rate of spruce needles than willow leaves. Thus, CT in spruce needles may have suffered a greater extent of chemical alteration, which formed partially altered CT with less protein binding capacity. Further, since the decomposition rate of spruce needles was slower than that of willow leaves, the partially altered CT remained extractable for a longer incubation period compared with the willow leave's CT.