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Binding affinity and capacities for ytterbium(3+) and hafnium(4+) by chemical entities of plant tissue fragments

Texas A&M University, College Station 77843

⁴ Correspondence:
Department of Animal Science, 2471 TAMU (phone: 979-845-5063; fax: 979-845-5292; E-mail:
w-ellis{at}tamu.edu).

The binding affinity of ytterbium (Yb³⁺) and hafnium (Hf ⁴⁺) to ligands of chemical entities of fragments of bermudagrass tissues and their resistance to exchanging Yb with other ligands and to displacement by protons were investigated. Chemical entities of acid resistant NDF (ARNDF), 0.1 N acid detergent fiber (0.1 N ADF), and permanganate cellulose (CELL) were prepared from fragments of bermudagrass hay (Cynodon dactylon [L.] Pers.) obtained by grinding to pass a 2-mm sieve. ¹⁷⁵Ytterbium and Yb, as YbCl₃, were initially bound to each preparation by soaking for 12 h in pH 5.5 borate buffer to obtain Yb bound onto ligands having affinity constants for Yb equal to or greater than that for the weakly stable borate ligand, Yb borate. The fraction of Yb borate was measured and fragments then sequentially exposed to acetate, citrate, nitrotriacetate (NTA), and EDTA ions to allow exchange of Yb from Yb borate with ligands having affinity constants for Yb equal to or greater than acetate (Yb acetate), citrate (Yb citrate), NTA (Yb NTA), and EDTA (Yb EDTA) ions. Binding of Yb borate indicated the existence of two species of ligands: strong ligands binding essentially 100% of added Yb at levels of 1 to 1,300 ppm (0.1 N ADF) and at 1 to 7,000 ppm (ARNDF); and weaker ligands binding 4 and 8% of the Yb, respectively, at levels of added Yb greater than 1,300 ppm and 7,000 ppm. Ytterbium acetate of ARNDF, but not 0.1 N ADF, was as resistant to exchange as Yb citrate. Ytterbium borate was exchanged extensively (85% or greater) with soluble ligands having affinity constants NTA. Ytterbium resistance to proton displacement at pH of 1.5 increased with Yb EDTA > Yb NTA > Yb citrate > Yb acetate. Very efficient binding of Yb to CELL suggested that such chemical preparations are not representative of native cellulose. Hafnium (4+) was strongly bound to plant tissues rendering both Hf and Hf-bound DM insoluble at a pH of 1.5 and insoluble in a modified NDF solvent without EDTA. It is concluded that Yb specifically applied as Yb acetate and Hf⁴⁺ are indelible markers for estimating sojourn time of undigested plant tissues at the normal pH of the rumen. Because of its resistance to proton displacement, Hf⁴⁺ would be an indelible marker for estimating sojourn time in more acidic postgastric segments of the gastrointestinal tract.

Key Words: Ytterbium • Plant Tissues • Markers

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