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Abstract: The stable physical properties of molecular imprints make them ideal artificial receptors, i.e., biosensor sensing elements, for detection systems against chemical and biological toxins, drugs, and environmental contaminants [B. Dave, B. Dunn, J. Selverstone Valentine, J. Zink, Anal. Chem. 66 (1994) 1120- 1127; O. Lev, M. Tsionsky, L. Rabinovich, V. Glezer, S. Sampath, I. Pankratov, J. Gun, Anal. Chem. 67 (1995) 22-30; R. Wang, U. Narang, P. Prasad, F. Bright, Anal. Chem. 65 (1993) 2671-2675; S.A. Piletsky, Y.P. Parhometz, N.V. Lavryk, T.L. Panasyuk, A.V. El'skaya, Sens. Actuators, B 18-19 (1994) 629- 631; M.P. Byfield, R.A. Abuknesha, Biosens. Bioelectron. 9 (1994) 373-400] and afford them advantages over traditional screening techniques such as purified receptors and antibodies which require elaborate preparative techniques and complex environments. Molecular imprints to the deadly castor bean Pectin Ricin (Ricinus communis, Toxin RCA(60)) and its 'A' and 'B' chains were prepared, and their respective binding constants determined using steady-state fluorescence. Stern- Volmer fluorescence quenching plots using iodide and acrylamide suggest imprint associated Ricin B chain tryptophans are more accessible to the solvent environment than those of the Ricin A chain. Scatchard analysis revealed two affinities for Ricin binding to Ricin imprints, i.e., K-d = 34 nM and 319 nM. Similarly, high affinity interaction of Ricin A and B chains with their respective imprints were observed (K-d congruent to 100 nM). Interestingly, Scatchard analysis of Ricin B chain binding to Ricin imprints revealed two apparent affinities (K-d = 0.23 and 25 nM) whereas, Ricin B chain binding to A chain imprints exhibited one high affinity constant (K-d = 8.9 nM). These data support the usefulness of intrinsic fluorescence and molecular imprints for detection of large proteins and biological toxins. (C) 2000 Elsevier Science B.V. All lights reserved