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The Dead-End Elimination Method, Tryptophan Rotamers, and Fluorescence Lifetimes

Mario Hellings ^*, Marc De Maeyer , Stefan Verheyden ^*, Qiang Hao , Els J. M. Van Damme , Willy J. Peumans ^¶ and Yves Engelborghs ^*

^* Laboratory of Biomolecular Dynamics, Catholic University of Leuven, Heverlee, Belgium; Laboratory of Biomolecular Modelling, Catholic University of Leuven, Heverlee, Belgium; Institute of Biosciences and Technology, Texas A&M University, System Health Sciences Center, Houston, Texas; Department of Molecular Biotechnology, University of Ghent, Ghent, Belgium; and ^¶ Laboratory of Phytopathology and Plant Protection, Catholic University of Leuven, Heverlee, Belgium

Correspondence: Address reprint requests to Dr. Yves Engelborghs, Katholieke Universiteit Leuven, Laboratory of Biomolecular Dynamics, Celestijnenlaan 200D, 3001 Heverlee, Belgium. Tel.: 321-632-7160; Fax: 321-632-7974; E-mail: yves.engelborghs{at}fys.kuleuven.ac.be; or Dr. Marc De Maeyer, Katholieke Universiteit Leuven, Laboratory of Biomolecular Modelling, Celestijnenlaan 200D, 3001 Heverlee, Belgium. Tel.: 321-632-7521; Fax: 321-632-7974; E-mail: Marc.DeMaeyer{at}fys.kuleuven.ac.be.

The Dead-End Elimination method was used to identify 40 low energy microconformations of 16 tryptophan residues in eight proteins. Single Trp-mutants of these proteins all show a double- or triple-exponential fluorescence decay. For ten of these lifetimes the corresponding rotameric state could be identified by comparing the bimolecular acrylamide quenching constant (k_q) and the relative solvent exposure of the side chain in that microstate. In the absence of any identifiable quencher, the origin of the lifetime heterogeneity is interpreted in terms of the electron transfer process from the indole C3 atom to the carbonyl carbon of the peptide bond. Therefore it is expected that a shorter [C3-C=O] distance leads to a shorter lifetime as observed for these ten rotamers. Applying the same rule to the other 30 lifetimes, a link with their corresponding rotameric state could also be made. In agreement with the theory of Marcus and Sutin, the nonradiative rate constant shows an exponential relationship with the [C3-C=O] distance for the 40 datapoints.

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