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Biophysical Journal 87:3162-3171 (2004)
© 2004 The Biophysical Society

Field-Dependent Effect of Crown Ether (18-Crown-6) on Ionic Conductance of {alpha}-Hemolysin Channels

Sergey M. Bezrukov *, Oleg V. Krasilnikov {dagger}, Liliya N. Yuldasheva {dagger}, Alexander M. Berezhkovskii {ddagger} and Claudio G. Rodrigues {dagger}

* Laboratory of Physical and Structural Biology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892 USA; {dagger} Department of Biophysics and Radiobiology, Federal University of Pernambuco, Recife, Brazil; and {ddagger} Mathematical and Statistical Computing Laboratory, Center for Information Technology, National Institutes of Health, Bethesda, Maryland 20892 USA

Correspondence: Address reprint requests to Dr. Oleg V. Krasilnikov, Universidade Federal de Pernambuco, Centro de Ciencias Biológicas, Depto. de Biofisica e Radiobiologia, Av. Prof. Moraes Rego, S/N, Cidade Universitária, Recife, Pernambuco, Brazil, CEP 50670-901. Tel.: 55-81-2126-8535; Fax: 55-81-2126-8560; E-mail: kras{at}ufpe.br.

Closing linear poly(ethylene glycol) (PEG) into a circular "crown" dramatically changes its dynamics in the {alpha}-hemolysin channel. In the electrically neutral crown ether (C2H4O)6, six ethylene oxide monomers are linked into a circle that gives the molecule ion-complexing capacity and increases its rigidity. As with linear PEG, addition of the crown to the membrane-bathing solution decreases the ionic conductance of the channel and generates additional conductance noise. However, in contrast to linear PEG, both the conductance reduction (reporting on crown partitioning into the channel pore) and the noise (reporting on crown dynamics in the pore) now depend on voltage strongly and nonmonotonically. Within the whole frequency range accessible in channel reconstitution experiments, the noise power spectrum is "white", showing that crown exchange between the channel and the bulk solution is fast. Analyzing these data in the framework of a Markovian two-state model, we are able to characterize the process quantitatively. We show that the lifetime of the crown in the channel reaches its maximum (a few microseconds) at about the same voltage (~100 mV, negative from the side of protein addition) where the crown's reduction of the channel conductance is most pronounced. Our interpretation is that, because of its rigidity, the crown feels an effective steric barrier in the narrowest part of the channel pore. This barrier together with crown-ion complexing and resultant interaction with the applied field leads to behavior usually associated with voltage-dependent binding in the channel pore.




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P. G. Merzlyak, M.-F. P. Capistrano, A. Valeva, J. J. Kasianowicz, and O. V. Krasilnikov
Conductance and Ion Selectivity of a Mesoscopic Protein Nanopore Probed with Cysteine Scanning Mutagenesis
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