Small molecules that recognize protein surfaces are important tools for modifying protein interaction properties. Since the 1980s, several thousand studies concerning calixarenes and host-guest interactions have been published. Although there is growing interest in protein-calixarene interactions, only limited structural information has been available to date. We now report the crystal structure of a protein-calixarene complex. The water-soluble p-sulfonatocalix[4]arene is shown to bind the lysine-rich cytochrome c at three different sites. Binding curves obtained from NMR titrations reveal an interaction process that involves two or more binding sites. Together, the data indicate a dynamic complex in which the calixarene explores the surface of cytochrome c. In addition to providing valuable information on protein recognition, the data also indicate that the calixarene is a mediator of protein-protein interactions, with potential applications in generating assemblies and promoting crystallization.

A crystal structure of lysozyme in complex with p-sulfonato-calix[4]arene (sclx4) reveals a linear assembly of protein tetramers glued together by protein-calixarene interactions. One interaction involves encapsulation of the highly exposed C-terminal Arg128. The other involves an intricate protein-bound complex of sclx4, Mg(2+) and a fragment of polyethylene glycol.

X-ray crystallography reveals how a calixarene can bind to dimethyllysine to form a complex with features similar to the aromatic cage motif of a chromodomain bound to a histone tail.

The assembly of redox proteins on electrodes is an important step in biosensor development. Recently, p-sulfonato-calix[4]arene was shown to act as "molecular glue" for the assembly and crystallization of cytochrome c (cyt c). Electrochemical data are presented for microscale cyt c-calixarene crystals grown on self-assembled monolayers (SAM)-modified Au electrodes. The crystals were characterized by cyclic voltammetry and exceptionally high concentrations of electroactive cyt c were obtained. The peak currents were found to increase linearly with the square root of the scan rate, thus allowing an evaluation of the rate constant for electron self-exchange. This study revealed high electroactivity accompanied by fast interprotein electron transfer in crystals, which may have implications for the construction of novel bioelectronic devices.

Die Assemblierung von Redoxproteinen auf Elektrodenoberflächen ist ein entscheidender Schritt für die Entwicklung von biohybriden Systemen. Kürzlichk onnte gezeigt werden, dass p-sulfoniertes Calix[4]aren als "molekularer Klebstoff" für die Assemblierung und Kristallbildung von Cytochrom c( Cyt c) dient. Hier präsentieren wir die ersten elektrochemischen Daten für Calixaren-Cyt-c-Kristalle im Mikromaßstab auf SAM-modifizierten Goldelektroden. Die Elektrochemie der Kristalle wurdem ittels Zyklovoltammetrie charakterisiert. Hierbei konnten enorme Konzentrationen an elektroaktivem Cyt cn achgewiesen werden. Die erhaltenen Peakstrçme nahmen linear mit der Quadratwurzel der Vorschubgeschwindigkeit zu. Dadurchw ar es mçglich, die Selbstaustauschrate (k ex )d er Cyt-c-Moleküle zu bestimmen. Durchd iese Studie kann nachgewiesen werden, dass auch Proteinkristalle eine hohe Elektroaktivität und einen schnellen intermolekularen Elektronentransfer aufweisen kçnnen. Es kann erwartet werden, dass diese Beobachtungen auche inen signifikanten Einfluss auf die Entwicklung neuer bioelektronischer Bauelemente haben.