Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2004, 43 (46), 6308—6311
Protein with Pores
"Biozeolites": Peptides as a new class of microporous organic solids
The catalysts, micro reaction chambers, molecular storage, and molecular sieves coveted by science and technology often consist of solids with microscopic cavities in which other molecules can lodge as "guests". The most important and versatile class of porous materials is the family of silicates known as zeolites. However, porous frameworks aren’t found exclusively in inorganic materials; organic materials can also be crisscrossed by narrow channels. A team of Canadian and Russian collaborators has now identified a new class of "biozeolites" made of simple peptides.
In natural life forms, cavities play important roles as ion channels and membrane pores. These consist of very complex structures made of proteins. For their experiments, Dmitry V. Soldatov, Igor L. Moudrakovski, and John A. Ripmeester chose to use a very simple type of protein. They limited themselves to two protein building blocks, the amino acids valine and alanine, and hooked them together. The result is two different dipeptides, depending on which amino and acid groups are coupled: alanyl-valine (AV) and valyl-alanine (VA). Both crystallize as microporous solids; the crystals consist of spiral dipeptide chains, each with an open channel in the center. These little channels are not straight, but twisted. What is unusual is that all of the channels are twisted in the same direction, to the right. The image and mirror image, in this case right- and left-turning spirals, are not identical—such structures are called chiral. Amino acids are also chiral, the naturally occurring form being the "left" version—which leads to right-handed channels in the dipeptide crystals. Materials with chiral channels are difficult to produce, but are highly desirable because they are used for the often decidedly difficult separation of the "left" and "right" versions of chiral molecules.
Although the AV and VA crystals have very similar structures and dimensions, there are distinct differences: the noble gas xenon is held much more tightly by VA channels than by the AV pores. The reason for this seems to be the slightly smaller pore diameter of the VA crystals; the smaller cavities result in more intensive interactions between the gas atoms and the pore walls.
If the diversity of possible small peptides is taken into consideration, a real cosmos of novel, highly robust porous materials seems to open up. The type, number, and order of the coupled amino acid building blocks could perhaps be used to tailor the pore properties of these "biozeolites" for particular applications. Nontoxic peptides would also be suitable for biomedical applications.