Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2003, 42 (9), 1012 - 1015
Amino acid detectors:
new approaches to protein analysis?
Walk in and it snaps shut -- molecules too can be caught like a mouse in a trap. The molecular mousetraps have been set by a collaborative effort involving the research teams of physical chemist J.L. Beauchamp and synthetic organic chemist Brian Stoltz at the California Institute of Technology.
The mousetrap is based on crown ethers, large ring-shaped molecules in which the atoms are arranged in a zigzag structure on two levels, reminiscent of a crown. The cavity in the center of the crowns can hold smaller molecules or molecular fragments. Such complexes are stable enough to survive the transition to the gas phase, a requirement for mass spectrometry. Which "guest molecules" are bound depends on the specific structure and size of the crown's cavity. Under the chosen experimental conditions, the end section of a side chain on the amino acid lysine fits perfectly into 18-crown-6, which consists of twelve carbon and six oxygen atoms. In order for the trap to snap shut once and for all, the researchers equipped the crown with an additional functional group that contains two nitrogen atoms. Under the conditions of mass spectrometry, the trap mechanism is activated when the two nitrogen atoms are removed as a nitrogen molecule. This bond rupture leads to the formation of an unsaturated, and thus highly reactive, carbon center, which promptly forms a strong (covalent) bond to the trapped lysine.
"Marking with crown ethers makes it possible to determine the number of lysines in smaller protein fragments," explains Beauchamp. "We could also follow the denaturing, or unfolding, process of larger proteins. Initially, only lysines on the surface are marked, then little by little the lysines on the interior of the protein become accessible as well. We can thus draw conclusions about the protein's structure." The researchers now want to synthesize more mousetraps specific to other amino acids. Their goal is the development of a new method of protein sequencing, which takes place directly in the gas phase within the mass spectrometer.
It is also conceivable that synthetic compounds of this "mousetrap" variety, which bind to specific amino acid sequences, will allow novel biomimetic reactions such as the cleavage of amide bonds, a reaction normally restricted to enzymes such as trypsin in solution, to be carried out in the gas phase.