In the 1660s, when the 28-year-old Robert Hooke looked through one of the first microscopes, he found, to his surprise, that plant tissue was divided up into little units. They reminded him of monk's chambers (it: cellula) and he called these units "cells." The manipulation of cells has since become an occupation of many scientists, and was the motivation behind recent work published by Natasha Patrito, MCIC, Claire McCague, MCIC, Swanda Chiang, and their supervisor Peter Norton, FCIC (all of The University of Western Ontario), and their collaborator Nils Petersen, FCIC (National Institute for Nanotechnology).
In two papers published in Langmuir, the team describes how one can graft a finely patterned film of acrylate polymer onto the surface of a humble block of polydimethyl-siloxane (PDMS) polymer. PDMS is cheap, flexible, chemically inert, but also water repellent, which explains its use as bathroom caulking or aquarium sealant. To make PDMS useful for bioanalytical applications, Patrito et al. devised a photolithographic scheme, making use of the fact that the hydrophilic acrlylate film can be grafted using UV radiation. The resulting pattern was very sharp, i.e., it showed ~1.5 micron resolution (Langmuir 22 (2006) 3453).
An even simpler patterning scheme was used to create hydrophilic, micron-sized spots on PDMS. Norton and co-workers sputtered aluminum though a stainless steel screen with ~200 micron holes that was laid on top of the PDMS polymer. The resulting aluminum spots were then etched away to expose the now activated and oxygen-rich PDMS underneath. When the entire surface was exposed to fibroblast cells, it was found that the cells accumulated and grew only in the activated PDMS spots--but not on the pristine PDMS. The cell cultures even preferred to grow in height rather than leaving the activated PDMS spot (Langmuir23 (2007) 715).
Norton and his collaborators see many potential applications for biomedical research, microfluidics, and lab-on-chips. And, of course, Hooke would have been thrilled to see cells put into cells.
Stark Control of Photodissociation Process
In a recent report on laser control of chemical reactions, Benjamin Sussman, Dave Townsend, Misha Ivanov, and Albert Stolow, ACIC (Steacie Institute for Molecular Sciences, NRC), described a new and conceptually simple scheme to change the outcome of the simplest chemical reaction one can imagine--the breaking of a chemical bond by laser excitation (Science 314 (2006) 279). When IBr is excited with femtosecond pulses of 520 nm laser light, one finds that about 75 percent of the bromine atoms are formed in their spin-orbit excited state and the remainder in the ground state. The branching ratio is determined by the interactions of two potential energy curves, each one leading to a different fragment state. Stolow and his co-workers found that the non-resonant field from an intense 150 fs laser pulse, that is fired during the excitation pulse, changes the potential energy curves enough that only 60 percent of the bromine atoms are formed in their excited state. When, on the other hand, the second pulse is delayed until the wavepacket reaches the crossing, the yield of the excited bromine atoms increases to over 95 percent. Laser control over chemical reactions is not a new concept, but this is the first time that the "control laser" energy does not have to be resonant with any of the molecule's transitions. Also, the authors state that this is the first example of control over neutral dissociation channels. The control is achieved simply by Stark shifting of the respective potentials. Because it does not matter which (non-resonant) colour the control laser has, the NRC team believes that laser control by the dynamic Stark effect should also be applicable to much larger molecules.
The Attractive Pull of Pt-Bound Pyrene
Despite the huge number of chiral homogeneous metal complexes that catalyze asymmetric processes with high enantioselectivities, the corresponding heterogeneous asymmetric catalysts are few and far between. One of the best and most well-known is the combination of metallic platinum with cinchona alkaloids, which catalyzes the hydrogenation of only one face of methyl pyruvate. Enantioselectivities obtained by this process are in the 95 + percent level and are the subject of much interest.
In a recent issue of Angewandte Chemie International Edition (45 (2006) 7404), Peter McBreen, ACIC, from the Universite Laval and post-doctoral fellows Stephane Lavoie and Gautier Mahieu report a detailed STM study describing what happens when pyrene and other aromatic molecules adsorb to the surface of platinum. In the presence of carbonyl containing molecules such as ethyl formate and methyl pyruvate, the Laval group has isolated and observed multiple hydrogen bonds between the two adsorbed species in which the aromatic C-H bonds act as hydrogen bond donors. This type of behaviour is usually observed only with highly acidic C-H bonds, however the McBreen group postulates that adsorption to the Pt surface results in a polarization of the aromatic ring and a resultant increase in C-H acidity. This type of effect is well precedented in homogeneous complexes between arenes and metals.
The results obtained by saturating the Pt (111) surface with pyrene and ethyl formate are shown in Figure 1. Pyrene molecules are observed to dot the entire Pt (111) surface and are each surrounded by ten molecules of ethyl formate (Figure 1c). The same effect is observed with methyl pyruvate (Figure 2) which is the exact substrate for the asymmetric hydrogenation.
[FIGURES 1-2 OMITTED]
If the aromatic molecule is changed from pyrene to methyl naphthalene, non-symmetric adsorption of ethyl formate is observed (Figure 3), indicating that the direction of the hydrogen bonding can be controlled by functionalization of the aromatic ring. These results are particularly important in the context of the asymmetric hydrogenation with the actual aromatic ligand, Cinchonidine, Figure 3B, depicted on the surface of platinum.
[FIGURE 3 OMITTED]
In addition to hydrogen bonding to the obvious ammonium group of the bicyclic amine, the McBreen study has shown that binding between the aromatic hydrogens of the adsorbed aromatic substituent and the carbonyl oxygen also occurs. Two point binding is a common feature in many homogeneous asymmetric systems since it reduces conformational freedom of the substrate, and directs which face of the molecule is reduced. The emerging picture provides a remarkably satisfying picture of the mechanism by which this combination of metal and ligand act in concert to affect the enantioselective reduction of alpha keto esters.
Cathleen Crudden, MCIC, and Hans-Peter Loock, MCIC, are both associate professors of chemistry at Queen's University in Kingston, ON.
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|Title Annotation:||research of cells (Biology); reports of medical research; research of plasma desorption mass spectrometry; research of hydrogen bonds|
|Comment:||Chemical shifts.(research of cells (Biology))(reports of medical research)(research of plasma desorption mass spectrometry)(research of hydrogen bonds)|
|Author:||Crudden, Cathleen; Loock, Hans-Peter|
|Publication:||Canadian Chemical News|
|Date:||May 1, 2007|
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