Richard S. Givens
- Professor Emeritus
1567 Iriving Hill Road
Lawrence, KS 66045
Mechanistic Organic Photochemistry and Catalytic Oxidation Reactions
Photochemical Reactions and Mechanistic Investigations of “Caged” Compounds
In our quest to discover fundamentally new light activated chemical reactions, we have uncovered very useful methods for the release of reagents and biological substrates. We have applied these reactions in such diverse fields as organic synthesis, physiology, and combinatorial chemistry. Photoremovable protecting groups or “caged” compounds, as the derivatives have become to be known, use light to break chemical bonds and essentially instantaneously release a reagent or biological substrate. Furthermore, since light can be focused on specific, minute locations with a predetermined intensity, light-activated processes provide exquisite control in triggering initiators or agonists for chemical or biological processes.
For a recent monograph on Caged Compounds, see Wiley's website.
As an example, the release of neurotransmitters in neuron cell preparations stimulates the cell or its components within a few milliseconds or less. Thresholds for the activation and effects of agonists and antagonists can be explored using caged glutamate. Correctly organized neuronal networks and delicately balanced synaptic excitatory-inhibitory interactions are fundamental for normal brain function. In one of our collaborations, the Kandler group investigates the developmental mechanisms by which converging excitatory (glutamatergic) and inhibitory (GABA/glycinergic) neuronal networks become spatially organized and functionally fine tuned. An example of this plasticity of neonatal neuronal connections has been registered in the time-controlled photorelease of glutamic acid (Glu) by photolysis of our caged pHP Glu. The Kandler group addresses these issues in a study of an inhibitory sound localization circuit in the mammalian brainstem. They recently used our pHP glutamate to functionally map the development of inhibitory connectivity patterns and thereby demonstrated that functional synapse elimination and strengthening are important events in the establishment of precisely organized inhibitory circuits (Kim and Kandler, 2003).
The rules and mechanism that govern the reorganization of inhibitory circuits in the auditory system or any other brain region are the research focus of many laboratories including Kandler’s. Although these mechanisms are currently poorly understood, it is quite intriguing that synaptic refinement takes place during the developmental stage in which individual inhibitory synapses not only release glycine but also release GABA (Nabekura et al., 2004) and while GABA and glycine are excitatory and increase postsynaptic [Ca+2]i. More detailed insight of the cellular consequences of these temporary properties of immature inhibitory synapses and their role on synaptic plasticity will be key to understand how the brain becomes wired up during development.
Fig. 1. Developmental refinement of the MNTB-LSO connections as determined by functional mapping with pHP glutamate. (a,b) shows an input map in a 3-day old (P) rat and (c,d) in a P14 rat. (a) Location of uncaging sites that elicit (colored circles) or do not elicit (open circles) synaptic responses in a LSO neuron. Uncaging sites are overlaid onto a video picture of the MNTB (outlined in black). Responsive sites are color coded according to peak amplitudes of postsynaptic LSO responses. Examples of synaptic responses elicited from three locations (marked with x) are illustrated in the lower traces. (b) Interpolated 3-D plot of input areas. The size of the MNTB is scaled to match the size at P14 (d). (c, d) MNTB-LSO input map form a P14 rat. Scale bars (a) and (c) are 100 µm. From Kim and Kandler, 2003 Nature Neuroscience, 6, 282-290.
Photostimulated Cross Linking of Type II collagen
Another collaborative project has addressed the difficulty in repairing torn ligaments and other collagenous materials. Professor George Timberlake, Department of Ophthalmology at the KU Medical Center, has worked with our group in the design and development of a photochemical method for closing incisions and wounds in the eye. A new tethered photoactive reagent has been synthesized that can be applied to collagen surfaces and irradiated for a few minutes to form new bonds that are often as strong as a nylon suture. This new material is being refined so that it can be applied to bind proteins to other materials or surfaces. The photochemical application involves the formation of reactive ketenes which have been tethered through a polyethylene glycol unit or a PAMAM dendrimer. The ketenes react with lysyl amino groups on collagen and with other reactive nucleophiles to form new covalent bonds, e.g. peptide linkages, thus fastening the two materials together. Figure 2 illustrates the sequence for the PAMAM derivatives as shown here.
Synthetic and Mechanistic Studies of Cobalt-catalyzed Oxidations of Hydrocarbons
The focus of our research program on the oxidation of hydrocarbons has been the discovery of new mild, environmentally beneficial methods to oxidize hydrocarbons with a Co(II) catalyst using air in a high pressure CO2–acetic acid environment. Substrates such as xylene, ethylbenzene, and cyclohexane, can be oxidized using O2 or air and a co-catalyst with Co(II) to form terephthalic acid (90% yield), acetophenone (80%) and adipic acid (40%), respectively. The mechanism of this “green chemistry” oxidation reaction is being examined using our ReactIR to follow the time-dependent change in the infrared spectra of the reactants, intermediates, and products.
[For further information, see Timberlake, G. T.; Yousef, A. L., Chiles, S. R.; Moses, R. A.; Givens, R. S., (2005) Photochem. Photobiol., 81, 1180–1185, below.]