Faculty Bibliography

The analysis of cell signaling requires the rapid and selective manipulation of protein function. We have synthesized photoswitches that covalently modify target proteins and reversibly present and withdraw a ligand from its binding site due to photoisomerization of an azobenzene linker. We describe here the properties of a glutamate photoswitch that controls an ion channel in cells. Affinity labeling and geometric constraints ensure that the photoswitch controls only the targeted channel, and enables spatial patterns of light to favor labeling in one location over another. Photoswitching to the activating state places a tethered glutamate at a high (millimolar) effective local concentration near the binding site. The fraction of active channels can be set in an analog manner by altering the photostationary state with different wavelengths. The bistable photoswitch can be turned on with millisecond-long pulses at one wavelength, remain on in the dark for minutes, and turned off with millisecond long pulses at the other wavelength, yielding sustained activation with minimal irradiation. The system provides rapid, reversible remote control of protein function that is selective without orthogonal chemistry.

A key technical barrier to furthering our understanding of complex neural networks has been the lack of tools for the simultaneous spatiotemporal control and detection of activity in a large number of neurons. Here, we report an all-optical system for achieving this kind of parallel and selective control and detection. We do this by delivering spatiotemporally complex optical stimuli through a digital micromirror spatiotemporal light modulator to cells expressing the light-activated ionotropic glutamate receptor (LiGluR), which have been labeled with a calcium dye to provide a fluorescent report of activity. Reliable and accurate spatiotemporal stimulation was obtained on HEK293 cells and cultured rat hippocampal neurons. This technique should be adaptable to in vivo applications and could serve as an optical interface for communicating with complex neural circuits.

We have observed reversible light-induced mechanical switching for individual organic molecules bound to a metal surface. Scanning tunneling microscopy (STM) was used to image the features of individual azobenzene molecules on Au(111) before and after reversibly cycling their mechanical structure between trans and cis states using light. Azobenzene molecules were engineered to increase their surface photomechanical activity by attaching varying numbers of tert-butyl (TB) ligands ("legs") to the azobenzene phenyl rings. STM images show that increasing the number of TB legs "lifts" the azobenzene molecules from the substrate, thereby increasing molecular photomechanical activity by decreasing molecule-surface coupling.

A concise, stereoselective, and convergent total synthesis of the unnatural enantiomer of the neodolastane diterpenoid heptemerone B has been completed. Saponification of (-)-heptemerone afforded (-)-guanacastepene E. The absolute stereochemistry of (-)-heptemerone B was thus established as 5-(S), the same as (-)-guanacastepene E. The longest linear sequence of the synthesis comprises 17 (18) steps from simple known starting materials. Our general synthetic approach integrates a diverse set of reactions, including an intramolecular Heck reaction to create one quaternary stereocenter and a cuprate conjugate addition for the establishment of the other. The central seven-membered ring was closed with an uncommon electrochemical oxidation, whereas the five-membered ring was formed through ring-closing metathesis. The absolute configuration of the two key building blocks was established through an asymmetric reduction and an asymmetric ene reaction.

The asymmetric total synthesis of (-)-bipinnatin J and its conversion into (+)-intricarene through a transannular 1,3-dipolar cycloaddition is described. In addition, the conversion of (-)-bipinnatin J into (+)-rubifolide and (+)-isoepilophodione B is reported. Biosynthetic relationships among furanocembranoids and the possible role of 1,3-dipolar cycloadditions in biosynthesis are discussed. [reaction: see text]

Ion channels are gated by a variety of stimuli, including ligands, voltage, membrane tension, temperature, and even light. Natural gates can be altered and augmented using synthetic chemistry and molecular biology to develop channels with completely new functional properties. Light-sensitive channels are particularly attractive because optical manipulation offers a high degree of spatial and temporal control. Over the last few decades, several channels have been successfully rendered responsive to light, including the nicotinic acetylcholine receptor, gramicidin A, a voltage-gated potassium channel, an ionotropic glutamate receptor, alpha-hemolysin, and a mechanosensitive channel. Very recently, naturally occurring light-gated cation channels have been discovered. This review covers the molecular principles that guide the engineering of light-gated ion channels for applications in biology and medicine.

The development of a new variant of the Friedel-Crafts reaction that yields 3-aryl enol triflates is described. The reaction is practical, is atom-economical, and works well with electron-rich arene substrates. [reaction: see text].