Alfred G. Redfield,
Professor Emeritus of Physics, Biochemistry, and
Rosenstiel Basic Medical Sciences Research Center
Member, US National Academy of Sciences
Studies of Macromolecules by Magnetic Resonance
Ph.D., University of Illinois
redfield @ brandeis.edu
We have developed a device that can be installed in our shared commercial 500 MHz NMR instrument, to move a sample from its usual position at the center of the magnet out to a lower-field position above the center, or outside, the magnet, and back, with a maximum round-trip time of 350 msec. This permits us to insert a low-field interval into any NMR pulse sequence, that can then be used to study relaxation processes at the lower field. At these lower fields, heterospecies nuclear dipolar relaxation between observed nuclear spins, in our case 31P or 13C, and nearby unpaired spin species, including those on introduced nitroxide-radical spin-labels, are enhanced. Most interestingly, we can characterize, and determine binding locations of weak non-covalently bound molecules to larger proteins and structures such as peripheral membrane proteins. The general method has been to measure the relaxation rate of a smaller chemical species that has an easily observed nuclear resonance signal (from naturally occurring 31P or protons, or substituted 13C) on a smaller molecule or phospholipid membrane, by the presence of a larger species that can be a spin labeled. In favorable cases distances between nuclear spins and spin-labeled sites on the two species can be usefully estimated. This has been a strong collaboration with Professor Mary Roberts, of Boston College, most recently finding evidence for existence of two binding sites, each for a different lipid, one of which of which is most likely an activity-enhancer site whose existence she had proposed. She and her group provided most of the biochemical impetus and all of the samples for most of our publications. The general approaches often resemble those using fluorescence, but our method is needed for weaker binding pairs of species, and/or where fluorescent labels would interfere with the behavior of one species.
Apparatus. Our apparatus uses a linear motor (visible towering above the main magnet of the University's shared 500 MHz magnet to the left of me in the photo) that, using a downward-extending carbon-composite rod, can move a standard liquid NMR sample, from its usual central position, up any distance even above the main magnet, under microprocessor control. In this way an interval can be introduced into any NMR sequence where the spin-lattice relaxation decay of any longitudinal nuclear coherence can be studied over a range of magnetic fields, down to 0.002 Tesla in favorable cases. Dipolar relaxation between nuclear spins and electrons is enhanced at low fields, so that this method is vastly superior to previous high-field methods, yet we maintain sensitivity and resolution comparable to the host 500 MHz spectrometer. The detailed field-dependence gives much information, This is not the first apparatus of its kind but it is the first which is reversibly installed on (within an hour), and removed from, a commercial instrument, thereby providing great economy (less than 20% of the cost of the host shared 500 MHz machine) and versatility.
My paper in J. Biol. NMR, 2012 52:159-177 provides a description of the new apparatus and references to descriptions of previous versions of it. It is extensively supplemented here by on-line material: <part 1> has various manuals and directions for users and support; <part 2> is a lengthy discussion directed toward potential builders of similar machines, with PDF figures in <figures for part 2>; and <part 3> has extensively commented program listings. A poster <EMBO 2009> provides useful photos and is up-to-date except that it used a stepper motor, instead of a servomotor, to drive the linear motor. A group of earlier files <Pneumatic> contains obsolete material some of which is referrenced in <part 2>, which was a supplement to my early paper in Magn. Reson. Chem, 41:753-768.
Applications of the apparatus are described in eleven research papers listed at the beginning of <Part1>.
I urge you to read some of them, they are all unique in both methodology and results.
Dr. Redfield is a Member of the National
Academy of Sciences and a Fellow of the American Academy
of Arts and Sciences.
reviewed: June 9, 2016.
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