Table Of ContentBiological Magnetic Resonance
Volume 21
EPR: Instrumental Methods
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Biological Magnetic Resonance
Volume 21
EPR: Instrumental Methods
Edited by
Lawrence J. Berliner
University of Denver
Denver, Colorado
and
Christopher J. Bender
Fordham University
Bronx, New York
SPRINGER-SCIENCE+BUSINESS MEDIA, LLC
ISBN 978-1-4613-4733-0 ISBN 978-1-4419-8951-2 (eBook)
DOI 10.1007/978-1-4419-8951-2
© 2004 Springer Science+Business Media New York
Originally published byKluwer Academic/Plenum Publishers, New York in 2004
Softcover reprint ofthe hardcover Ist edition 2004
10 9 8 7 6 5 4 3 2
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Preface
Practitioners ofelectronmagnetic resonance (EMR)spectroscopy arenowenjoying some
thing akin to a renaissance in this branch of magnetic resonance, much of which can be
attributedtoimprovedtechnologyformicrowave devicefabrication.Principleamongthese
advances are solid-state circuits, and in particular the monolithic microwave integrated
circuit (MMIC). These devices on the one hand are fast, enabling one to work in the
nanosecond time domain (e.g., pulsed EMR). MMICs also enable one to obtain the
performance characteristics ofelectron tubedevices (e.g.,reflexklystrons) inamonolithic
package(e.g.,Gunndevice), thusaffordingnewcapabilitiestothespectroscopistintheway
of multi- and high frequency techniques.
With these advances as a background, we felt that the time was appropriate for
devotingoneormore volumes inthissuccessfulseriestoEMRinstrumentation. With this,
ourfirstvolume devoted toinstrumentation, andintroductory chapteron general topics of
microwaveinstrumentation,asapplied toEMR, setsthestage.This general introductionis
followed by achapter that addresses the specificproblem of low-frequency spectrometer
design,whichisrelevanttosuchadvancedEMRtechniquesasimaging.Chapter3treats the
crucialproblemofsensitivity,andhowthisisaffectedbyspectrometeroperating frequency.
Chapter3isthereforeimportanttoanypractitionerofmulti-frequencyEMR. Thenext two
chapterstreattheoften-called 'advancedEMR' techniques,namelyelectron nucleardouble
resonance(ENDOR) andelectron spinecho(envelope)modulation(ESEEM).TheENDOR
chapter focuses on a topic that is absent from the EMR literature, namely the rf field
generating coil and how one renders it compatible with the microwave resonator for
optimumperformance.TheESEEMchaptertakesamoresystemicviewpointanddescribes
themannerinwhichthespinechoandspincoherenceeffectscanbeinvoked andmeasured.
Thesefirstfivechaptersprovidethepracticalbackground fortheremainingchapters, which
concern several of the current applications of EMR.
Theremaining three chaptersinthisfirstvolume oninstrumentationdeal withunique
applicationsofthemodemEMR spectrometer.Firstamongtheseisachapterdescribingan
algorithmicapproachtotheanalysisofspectralparametersthatisgenerally applicable. This
isfollowedbyachapteronEPRimagingand,finally,byapulsedelectronspinechomethod
of determining crystallographic distances between paramagnetic centers.
Finally, during thepreparationofthisvolume weweresaddenedtoreceive news that
our colleague Jerry Babcock had passed away. Jerry is known to many in the EMR
community for his work in flashkinetic and ENDOR spectroscopies, and to the scientific
community atlarge for hisinvestigationsinto theoxygen chemistry ofphotosynthesis and
oxidative phosphorylation. Besides the published work, however, many of us remember
v
vi PREFACE
Jerry as someone who brought enjoyment to cutting-edge scientific research. His enthu
siasmwasinfectious, andhebroughtoutthebestinthosewhoworkedinhislaboratory.This
canbebestexemplifiedbysomethingheusedtosaytohisgraduatestudents astheyneared
completion of their thesis; he would remind them that their work represented something
totally new,andthat they,astheperson whodid thework, knew the most aboutthattopic.
One could not help but get excited about science in Jerry's laboratory.
Theeditors and authorstherefore dedicate these volumes onEMR instrumentationto
the memory of Jerry Babcock.
Christopher J. Bender
Lawrence J. Berliner
CONTENTS
MICROWAVE ENGINEERINGFUNDAMENTALSAND
SPECTROMETERDESIGN
C.J.Bender .
EPRSPECTROMETERSAT FREQUENCIESBELOWX-BAND
G.R.Eaton and S. S.Eaton 59
FREQUENCYDEPENDENCE OFEPRSENSITIVITY
G.A. Rinard, R. W. Quine, S. S.Eaton, andG.R. Eaton 115
ENDORCOILSAND RELATEDRADIOFREQUENCYCIRCUITS
C.J.Bender 155
THE GENERATION AND DETECTIONOFELECTRON SPINECHOES
C.J.Bender 212
CONVOLUTION-BASEDALGORITHM: FROMANALYSISOF
ROTATIONAL DYNAMICSTO EPROXIMETRYAND PROTEIN
DISTANCEMEASUREMENTS
A. I. Srnirnovand T. I. Srnirnova 277
IDAND 2DELECTRONSPINRESONANCE IMAGING(ESRI)OF
TRANSPORTAND DEGRADATIONPROCESSESIN POLYMERS
M.V.Motyakinand S. Schlick 349
PEPTIDE-AGGREGATIONAND CONFORMATIONPROPERTIESAS
STUDIEDBY PULSEDELECTRON-ELECTRONDOUBLE
RESONANCE
Y.D.Tsvetkov 385
INDEX 435
Chapter 1
Microwave Engineering Fundamentals and
Spectrometer Design
Christopher J. Bender
Department ofChemistry, Fordham University.441E.Fordham Road, Bronx, New York 10458
USA
1. SCOPE & RATIONALE
This introductory chapter is intended asaconcise overview ofmicrowave
technology and the various devices that comprise a conventional electron
magnetic resonance spectrometer. It is hoped that it will serve the reader
with background information that isrelevant to the necessary understanding
ofinstrument operation, as well as be a precursor tothe chapters that follow
and describe many ofthe new advances in EMR spectrometer technology.
The three major sections of this chapter correspond to the functional
subsystemsthat would exist inatypical spectrometer, namely the microwave
source,transmission line devices, andthe receiver.
I prefer to approach this subject from an historical perspective because
doingso imposes a logical ordering ofthe facts, andtherefore much ofwhat
Lhave written in this chapter is distilled from the high frequency electrical
engineering and EMR literature dating back to the 1940s. The reference
section is consequently written as a bibliography of many now out-of-print
books and reviews because these may often be consulted for insights
regarding application-specific microwave components. Among the EMR
literature, there are several textbooks devoted exclusively to instrument
design and technique. Poole's (1967; 1983) classic treatise is still an
indispensable guide to EMR spectrometer design and technique, and its
second (1983) edition has been reissued by the Dover press. Algers' (1968)
C. J. Bender et al. (eds.), EPR: Instrumental Methods
© Kluwer Academic/Plenum Publishers, New York 2004
2 BENDER
textis a more elementary presentation ofspin resonance and includes useful
information on the construction 'arts' ofspectrometer design, and it likewise
has recently been reissued by University Microforms (Ann Arbor) after
many years of unavailability. Harvey's (1963) microwave engineering
textbook is an excellent advanced guide to systems design that includes
chapters on spectroscopy, and Wilhurst's (1968) text is a good general guide
to EMR instrumentation.
2. MICROWAVE SOURCES
2.1 Overview
Oscillatory signals can be obtained from static voltages by using reactive
circuit elements. A common textbook example is the charged capacitor or
energized inductor (i.e. an RC or RL circuit) that is switched at some
threshold voltage (Horowitz & Hill, 1989, p284ff). RLC circuits combine
two reactive elements in series or parallel, and the solutions to Kirchhoff's
equations resemble that ofthe harmonic oscillator. In other words, when an
RLCcircuit is closed, the response isoscillatory and characterizedbynatural
frequencies that are determined by the circuit elements (Reintjes & Coate,
1952, p115ff; Lancaster, 1980, Ch. 4). These RLC circuits are self
oscillatory, and it is a common engineering design practice to couple these
resonant circuits to an amplifier-like device in order to produce a signal that
is a high fidelity sine wave with a characteristic frequency. All ofthe high
frequency oscillators that are described in this section operate on this
principle.
In general, the ideal waveform will be sinusoidal because it is easy to
generate and not distorted by any linear network. This means that one can
pass a sinusoidal signal to a linear amplifier, and the output signal from the
amplifier will differ from the input signal only with respect to the amplitude
and phase. A second linear network can then be added so that it alters (or
samples) the amplified sine wave output and exactly duplicates (both
amplitude and phase) the original signal input to the amplifier. In this
manner one can therefore use a linear network as a means to provide a
feedback that maintains a constant signal input to the amplifier stage. A
practical oscillator consequently consists of a resonant circuit, a linear
amplifier, and a linear feedback network that compensates for damping
(resistive dissipative losses) inthe resonator.
Figure 1 depicts two representative oscillator circuits that illustrate the
interplay between a resonant RLC circuit and an amplifier. These Hartley
1.MICROWAVEENGINEERINGFUNDAMENTALSANDSPECTROMETERDESIGN 3
and Colpitts oscillators feature recognizable interactive resonator, amplifier
v
Figure /. The Hartley(left) and Colpitts (right) oscillators, illustratinghowapractical source
ofsine waves is obtained by coupling an amplifier to an RLC circuit, which determines the
frequencyoftheoscillation.
and feedback 'stages'; in some versions the RLC resonator is replaced by a
quartz crystal and a capacitor that permits a limited tuning range. The most
important feature of these circuits (and all oscillators) is the feedback
between the amplifier stage and the resonator circuit: the small (ca. 25%)
amount of the amplified signal is returned to the resonator in order to
overcome damping effects that would otherwise cause the signal amplitude
to decay. Ordinarily the two simple circuits depicted in Figure I would be
used to generate radiofrequency signals, but their design is easy to
comprehend for illustrative purposes. As for spectroscopic applications,
these oscillators find applications in nuclear quadrupole resonance
spectrometers, and the pioneering experiments ofEMR spectroscopy were
conducted with such oscillators. These low frequency (-150 MHz) EMR
experiments are described by Bruin (1961) and Kochelaev & Yablokov
(1995).
High frequency electromagnetic waves are generated from electrontubes
or solid-state devices (semiconductors). Experiments conducted during the
nineteenth century demonstrated that electromagnetic waves were emitted
from rarefied gas atmospheres subjected to an electrical discharge, and the
technological application ofthese devices followed from the discoveries that
the device could be used as transmitters or receivers of electromagnetic
radiation. By definition, an electron tube is a device whose function follows
