Presentation - acert

Report
PHYSICAL CHEMISTRY AWARD SYMPOSIUM
247th ACS National Meeting, Dallas, TX
Joel Hildebrand Award
March 18, 2014
ESR Perspective on
Complex Liquids
(A Retrospective)
Jack H. Freed
Dept. of Chemistry & Chemical Biology
Cornell University
Ithaca, New York 14853 USA
www.acert.cornell.edu
ESR Hyperfine Linewidths of Radicals in Solution
Alternating Linewidth - Spectrum
radical in 20C DMF
Asymmetric
Linewidth
Variation
Spectrum of
paradinotrobenzene
anion radical
-55C DMF
Alternating LW’s : Outof-phase correlation
between the HF
splittings of the two
nitroxides.
Necessitated New Paradigm for HF
Linewidths in Organic Radicals:
Freed – Fraenkel Theory:
Used Redfield Relaxation Matrix Based on WBR
(Wangsness-Bloch-Redfield Theory) includes
Degenerate HF Transitions. (JCP, 39, 326-48, 1963)
of para-dinotrodurene anion
Terms in the perturbation
H1(t).a
Anistropic Rotational Diffusion & ESR
Spectral densities jA(BC) (ω)are Fourier
Transforms of the time correlation fns of
the Wigner Rotation Matrix Elements:
Dm,m(L) .
For Anisotropic Brownian Motion: They depend on
eigenvalues of Rotational Diffusion Tensor (e.g.
Perrin, Favro) (Freed, JCP, 41, 2077, 1964).
The dependence on nuclear spin quantum
numbers MN & MH enable several
independent quantities to determine this
tensor.
An Analysis of the p-dinitrobenzene
anion linewidth (plus some
assumptions about internal dynamics)
yields:
[K+]2
These were preliminary results, but showed
ESR linewidths in principle provide enough
information to extract aanistropic diffusion
tensors.
Better assessment was made for the
simpler spectrum of peroxylamine
disulfonate (PADS), with a simple 3 line
14N ESR Spectrum
∥ /⊥ = .  ±  in ice clathrate cage
∥ /⊥= .  ±  in glycerol solvent
Freed, JCP, 56, 716 (1972)
Electron Spin Relaxation and Molecular
Dynamics in Liquids: Solvent Dependence
PD-Tempone
Analysis based on Stokes-Einstein type behavior
with
 =     + 
with ro = 3.2Å geometric effective spherical radius
re = effective rotational spherical radius
 = ∥ /⊥ Rotational Asymmetry
 ≡  / Rotational “Slip”
Non-secular spectral densities:
j(ω)≈ τR/[1+ε2τR2]-1, ε >1
τR vs. η/T over five orders of magnitude
Zager & Freed, JCP,77, 3344 (1982)
Electron-Spin Relaxation and Molecular Dynamics
in Liquids: Pressure Dependence
 vs vη/kBT for PD-Tempone in toluene-d8.
Variable pressure and temperature results.
Yields factor of
2 scatter in τR
60 Data Points: 52C >T> -40 C
1 bar ≤ P< 5 kbar
v = solute volume
η = solvent viscosity
β = isothemal compressibility
Zager & Freed, JCP, 77, 3360 (1982)
Removes
scatter in τR
Empirical Fit to 60 Data Points
τR/η/T) = a + bP+ cP2+dT+eT2+fPT
6 parameter fit gives R2 = 0.90
Plot of average value of τRT /ηβ
for each constant density group
(CDG) vs density, ρ
Led to Empirical Fit to all data points to τRT/ηβ=C(ρ- ) /ρ
With C=32 ×10-8 K × s × kbar/cP
 = 0.845 g/cm2 → The “expanded volume” =  ≡  −
Electron-Spin Relaxation and Molecular Dynamics in
Liquids: Pressure Dependence (continued)
Expanded Volume Model:  ∝


(−)
•  = − is a solvent reference volume such that as
the solvent volume  →  ( where  = − ), then
 → .
• This is an ideal reference state, not realized in real
systems because this model relates to purely viscous
motion, and as  →  the liquid is becoming more
gas-like, so inertial effects would take over. These
experiments on PDT exhibit purely viscous behavior.
• This expanded volume model takes into account in a
“natural” way the concept of slip of the rotating
molecule in the solvent.
Translational Diffusion: Heisenberg Spin
Exchange & ESR Spectra
When two molecules, each with an
unpaired electron spin, collide in solution
this yields an exchange interaction.
The ESR spectral line broadening depends
on the Heisenberg Exchange frequency:
Width vs.
concentration
for TCNE—
samples in
DME T = 15° C)
Where
J is the exchange interaction
τ1 is the lifetime of exchange pair
τ2 is the time between the biomolecular collisions.
For simple Brownian diffusion of
the radicals in solution:
τ2-1 = 4πdDfN
τ1-1 = (6D/d2)feu
Where
N = radical density
D = diffusion coefficient
d = interaction distance for exchange
f = (u/eu-1) and u = U(d)/kT are corrections for
intermolecular potential energy at contact
distance.
Width for .  ×
−  aqueous
solutions of PADS at
24°C as a function of
electrolyte
concentration.
Intermolecular Dipolar Interactions are not Important here.
Can show [(T2)-1 dipole/ (T2-1) exchange] = K(η/kT)2
“for strong exchange” = J2τ12 >> 1
 ∝ ℏ 
η = solvent viscosity
Eastman, Kooser, Das & Freed,JCP, 51, 2690 (1969) ; 52, 2511 (1970)
Translational Diffusion Coefficients by
ESR Imaging of Concentration Profiles: DID-ESR
Using 1D field gradients &
cw-ESR accurate
translational diffusion
coefficients ranging from
10-5 to 10-9 cm2/s were
measured in isotropic &
anisotropic fluids.
(−
 )
Concentration Profiles for Tempone diffusing
in a nematic phase at 300K at increasing times.
D ,PDT
D ,PDT
D  D =
1.41± 0.1
Nematic
D ,CSL
D ,CSL
ISOTROPIC/NEMATIC LIQUIDS:
D   to Nematic Director.
D   to Nematic Director
Sample Preparation
Smectic Liquid Crystal, S2
Small Probe: PDT D  D > 1
Large Probe: CSL D  D < 1
Lateral diffusion of CSL(
& 16PC (-  - ) in
phospholipid POPC vs.
cholesterol m.f. at different
temperatures .
CSL is Cholesterol
Analogue Spin Probe
Hornak, Moscicki, Schneider, Shin, Freed (JCP, 84, 1886 (1986);
Biophys. J. 55, 537 (1989); JCP 99, 634 (1993))
)
Along Spatial Axis to
Display The Spatial
Distribution:
Macroscopic Diffusion
Microscopic
vs.
Macroscopic
Diffusion
Coefficients
by ESR
SpectralSpatial
Imaging
Aligned POPC Membrane/16PC
•
•
Along Spectral Axis
to Display Spectral
Linewidth
Dependence on
Position: Heisenberg
Exchange
Spectral-Spatial Image in
Perspective
DID-ESR: Macroscopic Diffusion
Heisenberg Exchange broadening vs. concentration gradient:
Microscopic Diffusion
At 22°C :
Dmacro= (2.3 ± 0.4) X 10-8cm2/sec
Dmicro= (1.0 ± 0.4) X 10-7cm2/sec
Shin, Ewert, Budil, Freed BJ 59, 950 (1991)
Generalized Cumulant Expansions (GCE)
and Spin-Relation Theory (Freed, JCP 49 376 (1968))
1.
How to deal with break-down of
Motional Narrowing (WBR) Theory
Based on GCE method of Kubo.
2.
Leads to Relaxation Matrix to all
orders:
∞
This is a Complex
Expansion in powers of
 (t )  c
†
1
()
=
=
for t ≫ τc with R(n)
of order
†
 1 (t )
n
3.
n -1
c
Here H1(t) is the fluctuating timedependent portion of the Spin
Hamiltonian Operator and τc a
correlation time.
Also shows how to
introduce “finite time”
corrections when τc ≳ t .
The Stochastic Liouville Equation (SLE) and Slow
Motional ESR (with Bruno and Polnaszek, JPC 75, 3385 (1971) )

t
 - i  H ( t ),  
ρ : Spin Density Matrix
H(t): Random Hamiltonian

t
P (  , t) = -   P (  , t )



P(, t) : Probability of finding
 at t .
 time independent Markoff
Operator.
Leads to SLE:
ρ(,t): Joint Spin Density
Matrix As Well As Classical
Probability Density in .

Kubo (1969) showed
this with heuristic
argument.
Freed (1972) showed
this with generalized
moment expansion.
Hwang & Freed (1975)
developed this by
passing to semiclassical limit from
quantum stat. mech.
Leads to a “spin-force”
and/or “spin-torque”
back-reaction of spins
on bath. Confirms high
T limit.
Wassam & Freed (1982)
developed this from
even more general
many-body quantum
stat. mech.
ABSORPTION
Incipient
Slow Motion
Very
Slow
Motion
DERIVATIVE
Incipient
Slow Motion
Slow Motion
Very
Slow
Motion
Slow Motion
Line Shapes for S= ½, I= 1 (14N nucleus) with axially
symmetric g tensor, hyperfine tensor, and small ωn.
PADS in Frozen
D2O at -65°C. S. A Goldman
Very Slow Motion
--- Experimental
Calculated for Brownian
Diffusion
[K+]2
Electron Spin Relaxation of Nitroxide Probes in Solution:
Fast & Slow Motions and Search for a Model
(with Hwang, Mason & Hwang, JPC 79, 489 (1975))
Comparison of
experiment
and simulated
spectra in the
modeldependent
slow-tumbling
region for PDTempone in
toluene-d8
PD-Tempone
τR vs. η/T over five orders of magnitude
Non-secular spectral densities: j(ω)≈ τR/[1+ε2τR2]-1, ε >1
Brownian vs. Jump
Diffusion: Slow
Motional Fits.
Fluctuating Torques
(Fast Bath Modes) vs.
Slowly Relaxing
Structures (Slow Bath
Modes)
Efficient Computation of ESR Spectra and Related FokkerPlanck Forms by the Use of the Lanczos Algorithm (LA)
(with Moro, JCP 74 , 3757 (1981))
Spectrum from SLE:
I( ) 
1

R e{   [i (  1 - L    ]
-1
 }
L - Liouville operator associated with spin
Hamiltonian
 - Symmetrical diffusion operator
ν> - Vector of allowed spectral components
Distribution of the
eigenvalues for
calculation. Units
are in G; x & y axis
represent real &
imaginary parts of
the eigenvalues.
△ from Lanczos
algorithm;  exact.
The Lanczos algorithm : Let  ≡ L
By operating with A n times on ν> &
simple rearranging, an n-dimensional
orthonormal sub-set of the N >> n total
basis set is obtained such that An is tridiagonal with An= PnAPn-1 where Pn
projects out the “Relevant Sub-Space.”
This was the first significant
application of the LA to Complex
Symmetric (non-Hermitian) Matrices.
Leads to Order(s) of Magnitude
Reduction in Computer Space &Time.
Derivative
spectrum for
nonaxial g
tensor
Lanczos Steps rapidly
converge to solution
ESR and Spin Relaxation in Liquid Crystals
(with C.F. Polnaszek, JPC 79 2282, (1975))
Liquid Crystals Yield an Anisotropic
Environment:
Po (  )  exp ( - U (  )/kT )
 d  exp ( - U (  )/kT )
U() : Anistropic Potential
A challenge to diagonalization: Leads to nonsymmetric matrices. Render symmetric by
similarity transformation:
P (  ,t)  P0 (  )
- 1/ 2
P (  ,t)
Symmetrized Diffusion Operator:
 M R M +
M  R  MU
2 κT

TR T
(2 κT )
2
M: Vector Operator which generates an
infinitesimal Rotation.
T ≡ iMU() is the external torque derived
from the potential U().
Yielding:  P  t  -  P (  ,t)

Comparison of experimental (-----)
and theoretical (
) spectra for PDTempone in Phase V stresses the need
for SRLS model.
High Pressure ESR(J.S. Hwang and K.V.S. Rao, JPC 80, 1490 (1976))
More evidence for SRLS from High Pressure Experiments
10kbar
maximum
Graph of τR vs. pressure for PD
Tempone in phase V.
Comparison of experimental and simulated spectra at 45°C for
PD-Tempone in Phase V (a) 3450 bars (b)4031 bars ( - - - -)
experimental results; (· - · - ·) and (
) theoretical results for
different models.
SlowWave
Helix
ESR High Pressure Vessel
(Hydraulic)
General Theoretical Analysis Led to Expressions for SRLS
Spectral Density (JCP, 66, 483 (1977):
Where τR’-1= τR-1+
τx-1 and κ=1/5 for
isotropic
medium. Later
referred to as
“Model Free”
expression.
Electron Spin Relaxation & Ordering In Smectic
& Nematic Liquid Crystals
ESR spectra of 10-3 M P probe in smectic A phase
of S2 for various orientations θ between ňm & B.
The structures of some liquid crystals &
some ESR spin probes.
Meirovitch, Igner, Igner, Moro & Freed, 77, 3915 (1982)
ESR spectra calculated based on model of
cooperative chain distortions
MOMD ( Microscopic Order
Macroscopic Disorder) Model
The molecular motion is with respect to a local
“static” ordering potential, which is disordered
on a macroscopic scale.
A) ESR spectra of the
doxylstearic acids I(m,n) in
egg phosphatidylcholine
randomly oriented on
small glass beads
(phospholipid: spin-label
molar ratio 150:1). (B) ESR
spectra from rabbit small
intestinal brush border
vesicle membranes doped
with 12,3-DPPC.
Spectra simulated according to the MOMD model with
decreasing ordering & increasing motional rates from top to
bottom, illustrating typical temperature-induced spectral
evolution of the ESR response from lipid dispersions doped
with extended-chain doxy1 nitroxides.
Meirovitch, Nayeem & Freed,
JPC, 88, 3454 (1984)
SRLS (Slowly Relaxing Local Structure) Model
Reference frames which define the
structural and dynamic properties of
the combined system of spin-bearing
probe molecule and solvent cage: LF =
lab frame, DF = director frame, MF =
molecular frame, CF = cage frame, GF =
g tensor frame, AF = A tensor frame.
The SRLS model allows for the (slow) motion of the
local structure that is “frozen” in MOMD.
SRLS potential vint(β,γ) obtained from the best fit to the PDT
in toluene spectrum. This figure corresponds to the following
order parameters for PDT in the solvent cage:
(D200) = -0.437, (D202) = -0.482, (D400) = 0.271, (D200) = 0.253.
Polimeno & Freed, JPC, 99, 10995 (1995)
ESR Spectroscopy at 1 MM Wavelengths: FIR-ESR
(with Lynch & Earle, Rev. Sci. Instrum. 59, 1345, (1988))
First Quasi-Optical ESR Spectrometer – Transmission Mode
Fabry-Perot
cavity
M indicates
mirror
assembly
Newer: Quasi-Optical Reflection Bridge
Significant Increase in S/N
(with Earle & Tipikin, RSI, 67, 2502 (1996)
Fabry Perot
Resonator
Coupling Mesh
ESR Spectra of
PD-Tempone at
250 & 9.5 GHz in
solvents of
increasing
viscosity (a-e).
Paraboloidal
Focusing
Mirror
Detector
Focusing
Lens
*A motionally
narrowed
spectrum at
9 GHz looks
slow motional
at 250 GHz.
Corrugated
Wave-Guide
Polarization
Transforming
Reflector
Duplexing
Grid
Gaussian
Beam
TwoMirror
Telescope
Flat
Mirror
Source
Coupling
Lens
Paraboloidal
Focusing
Mirror
250 GHz Studies of
Molecular
Dynamics
Dynamic Cage Effects Above
the Glass Transition
(with Earle, Moscicki, Polimeno, JCP,
106, 9996, (1997))
PDT
MOTA
CSL
Experimental ESR
spectra taken at
250GHz covering the
entire temperature
range of liquid to
glassy behavior:
(a)PDT; (b) MOTA; and
(c) CSL.
OTP
Solvent
PDT
MOTA
CSL
OTP Cage
Rotational Diffusion Rates for Probes
dependent upon their size.
Relaxation of cage is the same
for all the probes.
Cage potential parameters below TM
(nominal melting temperature) depend on size &
shape of probe; above TM they all are zero.
Dynamics and Ordering in Mixed DMPC/DMPS Membranes
(with Barnes, BJ 75, 2532 (1998))
Nitroxide labeled CSL was studied in oriented membranes. A special “shunt”
Fabry-Perot resonator enabled study of both 0°C and 90°C orientation.
10°C
Gel
Phase
In PC:PS 80:20 , CSL shows typical characteristics: long axis of CSL parallel to bilayer
normal. As mole fraction of PS increases a second component grows in. A detailed
analysis shows that CSL senses a local, strongly biaxial environment.
Excellent Orientational
Resolution Enabled Key
Qualitative Features of
Model to be “read off”
the spectra before
detailed analysis.
Model in DMPS: A cutting motion
of CSL between domains of DMPS
Shunt Fabry-Perot Resonator
with Adjustable Interferometer
Multi-Frequency ESR & Molecular Dynamics in Biophysical Systems
(with Zhang, Fleissner, Tipikin, Liang, Moscicki, Lou, Ge, & Hubbell, JPCB, 105, 11053 (2001); 114, 5503 (2010).
Provides extensive experimental data to study microscopics of molecular dynamics.
The multi-frequency ESR studies to date cannot be adequately fit with simpler models,
but require the SRLS model, which provides adequate fit.
Complex Dynamics of Membranes
Complex Dynamics of SpinLabeled T4 Lysozyme
Standard MOMD fits are
in disagreement.
Only by the SRLS analysis
could results at both
frequencies be fit
simultaneously & with
physically sound axial
alignment of the acyl chains.
32°C
22°
12°
2°
Spectra At 4 Frequencies Were Fit Simultaneously To SRLS.
Yields 3 Distinct Components
Two-Dimensional
Fourier Transform ESR:
2D-ELDOR *
(with Gorcester, JCP 85, 5375 (1986); 88, 4678 (1988))
Absolute Value
2-D ELDOR of
PD-tempone in
toluene-d8 at
21°C.
Tmix= 3× 10-7 s.
Cross-peaks due
to Heisenberg
Spin Exchange.
Quadrature
Mixer
DC Block
Isolator
Modulator
TWT Amplifier
Spectrum after
LPSVD: Pure
2D- Absorption
representation.
GaAsFET
Pre-Amplifier
Pin Diode Limiter
2D-FT-ESR Spectrometer Block Diagram
* Original CW-ELDOR: Hyde, Chien, Freed JCP, 48,
4211 (1968)
2D-ELDOR
pulse
sequence:
3 /2 pulses
2D-ELDOR & Slow Motions
(with Lee, Patyal, Saxena, Crepeau CPL 221, 397 (1994))
with SRLS Analysis
(with Polimeno, JPC, 99, 10995 (1995))
Sc-
Sc+
The experimental technology for 2D-ELDOR had
progressed substantially & the detailed theory based on
the SLE was fully developed along with NLLS analysis.
By obtaining 2D-ELDOR spectra at 6-8 different mixing
times → actually a 3rd dimension to the experiment.
Time Domain in 2D-ELDOR Spectra
We found the spin-relaxation and motional dynamics information is very
extensive. Simple motional models could not fit data very well, so we
applied the SRLS model with considerable success:
In a complex fluid, one expects the molecular reorientation to be nonMarkovian. It is modeled in SRLS by both the Smoluchowski-type
diffusive rotation of the probe in a mean potential, and the diffusive
operator for the reorientation of the local structure (the cage) formed by the
molecules in the immediate surroundings of the probe.
Their collective motion constitutes a multi-dimensional Markov process.
Reference
Frames
for SRLS
LF – Lab Frame
DF – Director Frame
MF – Molecular Frame
CF – Cage Frame
GF – g-tensor Frame
AF – A tensor Frame
2D-ELDOR in Liquid Crystals: Multitude of Relaxation &
Dynamic Data
Optimum parameters obtained from fits to the SRLS Model (10
Tm=110 ns
CSL in
Macroscopically
Aligned Smectic A
phase of Liquid
Crystal 4O, 8
(59°C).
(Sastry, et al., JCP 105,
5753 (1996))
Tm=250 ns
such Parameters). Shows that in lower temperature phases the
dynamic cage freezes in to contribute to macro ordering
SRLS vs. Simple Fit of just
Brownian Reorientation in a
Macroscopic Aligning Potential
ESR Study of Heisenberg Spin Exchange in a Binary Liquid
Solution near the Critical Point






Heisenberg spin‐exchange
contribution ωHE to the ESR
linewidth of the di‐t‐butyl
nitroxide (DTBN) radical
dissolved in mixtures of
2,2,4‐trimethylpentane &
n‐perfluoroheptane.
The critical composition
χ(C8 H18) = 0.58.
Exhibits an anomaly in the
macroscopic kinematic
viscosity ν near Tc = 23.91°C
(for 4.3 x 10-3M DTBN).
In the critical region, ωHE is
not linear in T/ν. Instead, it is
linear in T/ν′
Here ν′ is the macroscopically
measured viscosity, but with
the ``anomalous portion''
subtracted out.
The experiments near the
critical region required
temperature stability &
control to within ±0.01°C at
the ESR sample.
Short range biomolecular
collisions are unaffected by longrange diverging hydrodynamic
viscous modes.
Lang & Freed, JCP, 56, 4103 (1972)
Divergence of Orientational Order Fluctuations about the
Nematic-Isotropic Weakly First-Order Phase Transition
(with Rao and Hwang PRL, 37, 515, (1976) & JCP 66, 4183, (1977))
Divergences are due to fluctuations in the
nematic order parameter as seen by the spin probe.
Divergence is symmetric
about Isotropic-Nematic
Phase Transition
ESR spectra of PD-Tempone in MBBA
near Tc = 41.4°C
Variation of B & C values with temperature.
Linewidth = A+BMI + CM2I
MI is 14N nuclear spin quantum number
Obeys Landau-de Gennes Mean Field Theory :
Isotropic Phase Diverges as (T – T*)-1/2
Nematic Phase Diverges as (T†- T)-1/2
Critical Fluctuations & Molecular
Dynamics at Liquid Crystalline
Phase Transitions
Structures of some nitroxide spin
probes.
Zager, Freed, CPL, 109, 270(1984)
Nayeem, Rananavare, Sastry & Freed, JCP 96, 3912 (1992)
 Nematic-Isotropic Transition: σ = ½ according to Mean
Field Theory for Weak First Order Transitions: Pretransitional nematic fluctuations affect probe rotational
dynamics.
 Nematic-Smectic Transitions σ = 1/3 according to scaling
laws analogous to the λ transition in He for 2nd order
transition: Pretransitional fluctuation in smectic order
affect position of probe in smectic layer: Expulsion of
probe to lower-density regions of transitory smectic layer.
Phase diagram
for mixtures of
4O, 6 & 6O,4
shown as a plot
of nematic
order
parameter S
versus the NA
phase transition
temperature,
TNA (x).
Tricritical Points in the Nematic to
Smectic Phase Transition
The straightline fit to
curve 1
yields an
exponent β2
= 1.00 ±
0.005, the
expected
mean-field
prediction.
Rananavare, Pisipati & Freed, CPL, 140, 255 (1987);
Rananavare, Pisipati & Freed, Liq. Crys. 3, 957 (1988)
The nematic
order
parameter S is
plotted versus
McMillan
ratio, M(n, m)
= TAN/TNI for
nO.m
homologues.
The straight
line fit yields
a β2 = 0.94±
0.12 and MTCP
= 0.959 ± 0.005.
Phase Diagram of He3He4 Mixtures. C is
Tricritical Point.
Dynamic Molecular Structure of Phase Domains in
Model & Biological Membranes by 2D-ELDOR
(with Chiang, Costa-Filho, JPCB, 111, 11260 (2007))
Excellent discrimination of the three
phases of mixed model membranes of
DPPC/Cholesterol with 16 PC
Absoption
Spectra in
Normalized
Contour
Mode: Shows
Homogeneous
Linewidths.
Yields this
Phase
Diagram
Effects on
PMV of
Crosslinking
of IgE
Receptors
% Population of Lo Phase:
decreases when receptors are
crosslinked.
Pure
Absorption
Components
for
coexisting
Lo & Ld
phases
The tie-line fields for co-existing lipid phases could also be
determined by ESR . (Smith & Freed, JPCB, 113, 3957 (2009)
The excellent
resolution
allowed the
analysis of the
effects of a
complex
biological
process upon
plasma
membrane
vesicles (PMV)
(with Chiang,
Costa-Filho &
Baird, JPCB,
115, 10462
(2011).)
Lipid-Gramicidin Interactions:
Dynamic Structure of the
Boundary Lipid by 2D-ELDOR
(A)Sketch of the effects of the presence of GA molecules
in lipid biolayer at concentrations lower & higher than
DPPC/GA = 15. (B) Molecular structure of the spin label
16-PC in its all-trans conformation (z-ordering).
2D-ELDOR, with its enhanced spectral resolution to
dynamic structure provides a reliable & useful way of
studying lipid-protein interactions.
2D-ELDOR contours for 16-PC at 35, 53, & 71°C,
& mixing time Tm = 1600 ns.
(A) 1:1 DPPC:GA; (B) 3:1 DPPC:GA; (C) 5:1
DPPC:GA; (D) pure DPPC.
Costa-Filho, Crepeau, Borbat, Ge & Freed, Biophys. J., 84,
3364 (2003)
The 2D-ELDOR spectra of the end-chain spin label 16-PC in
DPPC/GA vesicles is composed of two components, which
are assigned to the bulk lipids (with sharp auto peaks &
crosspeaks) & to the boundary lipids (with broad auto
peaks).
These spectra shows relatively faster motions & very low
ordering for the end chain of the bulk lipids, whereas the
boundary lipids show very high “y-ordering” & slower
motions. The y-ordering represents a dynamic bending at
the end of the boundary lipid acyl chain, which can then
coat the GA molecules.
Protein Structure Determination Using Long-Distance Constraints from
Double-Quantum Coherence (DQC) ESR: T4–Lysozyme
(with Borbat & Mchaourab , JACS 124, 5304 (2002))
DQC-ESR Pulse Sequence
/2 pulses = 3.2 ns
 pulses = 6.4 ns
T4L
Left: Time evolution of DQC Signal from
doubly labeled T4L; Right : their FT’s
Triangulation
Accounting for Flexibility
of Tether
Protein Superstructure: Bridging the Gap Between X-ray
Crystallography and Cyro-EM by Pulse-Dipolar ESR
(with Bhatnagar, Borbat, Pollard, Bilwes, Crane, Biochem. 49, 3824 (2010))
Structureless Protein Which Binds to Membranes: α – Synuclein
(with Georgieva, Ramlall, Borbat, Eliezer, JBC, 285 , 28261 (2010))
The End

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