Noncovalent spin labeled proteins (ovalbumin, bovine serum albumin, hemoglobin, and cytochrome c) were investigated in order to follow the different type of interactions between the nitroxide radical of 3-carbamoyl- 2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy spin label and functional groups of heme and nonheme proteins as well as the pH influence on molecular motion of the label with respect to these proteins. EPR spectra were recorded at room temperature and the computer simulation analysis of spectra was made in order to obtain the magnetic parameters. Noncovalent labeling of proteins can give valuable information on the magnetic interaction between the label molecule and the paramagnetic center of the proteins. The relevance of this interaction can be obtained from line shape analysis: computer simulations for nonheme proteins assume a Gaussian line shape, whereas for heme proteins, a weighted sum of Lorentzian and Gaussian components is assumed. In the framework of the “moderate jump diffusion” model for rotational diffusion, the rotational correlation time is strongly influenced by pH, because of the electrostatic interactions and hydrogen bonding.

1. Rotational Correlation Times of
3-Carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy Spin Label
with Respect to Heme and Nonheme Proteins
S. Cavalu*,† and G. Damian‡
Faculty of Medicine and Pharmacy, Department of Biophysics, University of Oradea,
P-ta 1 Decembrie No. 10, Oradea, 3700, Romania, and Faculty of Physics, Babes-Bolyai University,
Ro-3400 Cluj-Napoca, Romania
Received March 26, 2003; Revised Manuscript Received August 26, 2003
Noncovalent spin labeled proteins (ovalbumin, bovine serum albumin, hemoglobin, and cytochrome c) were
investigated in order to follow the different type of interactions between the nitroxide radical of 3-carbamoyl-
2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy spin label and functional groups of heme and nonheme proteins as
well as the pH influence on molecular motion of the label with respect to these proteins. EPR spectra were
recorded at room temperature and the computer simulation analysis of spectra was made in order to obtain
the magnetic parameters. Noncovalent labeling of proteins can give valuable information on the magnetic
interaction between the label molecule and the paramagnetic center of the proteins. The relevance of this
interaction can be obtained from line shape analysis: computer simulations for nonheme proteins assume
a Gaussian line shape, whereas for heme proteins, a weighted sum of Lorentzian and Gaussian components
is assumed. In the framework of the “moderate jump diffusion” model for rotational diffusion, the rotational
correlation time is strongly influenced by pH, because of the electrostatic interactions and hydrogen bonding.
Introduction
The successful application of spin labeling to protein
structure investigations is limited by the possibility to
chemically change specific side chains in proteins. However,
useful information on protein properties can be obtained by
noncovalent spin labeling if the affinity of the protein for
the label molecules is great enough to affect their motional
freedom.1-5
The rate of rotation (or tumbling) of the spin
label influences the line shape of its EPR spectrum.
Therefore, the EPR signal of a spin label covalently or
noncovalently bonded to a biomolecule can yield a range of
information about its structural environment in conventional
ESR and allows for full spectral coverage.6,7
At the same time, EPR has been an invaluable tool for
probing microscopic molecular motions in a variety of
systems, including isotropic solvents,8
liquid crystals,9
model
membranes, and biomolecules.10-12
In the present work, noncovalent spin labeled bovine serum
albumin (BSA), ovalbumin, bovine hemoglobin (BH), and
cytochrome c, with Tempyo spin label (3-carbamoyl-2,2,5,5-
tetramethyl-3-pyrrolin-1-yloxy) were investigated both in
liquid and lyophilized samples, in the pH range 2.5-11, to
obtain useful information related to the interaction between
the nitroxide group and the functional site of the proteins.
Interactions of the spin label with heme or nonheme
proteins might affect the spin label spectra, and at the same
time, it is well-known that the pH strongly influences the
conformation of proteins leading to significant changes in
the type and degree of these interactions. In this pH range,
we followed the effect of protein conformational changes
on the interactions between the nitroxide and the functional
groups of proteins and also the pH influence on molecular
motion emphasized by the EPR spectra of the spin label.
All structural and functional properties of proteins derive
from the chemical properties of the polypeptide side-chain.
The ionizable groups of the side-chains of charged amino
acids are often involved in biochemical transactions (binding,
catalysis). Therefore, pH usually has rather dramatic effects
on the function of proteins. Polar residues are both buried
as well as on the surface of the protein. They either form
hydrogen bonds with other polar residues in the protein or
with water, but nonpolar residues do not interact favorably
with water.
Materials and Methods
Powdered bovine serum albumin, ovalbumin, hemoglobin
(>95% methemoglobin) and cytochrome c from Sigma
Chemicals, were used without further purification. Proteins
were hydrated in phosphate buffer physiological saline at a
final concentration of 10-3
M. Tempyo spin-label (3-
carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy), from
SIGMA Chemicals (Figure 1), was added to the liquid
samples of each protein in a final concentration of 10-3
M
(protein/spin label molar ratio 1:1) and the pH values were
adjusted to the desired value in the range 2.5-11. A total of
5 mL was taken from each sample and was lyophilized for
30 h at -5 °C and used for the EPR measurement, at room
temperature.
* To whom correspondence should be addressed. E-mail: scavalu@
rdslink.ro.
† University of Oradea.
‡ Babes-Bolyai University.
1630 Biomacromolecules 2003, 4, 1630-1635
10.1021/bm034093z CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/16/2003

2. EPR spectra for both liquid and lyophilized samples were
recorded at room temperature with a JEOL-JES-3B spec-
trometer, operating in X-band (9.5 GHz), equipped with a
computer acquisition system. Samples were placed in quartz
capillary tubes. The spectrometer settings were as follows:
modulation frequency 100 kHz, field modulation 1 G,
microwave power 20mW. The computer simulation analysis
of spectra, for obtaining the magnetic characteristic param-
eters, was made by using a program that is available to the
public through the Internet (http://epr.niehs.nih.gov)
The line shape of an EPR spectrum depends on, among
other factors, the orientation of the paramagnetic center with
respect to the applied magnetic field. In a powder, or a frozen
aqueous solution, the paramagnetic centers will be fixed with
a random distribution of orientations, and in the case of the
anisotropic g factor and hyperfine interactions, this will lead
to a broadened EPR spectrum, because all orientations
contribute equally. In the liquid state, however, the para-
magnetic centers are not fixed but undergo rotational
fluctuation. In the case of fast rotation, the anisotropic
interactions are thereby averaged to zero, giving rise to sharp
EPR lines. If the velocity of the rotational motion decreases,
the EPR spectrum will approach that of the powder spectrum.
Therefore, a rotational correlation time for a paramagnetic
molecule can also be determined by EPR.
For isotropic motion in the rapid tumbling limit, the spectra
will be isotropic with the averages of the principal compo-
nents of the g values and hyperfine splitting factor, aN
. The
rate of the isotropic motion determines the relative widths
of resonances and the width, ∆Hm, of an individual (hyper-
fine) line, in the first approximation can be written as a
function of the z component of the nitrogen nuclear spin
number (m ) -1, 0, 1):13
where the A coefficient includes other contributions than
motion. The terms B and C are functions related to the
rotational correlational time (τ) and can be defined as a
function of peak to peak line width of the central line, ∆H0
[G], and the amplitudes of the mth line Im:11,12
in which
and ωN ) 8.8 × 10-6〈aN〉, aN is the isotropic hyperfine
splitting and ωe is the ESR spectrometer frequency in angular
units.
In the range from 5 × 10-11
to 10-9
s (motion in the rapid
tumbling limit) and a magnetic field above 3300 G, ∆g and
∆aN
vanish, and the correlation times τB and τC are directly
related to the B and C coefficients by the following simple
relations:4
where K1 ) 1.27 × 10-9
and K2 ) 1.19 × 10-9
. The average
correlation times is
The slow motion of the spin probe lead to a broadening of
the EPR lines. In this case, the rotational correlation time,
τ, is larger than 10-9 s, and thus, eq 8 is not applicable.
The isotropic nitrogen hyperfine splitting changes to a
powder like spectrum, with the peak-to-peak distance
between the external peaks of the spectrum (2a′zz(N)) de-
pending on the magnitude of the rotational correlation time,
τ. Another line shape theory for slow isotropic Brownian
rotational diffusion of spin-labeled proteins has been devel-
oped by Freed and co-workers.8 Thus, the correlation time
can be evaluated from the ratio of the observed splitting
between the derivate extrema a′zz and principal value azz,
determined from the rigid matrix spectrum:4,6
The R and β parameters are empirical constants depending
on the kind of the diffusion process and are tabulated in e.g.
Poole and Farach.14 For a small spin probe, the intermediate
jump diffusion is preferable.14
Results and Discussion
In the low concentration liquid state of proteins solution,
the Tempyo spin label, which is a relatively small molecule,
gives rise to a spectrum with narrow lines and constant
hyperfine splitting, typical for fast isotropic rotational motion
(Figure 2) with very low rate of migration between protein
molecule and water. For this kind of rotation, the rotational
correlation time can be estimated from the intensity ratio of
the low-field and high-field N lines using a semiempirical
formula (eq 8). For the Tempyo spin label in nonheme
proteins (BSA and ovalbumin) aqueous solution, the calcu-
lated rotational correlation times was 2.5 × 10-10 s, whereas
Figure 1. Chemical structure of the Tempyo spin label.
∆Hm ) A + Bm + Cm2
(1)
B )
1
2
∆H0( I0
I1
-
I0
I-1
)) 0.103ωe[∆g∆a
N
+
3(δg)(δa
N
)]τB[1 +
3
4
(1 + ωe
2
τB
2
)-1
](2)
C )
1
2
∆H0( I0
I1
+
I0
I-1
- 2)) 1.181 × 106
[(∆a
N
)2
+
3(δa
N
)2
]τc[1 -
3
8
(1 + ωc
2
τc
2
)-1
-
1
8
(1 + ωe
2
τc
2
)-1
](3)
∆a
N
) azz
N
-
1
2
(axx
N
+ ayy
N
),δa
N
)
1
2
(axx
N
- ayy
N
) (4)
∆g ) gzz -
1
2
(gxx - gyy), δg )
1
2
(gxx - gyy) (5)
τB ) τz ) K1B (6)
τC ) τx,y ) K2C (7)
τ ) (τBτC)1/2
(8)
τ ) R(1 -
a′zz
azz
)
β
(9)
Rotational Correlation Times of Nitroxide Radical Biomacromolecules, Vol. 4, No. 6, 2003 1631

3. with respect to heme protein (BH and cytochrome c)
solutions, the respective value was around 2. × 10-10
, which
is consistent with fast rotation as expected for a small
molecule.
No detectable changes were observed in the EPR spectra
of the aqueous solution of the Tempyo spin label without
proteins, at different pH values.
The characteristic EPR spectra of a Tempyo spin label in
lyophilized samples is due primarily to anisotropy in the
nitrogen hyperfine coupling typical for slow rotation. For
slow rotations, the EPR spectrum of spin labels, depends in
a much more complicated fashion on the combined influ-
ences of molecular motion and magnetic interactions. Figures
3-5 display the experimental and simulated spectra for
Tempyo spin label with respect to BSA, ovalbumin, BH,
and cytochrome c, lyophilized at pH ) 2.5, 6.7, and 11,
respectively. To find the magnetic parameters, the experi-
mental EPR spectra were simulated. The best fit of experi-
mental EPR spectra of the Tempyo spin label in lyophilized
BSA and ovalbumin was obtained assuming a single
paramagnetic species and a Gaussian line shape correspond-
ing to static dipolar interactions. The magnetic parameters
are listed in Tables 1 and 2, respectively.
The main feature of the EPR spectra of the Tempyo spin
label in lyophilized haemoglobin and cytochrome c exhibits
the characteristics to the slow motion of the spin label but
with more broadening of the line shape. The simulation of
the experimental EPR spectra can be obtained by assuming
the presence of two functional groups in heme proteins,
associated with two nonequivalent paramagnetic species.15
Computer simulations, indicate a weighted sum of mixed
Gaussian line shapes (static dipolar interactions) and Lorent-
zian line shapes (spin-spin interactions). To support this
affimation, in Figures 6 and 7 are presented the EPR spectra
of the Tempyo spin label in lyophylized BH at low and high
pH, in which are displayed the contributions of each species.
Generally, the broadening of the Gaussian line shape is due
to the static dipolar interactions of the spin label molecules,
whereas the broadening of the Lorentzian line shape is due
to spin-spin interactions.16-17
From the computer analysis
of spectra, we suggest that at low pH (pH ) 2.5) the main
contributions are due to species with a Gausian line shape
(∼80%). This contribution decreases to ∼50% at high pH
(pH ) 11). The first species, with Gaussian line shape and
poor resolved hyperfine splitting, is not influenced by the
Figure 2. EPR spectrum of the Tempyo spin label at low pH.
Figure 3. EPR spectra, experimental (solid line) and simulated
(dotted line), of the Tempyo spin label proteins at pH ) 2.5.
Figure 4. EPR spectra, experimental (solid line) and simulated
(dotted line), of the Tempyo spin labeled proteins at pH ) 6.7.
Figure 5. EPR spectra, experimental (solid line) and simulated
(dotted line), of the Tempyo spin labeled proteins at pH ) 11.
1632 Biomacromolecules, Vol. 4, No. 6, 2003 Cavalu and Damian

4. presence of the heme iron, and therefore, we assume to be
located far from the heme group. The second species with a
Lorentzian line shape and a well resolved hypefine structure
is located, probably, near the heme group, giving rise to a
spin-spin interaction between the nitroxide radical and the
paramagnetic iron of the heme group. Our results are in
accordance with covalently labeled methemoglobin and other
porphyrins in frozen samples under 50 K.18,19
In these
previous studies, spectra of covalently labeled methemo-
globin were analyzed by using perturbation calculations in
order to estimate the iron to nitroxyl distances, and it was
suggested that plausible distances are in the range of 14.5-
17.5 Å. The g tensor and A tensor components used in the
simulation for the best fit values of the simulation of the
effective powder spectrum are presented in Tables 3 and 4
(for Tempyo-BH and Tempyo-cytochrome c, respectively).
Because of increased spatial restrictions of the protein
structure in the vicinity of label, by lyophilization, the
mobility of the spin label is slow on the EPR time scale
(∼5 × 106
s-1
), leading to a broadening of the EPR lines,
with the peak-to-peak distance between the external peaks
of the spectrum (2a′zz(N)) depending on the magnitude of
the rotational correlation time, τ. Generally, a broadening
of the peaks in an EPR spectrum is indicative of immobiliza-
tion of the spin label, whereas sharpening of the peaks points
to an increase in label mobility. By comparison of the
apparent nitrogen hyperfine splitting (termed a′zz(N)) with
the nitrogen hyperfine splitting obtained from their rigid limit
values (a′zz(N)), the rotational correlation times can be
calculated using eq 9. The values of R and β coefficients
depend on the motional model. The study of the influence
of the different diffusional models on the spectral line shape
in the regime of the slow motional spin label by high EPR
fields has showed that jump diffusion mainly affects the line
widths at the same motional rates.20
In our calculations, the
intermediate jump diffusion model was considered, with
coefficients values of R ) 5.4 × 10-10
s and β ) -1.36.21
In Figure 8 are plotted the average of the correlation time
for differents values of the pH. As shown in the figure, the
pH influences the rotational correlation time of Tempyo with
respect to all these proteins. In the acid pH range, the NH2
groups of the label molecule as well as those of the amino
acids residues are protonated. The fact that τ shows greater
values in this range followed by a significant decrease in
the basic pH range indicates a low mobility of spin label in
acid environment, whereas an increasing of mobility can be
noticed in the basic pH range. From the pH dependence of
correlation time (involving the mobility of the label as well),
we assume that in an acid environment the mobilities of spin
label molecules are reduced because if the formation of the
hydrogen bonds between the NH2 group of the spin label
and the side chains of neighboring amino acids. In the case
of BSA, one can correlate this observation with the fact that
Table 1. Magnetic Parameters Values of the Tempyo Spin Label in Lyophilized Bovine Serum Albumin at Various pH Values
pH gxx gyy gzz Axx(G) Ayy(G) Azz(G)
11 2.0103 ( 4.3 × 10-4 2.0084 ( 4.3 × 10-4 2.0044 ( 4.3 × 10-4 4.7 ( 0.2 8.5 ( 0.4 35.5 ( 1
6.7 2.0011 ( 4.3 × 10-4 2.0074 ( 4.3 × 10-4 2.0050 ( 4.3 × 10-4 6.2 ( 0.3 6.6 ( 0.3 33.2 ( 0.9
2.5 2.0124 ( 4.3 × 10-4 2.0066 ( 4.3 × 10-4 2.0054 ( 4.3 × 10-4 8.3 ( 0.4 5.3 ( 0.3 36.0 ( 1.1
Table 2. Magnetic Parameters Values of the Tempyo Spin Label in Lyophilized Ovalbumin at Various pH Values
pH gxx gyy gzz Axx(G) Ayy(G) Azz(G)
11 2.0111 ( 4.1 × 10-4 2.0069 ( 4.1 × 10-4 2.0041 ( 4.1 × 10-4 4.3 ( 0.2 7.7 ( 0.3 34.1 ( 0.9
6.7 2.0012 ( 4.1 × 10-4 2.0063 ( 4.1 × 10-4 2.0053 ( 4.1 × 10-4 7.3 ( 0.3 7.2 ( 0.2 32.8 ( 0.8
2.5 2.0014 ( 4.1 × 10-4 2.0098 ( 4.1 × 10-4 2.0055 ( 4.1 × 10-4 4.6 ( 0.2 9.7 ( 0.4 35.6 ( 1.1
Figure 6. Experimental EPR spectrum and its subspectra of the
Tempyo spin label in lyophilized hemoglobin at pH ) 2.5.
Figure 7. experimental EPR spectrum and its subspectra of Tempyo
spin label in lyophilized hemoglobin at pH ) 11.
Rotational Correlation Times of Nitroxide Radical Biomacromolecules, Vol. 4, No. 6, 2003 1633

5. serum albumin undergoes reversible isomerization in the pH
range 2.7-7 from the expanded form characterized by 35%
R-helix content to normal the form characterized by 55%
R-helix content accompanied by a decrease in β-sheet.22,23
It is well-known that the β-sheet conformation favors the
formation of hydrogen bonding.
By comparing the results in Figure 8, we can notice that
the mobility of Tempyo is greater with respect to that of the
heme proteins, which is not surprising if we take into account
that hydrogen bonding opportunities depend on the β-sheet
content: in hemoglobin, the β-sheets represent 50%, whereas
in BSA, the percentage varies from 70% to 45%, depending
on pH. On averrage, τcyto < τHb < τovalb < τBSA.
As shown in Figure 8, the mobility of Tempyo increases
in an acid environment, followed by a slow decrease. We
suggest that in the basic pH range, where the label is not
subject to strong electrostatic interactions, dipolar and spin-
spin interactions are manifested almost with the same
contribution in the brodening of the spectrum.
Conclusions
EPR spectroscopy is very useful to study the mobility of
nitroxide radicals with respect to heme or nonheme proteins
in different environmental conditions. Noncovalent labeling
of proteins can give valuable information on the magnetic
interactions between the label molecule and the paramagnetic
center of the proteins. The relevance of this interaction can
be obtained from line shape analysis: computer simulations
for a nonheme protein assume a Gaussian line shape, whereas
for a heme protein, a weighted sum of Lorentzian and
Gaussian components is assumed. The contribution of the
each line shape to experimental spectrum depends on the
pH. We can conclude that, on averrage, τcyto < τHb < τovalb
< τBSA.
References and Notes
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Table 3. Magnetic Parameters Values of the Tempyo Spin Label in Lyophilized Hemoglobin at Various pH Valuesa
pH gxx gyy gzz Axx(G) Ayy(G) Azz(G)
11 2.0124 ( 4.3 × 10-4 2.0077 ( 4.2 × 10-4 2.0037 ( 4.2 × 10-4 4.8 ( 0.2 8.7 ( 0.4 34.0 ( 1 (L)
2.0112 ( 4.3 × 10-4 2.0050 ( 4.2 × 10-4 2.0061 ( 4.2 × 10-4 6.4 ( 0.3 7.8 ( 0.7 33.2 ( 0.9 (G)
6.7 2.0011 ( 4.2 × 10-4 2.0077 ( 4.2 × 10-4 2.0042 ( 4.2 × 10-4 3.3 ( 0.2 8.2 ( 0.4 33.7 ( 0.9 (L)
2.0122 ( 4.2 × 10-4 2.0048 ( 4.2 × 10-4 2.0056 ( 4.2 × 10-4 7.4 ( 0.3 8.9 ( 0.4 35.8 ( 1.1 (G)
2.5 2.0127 ( 4.2 × 10-4 2.0082 ( 4.2 × 10-4 2.0041 ( 4.2 × 10-4 4.1 ( 0.2 5.4 ( 0.2 35.6 ( 1.1 (L)
2.0117 ( 4.2 × 10-4 2.0029 ( 4.2 × 10-4 2.0051 ( 4.2 × 10-4 7.1 ( 0.3 9.4 ( 0.4 36.5 ( 1.1 (G)
a (L), Lorentzian line shape; (G), Gaussian line shape.
Table 4. Magnetic Parameters Values of the Tempyo Spin Label in Lyophilized Cytochrome c at Various pH Valuesa
pH gxx gyy gzz Axx(G) Ayy(G) Azz(G)
11 2.0155 ( 4.2 × 10-4 2.0109 ( 4.2 × 10-4 2.0056 ( 4.2 × 10-4 4.8 ( 0.2 8.7 ( 0.4 33.0 ( 0.9 (L)
2.0124 ( 4.2 × 10-4 2.0140 ( 4.2 × 10-4 2.0087 ( 4.2 × 10-4 6.4 ( 0.3 7.8 ( 0.3 34.2 ( 0.9 (G)
6.7 2.0141 ( 4.2 × 10-4 2.0118 ( 4.2 × 10-4 2.0049 ( 4.2 × 10-4 6.0 ( 0.3 5.9 ( 0.3 33.0 ( 0.9(L)
2.0136 ( 4.2 × 10-4 2.0113 ( 4.2 × 10-4 2.0057 ( 4.2 × 10-4 5.2 ( 0.2 3.7 ( 0.2 35.3 ( 1 (G)
2.5 2.0183 ( 4.2 × 10-4 2.0092 ( 4.2 × 10-4 2.0025 ( 4.2 × 10-4 3.3 ( 0.2 7.1 ( 0.4 32.1 ( 0.8 (L)
2.0124 ( 4.2 × 10-4 2.0129 ( 4.2 × 10-4 2.0087 ( 4.2 × 10-4 7.8 ( 0.4 6.4 ( 0.3 33.4 ( 0.9 (G)
a (L), Lorentzian line shape; (G), Gaussian line shape.
Figure 8. Correlation times (τ) as a function of pH for Tempyo spin
label in lyophilized cytochrome c (9), hemoglobin (b), ovalbumin (2),
and bovine serum albumin (1).
1634 Biomacromolecules, Vol. 4, No. 6, 2003 Cavalu and Damian

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