2. osmotic blood pressure.7
It has now been deter-
mined that serum albumin is chiefly responsible
for the maintenance of blood pH.
The substantial information on bovine serum
albumin (BSA) has led to some contradictory re-
sults and discussions. The previously reported
Raman spectra8
showed an intensity increase at
1235 cmϪ1
and a decrease at 940 cmϪ1
for the
BSA gel, indicating a drop in the ␣-helix content
accompanied by -sheet formation. An investiga-
tion of thermal, acid, and alkali denaturation of
BSA by Raman spectroscopy reported that heat-
ing to 70°C or a change in pH below 5 or above 10
caused an increase of the 1246 cmϪ1
band and a
decrease of the 938 cmϪ1
band. The interpretation
was that there is a decrease in the ␣-helix content
accompanied by an increase in  sheets.9
In another reported Raman spectrum of BSA10
the interpretation was made with the aid of pre-
vious amino acid data, highlighting a relationship
between the intensity of a given amide I band and
the amount of structural conformation to which it
is assigned. Further assignments10
were found to
be not relevant for the conformation of the pro-
tein.
Conformation-sensitive Raman bands of glob-
ular proteins were also reported11
in which the
amide I, amide III, and some skeletal modes are
assigned and used to compare and differentiate
among the ␣-helix, -sheet, and random coil con-
tent. The Raman active amide I mode for the
␣-helix conformation appeared at the same fre-
quency as in the IR spectra (1650 to 1657 cmϪ1
).
Strong splitting of the amide I mode was observed
for the antiparallel -sheet conformation.11
An
intense amide III band at 1235 cmϪ1
could be
assigned to the antiparallel  sheet, and the dis-
ordered conformation appeared at about 1245
cmϪ1
in the amide III region of the Raman spec-
trum.11
The lack of any strong Raman bands in
the range from 1200 to 1300 cmϪ1
was reported as
evidence for the ␣-helix conformation.
In the present Raman and SER spectroscopic
studies the conformational change of BSA was
investigated in a pH range of 6.7–11. All these pH
values are greater than the isoelectric pH value,
at which point the ONH2 groups are recovered
and OCOOH groups become anionic (OCOOϪ
).11,12
Cytochrome c is an essential component of the
mitochondrial respiratory electron transport
chain.13
The structure of cytochrome c is very
similar to hemoglobin in regard to the heme
group, the active site of this protein. This group is
not attached to the protein by only one axial li-
gand (⑀-imidazole nitrogen of histidine) of the iron
atom as in hemoglobin. In both oxidation states
there is a second axial ligand, the sulfur atom of
methionine 80, which is part of the protein. Ad-
ditionally, the heme is linked to cysteine 14 and
cysteine 17 by two thioether bonds.13,14
The iso-
electric pH value of this protein is 10.6.
A typical stable, nitroxyl-free radical widely
used as an electron spin resonance spin label,
2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy-3-carboxamide
(tempyo), was applied as a label on the solutions
of proteins at pH 6.7, 8.1, 9.5, and 11. In order to
study the magnetic interaction between the spin
label and the functional group of these proteins
(spin–spin and exchange phenomena) and the
motional effect in spin label spectra, samples for
electron paramagnetic resonance (EPR) spectros-
copy were prepared and subjected to a parallel
investigation (data not shown). The EPR investi-
gations followed the pH influence on the rota-
tional correlation time of tempyo with respect to
these proteins in the framework of the “moderate
jump diffusion” model for rotational diffusion.15,16
The localization of the tempyo label in the pro-
tein–tempyo complex structure could influence
the vibrational structure and, consequently, the
adsorption behavior of the proteins.
Therefore, in this article we present vibra-
tional Raman and SERS investigations on the
tempyo labeled BSA and ferrocytochrome c in a
pH range between 6.7 and 11, which is where the
mobility of tempyo is assumed to be sensitive with
respect to these proteins. The extension of the
study to SER spectroscopy was performed to
check the influence of the tempyo label on BSA or
cytochrome c adsorption and to study binding
sites and binding mechanisms of tempyo in the
tempyo–protein complex, if any exist.
MATERIALS AND METHODS
Chemicals
The BSA, cytochrome c, and tempyo spin label
powder were purchased from Sigma and used
without further purification. The cytochrome c
was reduced with a small excess of sodium dithio-
nite. Samples were prepared in phosphate buffer
physiological saline at a final concentration of
10Ϫ3
mol/L. Noncovalent labeling of each sample
was made in a 1:1 protein/tempyo spin label mo-
lar ratio. The pH range was adjusted between 6.7
and 11. A small amount (5 mL) of each sample
was lyophilized for 30 h at Ϫ50°C and than used
as the powder Raman sample.
342 CAVALU ET AL.
3. Colloidal silver substrate was prepared accord-
ing to the Lee–Meisel procedure.17
The maximum
absorption of the freshly prepared colloid was cen-
tered at 423 nm. Proteins were rehydratated in
buffer solutions for each pH value. A small
amount (10 L) of 10Ϫ2
mol/L protein solution
was added to 2 mL of colloidal silver, resulting in
a final sample concentration of 5 ϫ 10Ϫ5
mol/L.
Apparatus
A micro-Raman setup was employed to record the
Raman spectra of the lyophilized powder samples.
The 514.5-nm line of an argon ion laser (model
166, Spectra Physics) was applied for excitation.
The scattered light was collected in backscatter-
ing geometry by means of a 50ϫ objective (Olym-
pus ULWD MSPlan50). A LabRam Dilor with
1800 grooves/mm diffractive grating was used for
dispersing the scattered light. The detection sys-
tem consisted of a charge-coupled multichannel
detector.
A 10ϫ objective, a laser power of 1 mW, and an
exposure time of 1000 s with four overlaps were
used for the SERS spectra of BSA. When using
high power in the excitation of the SERS spectra,
the SERS signal was untrustworthy. When keep-
ing the low incident power (up to 1 mW), they
were reproducible. (Three or four experiments
were reproduced.)
In cytochrome c the acquisition of a single spec-
trum typically takes about 100 s, and four repeats
on each sample were done. Each Raman spectrum
is the result of four accumulations with 100-s
exposure time using a laser power of about 12
mW. The spectral resolution was 3 cmϪ1
.
RESULTS AND DISCUSSION
BSA Analysis
The Raman spectra of pure and tempyo labeled
BSA at different pH values and in the spectral
range of 500–1800 cmϪ1
are presented in Figure 1.
In comparison with previous Raman reports on
BSA,11
the spectral region between 1200 and
1300 cmϪ1
is poorly represented, indicating the
dominant presence of ␣-helix content. The second-
ary structure (␣ helix, -pleated sheet, and ran-
dom coil)10–12,18
is observed in the Raman spectra
through the contribution of the strong amide I
band (1653 cmϪ1
) and weak amide III band (1270
cmϪ1
). The phenyl stretching bands at 998, 1027,
and 1603 cmϪ1
are present in the Raman spectra,
indicating the aromatic amino acid residues. The
CH2 and CH3 scissoring modes are located at
1444 and 1461 cmϪ1
, respectively, similar to that
of the ovalbumin Raman spectrum.19
In addition,
bands from the side chains of some of the amino
acids residues (tyrosine, tryptophan, phenylala-
nine, etc.) are present.
Besides the common amide bands, the supple-
mentary17
disulfide bridges are characteristic for
BSA. The characteristic frequencies of COS and
SOS groups are present as weak–medium bands
in the 500–750 cmϪ1
spectral region. Comparing
the spectra from Figure 1 (spectra a–e), the only
major difference observed is the presence of a new
band at 1042 cmϪ1
for the pH 6.7 and 8.1 values.
This band was uncertain for any assignments;
moreover, the tempyo contribution to this was
excluded after studying the Raman behavior of
pure tempyo.
The label does not bring any contribution to the
vibrational Raman structure of the protein, in
spite of its large Raman cross section. This is
Figure 1. Raman spectra of pure (spectrum a) and
tempyo labeled BSA at pH 6.7 (spectrum b), 8.1 (spec-
trum c), 9.5 (spectrum d), and 11 (spectrum e). A
514.5-nm laser line at 100-mW power was used.
RAMAN AND SERS OF BSA AND CYTOCHROME c 343
4. probably due to the folding complexity of the pro-
tein, which limits the scattering effect of the
small tempyo label.
The SERS spectra of tempyo labeled BSA at
different pH values (Fig. 2) strongly differ from
their corresponding Raman spectra in both the
band positions and relative intensities. These dif-
ferences could have at least two possible explana-
tions: SERS and normal Raman probably span a
different portion of the protein structure, and
thus the bands of different wavenumbers and rel-
ative intensities are observed; further, based on
SERS theory,12,20
the molecules can be phy-
sisorbed or chemisorbed, the last case being char-
acterized through the drastic spectral change on
passing from Raman to SERS. When the chemi-
sorption takes place, the molecule of interest to-
gether with the nanometric surface builds a so-
called “metal–molecule SERS complex” where a
charge transfer contribution to the total enhance-
ment mechanism20
is present. According to the
literature,21,22
the amide I bands corresponding
to the ␣-helix, -sheet, and random coil conforma-
tions occur in the 1658–1640, 1680–1665, and
1666–1660 cmϪ1
ranges, respectively. On the
other hand, the amide III band positions of these
conformations were localized in the 1310–1260,
1242–1235, and 1250–1240 cmϪ1
regions, respec-
tively. In our SERS spectra of tempyo labeled
BSA the amide I band can be observed at 1645
cmϪ1
, which could be supposed as a Raman shift
toward a lower wavenumber, and the amide III
band is localized at 1304 cmϪ1
. A very strong
SERS band was observed at 1357 cmϪ1
. At this
position there is no correspondence in the Raman
spectra (Fig. 1). It is difficult to believe that the
weak band at 1400 cmϪ1
from the Raman is
shifted to such lower wavenumbers in SERS. It
would be more convincing if there was a SERS
contribution derived from the large Raman band
at 1334 cmϪ1
.
On comparing the SERS spectrum of pure BSA
with the SERS spectra at various pH values, only
one notable difference could be observed (Fig. 2):
the presence of the weak and medium phenyl
bands at 1615 and 1024 cmϪ1
, respectively. In the
previous published SERS spectrum of BSA23
the
authors reported that only those vibrations corre-
sponding to aromatic side chains are visible, in
addition to the amide I and III major bands. On
this basis they suggested a random coil conforma-
tion on the surface. However, it is not appropriate
to compare our SERS spectra with those obtained
in completely different conditions of surface, sur-
face plasmon resonance, or sample drying.
In our study we suggest a carboxylate group
interaction with the colloidal surface through the
presence of the enhanced amide I band and less
through the aromatic side chains for all the pH
values investigated. The secondary structure of
the title compound is obviously modified by the
adsorption induced structural changes.
These remarks suggest that the ␣-helix confor-
mation is probably the SERS binding site of BSA.
Moreover, in the pH range that was studied the
tempyo labeled BSA was found to be stable when
adsorbed onto the colloidal silver particles (with
insignificant differences from the pure BSA). The
pure tempyo was found to be SERS inactive, in-
dependent of the concentration in the colloidal
final sample or laser power.
Cytochrome c
Excitation with the 514.5-nm line falls into one of
the absorption bands (408, 520, and 544 nm) of
cytochrome c, leading to the RR spectra of the
sample. Figure 3 presents the RR spectrum of
pure cytochrome c in the solid state in comparison
with the spectra of cytochrome c at pH 6.7, 8.1,
9.5, and 11.
Figure 2. SERS spectra of pure (spectrum a) and
tempyo labeled BSA at pH 6.7 (spectrum b), 8.1 (spec-
trum c), 9.5 (spectrum d), and 11 (spectrum e). A
514.5-nm laser line at 100-mW power was used.
344 CAVALU ET AL.
5. The very strong at 1580 (CAN) and 1308 cmϪ1
(porphyrin stretching); the strong bands at 1530
(CAC), 1393, and 1356 cmϪ1
(ACON); and the
medium bands at 1225 ␦(COH), 1166, and 1124
cmϪ1
are observed in concordance with the previ-
ously reported results on cytochrome c.24–26,29
The band at 1225 cmϪ1
is believed to arise from
an in-plane bending vibration of the methine hy-
drogens. In our RR spectra this vibration seems to
not be affected by the pH variation, even if H-
NMR and visible absorption studies30
showed
that pH, temperature, or ionic strength perturba-
tions readily displaced the methionine ligand. It
was suggested that the loss of the methionine
ligand results in the loss of the electron transport
capability of cytochrome c. When comparing the
RR spectra of cytochrome c in both oxidation
states, ferricytochrome c exhibited a very differ-
ent Raman spectrum from ferrocytochrome c,
which is much more sensitive to RR.26
For the ferricytochrome (oxidized form, Fe3ϩ
)
three bands of medium intensity at 1560, 1585,
and 1638 cmϪ1
are reported26
and interpreted as
CAC and CAN stretching vibrations of the por-
phyrin macrocycle. Other RR spectra27
of oxidized
and reduced cytochrome c in solution correlated
the spectral band at 1584 cmϪ1
with the existence
of low spin iron in both oxidation states and the
position of the spectral band at 1375 or 1360 cmϪ1
with oxidized or reduced cytochrome c, respec-
tively. A general shift toward a lower frequency
was concluded upon reduction of Fe3ϩ
to Fe2ϩ
. On
the other hand, chemical modification of the heme
protein resulting from the conversion of iron from
a low to a high spin state was correlated with a
1584–1566 cmϪ1
shift, which is ascribed to the
movement of the iron atom out of the heme plane
upon the increasing of its spin state.29,31
Based on
the previous observations, we conclude that the
strong band located at 1580 cmϪ1
(Fig. 1, spec-
trum a) indicates the presence of low spin Fe2ϩ
.
The tempyo spin label was expected to reveal a
scattering contribution in the cytochrome c–tem-
pyo complex, based on its Raman spectrum (Fig.
4, spectrum a). In Figure 4 one can observe that
the label does not make any contribution to the
vibrational structure of this protein, in spite of its
large Raman cross section. This is probably due to
the folding complexity of the protein, as in the
BSA, which limits the scattering effect of the
small tempyo label. We suppose that the label is
bound within a certain distance from the heme
group in the basic pH range of 6.7–11.
A comparison of Figures 3 and 4 reveals the
Figure 3. Resonance Raman spectra of pure (spectrum a) and lyophilized cytochrome
c at pH 6.7 (spectrum b), 8.3 (spectrum c), 9.5 (spectrum d), and 11 (spectrum e). The
excitation was at 514.5 nm, and there was 12-mW power on the powder sample.
RAMAN AND SERS OF BSA AND CYTOCHROME c 345
6. very stable conformation of the cytochrome c
heme group in the pH range from 6.7 to 11, in
spite of the fact that the amino acid residues are
very sensitive to the pH variation. Furthermore,
the RR spectra do not reflect changes in the pro-
tein content around the porphyrin ring.
Figures 5 and 6 respectively present the
SERRS spectra of pure cytochrome c at the corre-
sponding pH values with or without tempyo spin
label. The first SERRS of cytochrome c24
showed
that this spectrum appeared to be very similar to
its solution RR spectrum. Other previous works,
which were based on a comparison between the
SERRS and RR spectra of the oxidized form, con-
cluded that the heme was detached from the pro-
tein at the level of the silver surface and formed
oxo-dimers.25
When passing from RR to SERRS in the
present work, large differences can be observed in
the band positions and relative intensities. The
dominant SERRS bands are observed at 1633
(CAN), 1581 (CAN), 1562, 1399, 1369, 1166, and
1127 cmϪ1
. In contrast to the corresponding RR
spectra, the in-plane bending vibration of the me-
thine hydrogens (1227 cmϪ1
) is poorly repre-
sented but it seems to be pH dependent. The
blueshift from 1356 (RR) to 1369 cmϪ1
(SERRS)
and the enhancement indicate an interaction of
the N atoms from (ACON) bonds of the porphyrin
ring through the bonds with the metal surface.32
According to the literature,33
porphyrin macro-
cycles in cytochromes and other porphyrin com-
pounds bind edge-on to silver surfaces via propi-
onate functional groups. Other reports1
showed
that the protein envelope prevents the heme
group from coming in direct contact with the sil-
ver electrode surface. If the heme group were
located far from the surface, the local electromag-
netic enhancement of the heme modes would be
less representative. Because the resonance contri-
bution in the signal intensity is the only one re-
maining, this would be similar to the RR signal,
which is not observed in our SERRS spectra.
The SERRS spectral features of cytochrome c
are certainly attributable to the adsorption in-
duced structural changes in the heme pocket, re-
sulting in the corresponding enhancement of the
porphyrin cytochrome c–Ag complex modes, inde-
pendent of the presence of the tempyo label. The
pure tempyo was found to be SERS inactive once
more and independent of the protein concentra-
tion in the colloidal final sample or the laser
power.
Figure 4. The Raman spectrum of tempyo (spectrum a) and resonance Raman spectra
of the lyophilized tempyo labeled cytochrome c at pH 6.7 (spectrum b), 8.3 (spectrum c),
9.5 (spectrum d), and 11 (spectrum e). The excitation was at 514.5 nm, and there was
12-mW power on the sample.
346 CAVALU ET AL.
7. CONCLUSIONS
The suspected tempyo induced motional effects or
conformational changes of tempyo labeled BSA in
the basic pH region between 6.7 and 11 were
concluded to be absent from the Raman spectra.
Pure or tempyo labeled BSA in the basic pH range
(6.7–11) was found to be adsorbed on the silver
Figure 5. SERRS spectra of pure (spectrum a) and lyophilized cytochrome c at pH 6.7
(spectrum b), 8.3 (spectrum c), 9.5 (spectrum d), and 11 (spectrum e). The excitation was
at 514.5 nm, and there was 12-mW power on the sample.
Figure 6. SERRS spectra of tempyo labeled cytochrome c at pH 6.7 (spectrum a), 8.3
(spectrum b), 9.5 (spectrum c), and 11 (spectrum d). The excitation was at 514.5 nm, and
there was 12-mW power on the sample.
RAMAN AND SERS OF BSA AND CYTOCHROME c 347
8. colloidal particles, and there was a chemical con-
tribution to the total enhancement mechanism.
The amide I and III band contributions in the
SERS spectra suggest that the ␣-helix domain of
the free or labeled protein is closer and interacts
with the colloidal surface.
In the studied pH range, the RR spectra of
cytochrome c (pure or labeled) indicate a stable
conformation of the heme group. The tempyo la-
bel was supposed to be bound within a certain
distance from the heme group in the basic pH
6.7–11 range. From the SERRS study of the tem-
pyo labeled cytochrome c on the silver surface, a
chemical contribution to the total enhancement
was concluded. The enhancement of the bands
assigned to the porphyrin macrocycle stretching
modes allowed the supposition of the N-adsorp-
tion from the porphyrin ring to the colloidal sur-
face. The adsorption of the cytochrome c on the Ag
surface under resonance conditions is indepen-
dent of the pH in the range from 6.7 to 11 or the
presence of the tempyo spin label.
The financial support from the World Bank is grate-
fully acknowledged.
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