Comparison of the core size distribution in iron dextran complexes using Mössbauer spectroscopy and X-ray diffraction

$Page 1: Comparison of the core size distribution in iron dextran complexes using Mössbauer spectroscopy and X-ray diffraction$
0162-0134/99/$ - see front matter q 1999 Elsevier Science Inc. All rights reserved.PII S0162- 0134 (99 )00020 -3

Friday Apr 30 11:55 AM StyleTag -- Journal: JIB (Journal of Inorganic Biochemistry) Article: 6157

Journal of Inorganic Biochemistry 73 (1999) 227–233

Comparison of the core size distribution in iron dextran complexes usingMossbauer spectroscopy and X-ray diffraction¨

Brian Knight a, Lawrence H. Bowen a,1, Robert D. Bereman a,*, Shuyen Huang b,Eddy De Grave c,2

a Department of Chemistry, Box 8204, North Carolina State University, Raleigh, NC 27695-8204, USAb TAP Holdings Inc., 2355 Waukegan Road, Deerfield, IL 60015, USA

c Department of Subatomic and Radiation Physics, University of Ghent, B-9000 Ghent, Belgium

Received 27 April 1998; received in revised form 25 January 1999; accepted 1 February 1999

Abstract

Mossbauer spectra of a series of iron dextran complexes in the intermediate temperature range where both sextet and doublet coexist may¨be used to obtain a qualitative description of the distribution of core sizes in these samples. Eight samples from five suppliers have beenexamined at 100 and 77 K. These differ markedly in the relative doublet contribution to the total spectral area and also in the hyperfine fieldscharacterizing the sextets. The results indicate three distinct types of distribution. One sample from each type has also been examined at 4 K,where the doublet component has vanished and the hyperfine field distribution has become narrow and symmetric. These data are comparedwith estimates of average core diameters from X-ray line broadening. q 1999 Elsevier Science Inc. All rights reserved.

Keywords: Mossbauer spectroscopy; Iron dextran; Akaganeite; Core size; Nanophase; Hyperfine-field distribution; Line broadening¨

1. Introduction

A polysaccharide such as dextran interacts strongly withferric oxide and can solubilize the oxide in aqueous solutionat neutral pH. The soluble organic component with an inor-ganic core mimics the structure of the iron storage proteinferritin, and materials of this kind are used for the treatmentof anemia. The inorganic core has to be small in order for thematerial to be soluble, and considerable effort has beenrequired to study the nature of the core due to its poor crys-tallinity. These studies have concentrated on determining itsmineralogy and size.

Two techniques which have been used in the past to char-acterize polysaccharide–iron complexes are Mossbauer spec-¨troscopy and X-ray diffraction. These were applied to severalcommercial samples of iron dextran [1,2] and to an oralhematinic (Niferex) [3]. These two techniques probe thesolid state structure of the iron oxide core, but not the dextrancoating. The X-ray diffraction technique is sensitive to long-range crystalline order and thus allows the distinction

* Corresponding author. Tel.: q1-919-515-9370; fax: q1-919-515-8909;e-mail: robert–[email protected]

1 Deceased.2 Research Director, Fund for Scientific Research, Flanders.

between different mineralogical forms of iron oxide. In thecase of nanophase particles (i.e. the cores) there is consid-erable broadening of the diffraction lines which makes thedetermination of line position difficult. However, this broad-ening is directly related to the length of the diffracting zoneand thus provides a measure of the size of the core. Mossbauer¨spectroscopy is complementary in that it provides parameterssensitive to the local environment around the iron. Theseparameters are center shift (d), quadrupole splitting (DEQ),and magnetic hyperfine splitting (H). DEQ and H are a meas-ure of the electric field gradient and internal magnetic fieldat the Fe-57 nucleus, respectively.

Both Imferon and Niferex had similar core structure, ahydrous iron oxide of about 3 nm diameter (Imferon [2])and 4 to 5 nm (Niferex [3]) resembling akaganeite, -b

FeOOH, in their X-ray diffraction patterns. The Mossbauer¨

spectra [1–3] showed characteristic magnetic ordering at lowtemperature and gradual collapse of the sextet over a widerange of temperature, beginning below 30 K and continuingin some cases above 100 K. When particles of magneticmaterial are very small and form single domains, the availa-bility of thermal energy at a specific temperature may besufficient to overcome the anisotropy barrier. Then, the mag-netic vector associated with the domain can flip between the

B. Knight et al. / Journal of Inorganic Biochemistry 73 (1999) 227–233228


Table 1Samples of iron dextran examined in this study

Sample designation Supplier Wt.% Fe Wt.% Cl

A1-DS a A 15.9 1.7A1-DP A 15.9 1.7A2 A 16.8 1.8B1 B 15.3 1.8B2 B 15.2 1.6C1 C 16.2 1.9D1 D 30.6 3.8E1 E 28.4 6.3

a Bulk drug used to make A1-DP lyophilized powder. The wt.% Fe and Clare determined for the bulk drug for A1-DP and A2.

Table 2Mossbauer data a at 100 and 77 K¨

Sample 100 K 77 K

%D b Hmax Have %D b Hmax Have

A1-DS 45 457 379 33 465 392A1-DP 47 451 384 33 459 389A2 48 450 372 37 457 385B1 47 451 372 34 457 385B2 39 455 363 27 461 382C1

c 16 423, 180 327 6 442 348D1

c 24 410, 190 301 13 435 336E1

c 63 420, 182 382 44 433 305

a The lower Hmax is an artifact of the fitting.b %D is the relative doublet contribution to the total spectral area.c These samples have such broad distributions of fields at 100 K that twomaxima are observed.

two easy directions at a rate higher than the Larmor frequencyof the excited Fe-57 and the resultant magnetic field of thenucleus would be zero. In that case, the sextet would collapseto a doublet. This collapse is most likely due to superpara-magnetism of the nanophase particles, and appears to be quitesensitive to minor variations in core size [1].

The availability of a number of new commercial iron dex-tran drug products prompted the present study to see if Moss-¨bauer spectroscopy could detect variations in core structureamong these. The spectra were obtained at 77 and 100 K,temperatures where doublet and sextet coexist, and also insome cases at 4 K, where the doublet has vanished.

The data have been interpreted by a simplified model ofsuperparamagnetism which provides insight into the varia-tion in size distribution in the iron oxide core of theseproducts.

2. Experimental

Mossbauer spectra were obtained at 100 and 77 K using a¨closed-cycle helium cryostat maintained at the required tem-perature to within about 0.1 K. The spectrometer was anAustin Science Associates constant-acceleration drive withvelocity calibrated by laser interferometry. The source wasabout 25 mCi 57Co(Rh) and was kept at room temperature.Spectra at 4 K were obtained with the sample cooled in aliquid helium cryostat, using a Wissel spectrometer and about50 mCi source. The spectra were fitted with a distribution ofhyperfine fields for the sextet and a distribution of quadrupolesplittings for the doublet portion of the spectrum [1,3].

X-ray diffraction patterns were obtained with a Rigaku D/Max-B diffractometer using Cu K radiation and a diffracteda

beam graphite monochromator. Sample preparationconsistedof grinding the sample with acetone and allowing the result-ing suspension to dry on an oriented quartz plate. The quartz(112) line from the plate was used as an internal standardboth to correct peak positions and to estimate the contributionof instrumental line broadening to the width of the peak(0.158).

Seven lyophilized powders from five different supplierswere examined in the present work (Table 1). For one ofthose, the bulk drug used to make the drug product solutionwas also studied. Elemental analysis indicates five of thelyophilized powders and the bulk drug have 15–17% Fe and1.6–1.9% Cl, while the other two powders have appreciablyhigher Fe and Cl.

3. Results

The Mossbauer spectra at room temperature (RT) wereälmost identical for all these samples and consist of a simpledoublet with ds0.36 mm sy1 relative to iron metal,DEQ(max)s0.68 and DEQ(ave)s0.76 mm sy1, whereDEQ(max) corresponds to the quadrupole splitting of maxi-

mum probability. The d indicates that the iron is Fe3q inoctahedral coordination to oxygen, while the DEQ valueshavea range corresponding to a distribution of particle sizes. Thevalues are similar to those of other iron dextran complexespreviously studied [1].

At low temperature, the small particles order magnetically,and the Mossbauer spectrum changes from a doublet to a¨sextet. It is the intermediate temperature region where bothdoublet and sextet exist that is the most sensitive to smallvariations in the core size. The Mossbauer results are givenïn Table 2 for both 100 and 77 K. In this table, the hyperfinefield distributions are characterized by two parameters: Hmax,the field of maximum probability, and Have, the average field.Example spectra are shown in Figs. 1 and 2.

The first five samples in Table 2, those from suppliers Aand B, have Mossbauer parameters that are similar for both¨temperatures: at 100 K the percent doublet is 45"4%, Hmax

is 453"3 kOe, and Have is 374"8 kOe; at 77 K the percentdoublet is 33"4%, Hmax 460"3 kOe, and Have 387"4 kOe.The drug substance A1-DS and the drug product from it, A1-DP, have essentially the same parameters. Sample B2 has asomewhat lower doublet contribution than the others,although the differences are within 2s.

The other samples are quite different. Samples C1 and D1

have a much lower percent doublet than the above, while

B. Knight et al. / Journal of Inorganic Biochemistry 73 (1999) 227–233 229


Fig. 1. Mossbauer spectra of selected samples at 77 K: (a) sample C1, (b)¨sample A1-DP, (c) sample E1.

Fig. 2. Mossbauer spectra of selected samples at 100 K: (a) sample C1, (b)¨sample A1-DP, (c) sample E1.

Table 3Mossbauer data at 4 K¨

Sample Hmax Have

A1-DP 490 483D1 480 478E1 469 466

sample E1 has a much larger percent doublet. At 77 K allthree of these have Hmax and Have much lower than samplesfrom suppliers A and B. The same is true for samples C1 andD1 at 100 K. Sample E1 has such a small and broad sextetcomponent at 100 K (Fig. 2) that its Hmax and Have are notwell-defined at that temperature and thus cannot be comparedto the others. The distinctive behavior of sample E1 is likelyto be related to its high Cl/Fe ratio (Table 1).

In order to explore the behavior at temperatures where onlya sextet is present, three samples (A1-DP, D1 and E1) repre-sentative of the different types were examined at 4 K. Dataare given in Table 3 and a sample spectrum is shown inFig. 3.

The X-ray diffraction patterns consist of broad peaks indic-ative of akaganeite, -FeOOH [4], of very poor crystallinity.b

Even though the patterns have considerable background scat-tering from X-ray fluorescence, nonlinear curve fitting canbe used to determine peak positions and linewidths. Theresults in Table 4 are based on curve fitting of the two most



Fig. 3. Mossbauer spectrum of sample A1-DP at 4 K. There is a slightäsymmetry in that the peak at highest velocity is broader and less intensethan the one at lowest velocity. This is in qualitative agreement with the lowtemperature spectra of akaganeite [14], and has been fitted with a correlationbetween hyperfine field and quadrupole/center shifts.

Table 4Mean crystalline dimension a (MCD) perpendicular to the planes for thetwo most prominent X-ray diffraction peaks, d(310);0.33 nm andd(211);0.25 nm

Sample MCD(310) (nm) MCD(211) (nm)

A1-DP 2.7 3.9A2 3.5 4.0B1 3.4 4.9B2 4.0 4.4C1 4.2 5.2D1 3.9 4.8E1 2.7 5.3

a The reproducibility in determining MCD is only about "0.5 nm.

Fig. 4. Sample X-ray diffraction patterns showing the region from 24 to 398

2u fitted with two peaks corresponding to the (310) and (211) lines ofakaganeite: (a) sample C1, (b) sample E1.

prominent lines at 2u;278 and ;358 to a sum of Lorentziansaccounting for the K 1 and K 2 radiation [5]. Duplicatea a

patterns were taken for most of these samples to check forreproducibility. The d spacings for these lines are within0.002 nm of those for pure akageneite, d(310)s0.333 nmand d(211)s0.255 nm [4]. The fitted linewidths (see Fig.4) were used with the Scherrer equation [6] to estimate themean crystalline dimensions (MCDs) reported in Table 4.These are the diameters of the iron core perpendicular to theplanes. All samples are nanophase and are also distorted fromspherical, because the MCD(211) is measurably greater thanMCD(310). The C1 core is the largest, but the D1 core isalmost as large. If the four samples from suppliers A and Bare assumed to be the same size, they give average values ofMCD(310)s3.4"0.5 nm and MCD(211)s4.3"0.5 nm.Surprisingly, although the Mossbauer spectra of sample E1¨

are considerably different from those of the others, the MCDsare within the range of the other samples. The X-ray patternof sample E1 was distinguished from all the others in havingmuch lower background counts in the region around the 27and 358 lines (see Fig. 4).

4. Discussion

The variations observed in Mossbauer parameters (Table¨2) indicate distinct differences between the commerciallyavailable iron dextrans. These differences are related to thedistribution of core size, not to the basic crystallographicstructure, which is nanophase akageneite for all. It should benoted that interstitial water plays an important role in deter-mining the magnetic properties of bulk akaganeite [7],although there is no evidence that the water content of theiron dextran cores varies in the present case. According tothe X-ray diffraction linewidths, sample C1 has the largestcore size. This difference is reflected in a much smaller dou-blet component than for the others, only 6% at 77 K and 16%at 100 K. In general, a large particle size should also corre-spond to large hyperfine fields. Surprisingly, the two sampleswith the largest core size according to the doublet area, sam-ples C1 and D1, have hyperfine fields Hmax appreciably lowerthan those of samples A and B. The 100 K spectra of samplesC1 and D1 have a very broad distribution of fields such thatthere appear to be two maxima in the probability distribution,although the lower one is an artifact of the fitting. The E1

sample also has a low Hmax, as well as a large percent doubletat both 100 and 77 K. The first five samples in Table 2, thosefrom suppliers A and B, have very similar hyperfine fields



Table 5Values of temperature TB, the particle diameter corresponding to this tem-perature, dB, and particle diameter corresponding to maximum probability,dmax

a

Sample TB (K) dB (nm) dmax (nm)

A1-DP 190 4.9 4.5A2 190 4.9 4.4B1 190 4.9 4.4B2 220 5.1 4.6C1 290 5.6 4.1D1 260 5.4 3.9E1 150 4.5 4.2

a Sizes calculated using Ks170 kJ my3. This value of K corresponds tods3.9 nm for collapse at 100 K and 3.6 nm for 77 K.

and percent doublets at both temperatures, indicating a strongsimilarity in the nature of their cores. Thus, there are threedistinct types of iron dextran according to these Mossbauer¨spectra: the samples from suppliers A and B, samples C1 andD1, which have smaller percent doublet and hyperfine fieldat a given temperature, and sample E1, which has smallhyper-fine field, but large percent doublet. In the discussion below,an attempt is made to rationalize these differences in termsof a simplified model of superparamagnetism.

Superparamagnetic relaxation of small particles is an expo-nential function of particle volume. According to Neel [8],´the relaxation time is given by

tst exp(KV/kT) (1)0

where t0 is of order 10y10 s, K is the anisotropy constant, kthe Boltzmann constant, and V the particle volume. Below acertain temperature TB the relaxation time is long and theMossbauer spectrum will be a sextet. Above TB, fast relaxa-¨tion causes the sextet to collapse to a doublet [9]. There aretwo ways that have been used to define TB, sometimes calledthe blocking temperature. One way is to measure the temper-ature at which equal areas of sextet and doublet are present(mean blocking temperature). An alternative is the upperblocking temperature where the sextet completely collapsesto a doublet. This is the definition used for TB in the presentwork. Thus, the mean blocking temperature here correspondsto half the particles below their TB and half above. The TB forthe sample corresponds to the highest temperature at whichany sextet is observed.

A distribution of particle volumes in a sample will producea range of relaxation times and thus a range of temperatureswhere the sextet and doublet coexist. The relative proportionof doublet will then increase as temperature increases. In asimplified model, particles with a given volume will givespectra which collapse to a doublet at a certain temperature.The smaller particles will produce a doublet at lower tem-perature. The temperature variation of doublet area strictlydepends on the distribution of KV, not just V alone. Theeffective anisotropy constant K depends on particle size. Ingeneral, K is much larger for small particles; for example, Kfor 6.3 nm Fe is 8 times that of macroscopic crystals, K for 4nm Ni is 100 times that of bulk [9]. In addition to the collapseof the sextet at TB, there is a decrease in hyperfine field belowthis temperature, given for KV4kT [9] by

H sH (1ykT/2KV) (2)obs 0

where H0 is the field for macroscopic crystals. The aboveequations have been applied to interpretation of the Moss-¨bauer spectra of 6, 10 and 12 nm Fe3O4 particles [9–11], andto the spectra of the iron storage protein ferritin [12].

The temperature where the sextet completely disappears,TB, can be estimated by extrapolation of the percent doubletat 77 and 100 K (Table 2). This temperature (Table 5) isabout the same for A1-DP, A2 and B1 at TB;190 K, consid-erably higher for C1 and D1, and lower for E1. The field ofmaximum probability, Hmax, is well-measured at 77 and 4 K

(there is some ambiguity at 100 K because the sextet is sobroad). This field, Hmax, should be related to a particlevolumeof maximum probability through Eq. (2). The value of K forEqs. (1) and (2) is a priori not known. Williams et al. [12]report that K should be 1–10 kJ my3 for all the FeOOH andrelated hydrous oxides. For ferritin with average diameter 6.5nm, they give Ks6.7 kJ my3. Mørup et al. [11] quoteKs130 kJ my3 for 6.0 nm Fe3O4. Spectra of the latter col-lapse to a doublet at about 200 K, while the ferritin collapsesat much lower temperature, about 50 K. The present samplesapparently have K values more like the value for Fe3O4

because of their high temperature for collapse to a doublet,that is, K)100 kJ my3.

The assumption has been made that t/t0 in Eq. (1) isabout 50 for the disappearance of the sextet, thus KV/kTs4.Although this is somewhat arbitrary, it is reasonable to pre-sume that t/t0 must be considerably greater than unity forthe sextet to completely disappear. Rancourt and Daniels[13] indicate that, below t/t0;20, transitional spectra withboth sextet and doublet will be seen. In order to estimate K,it is noted that the 100 K spectra of the samples from supplierA have about equal areas of sextet and doublet (Table 2).Thus, the volume calculated by Vs400k/K should corre-spond to the average for these samples. Assuming sphericalparticles, this volume should be (4 /3)(d/2)3, where d isp

the average particle diameter. The latter was assumed to beabout the same as MCD(211) for these samples (Table 4),or about 3.9 nm. The value of K required for this size is about170 kJ my3.

Values of TB, the temperature for collapse of the sextet,and, from KVB/kTBs4, the particle diameter correspondingto complete collapse of the sextet, dB, are given in Table 5.This diameter is a measure of the upper limit to the sizedistribution (if K is indeed the same for all) and is about thesame for A1-DP, A2 and B1, much larger for C1 and smallestfor E1.

The value of the hyperfine field of maximum probability,Hmax (Table 2), should be related to a particle volume ofmaximum probability, Vmax and corresponding diameter dmax

through Eq. (2).



In order to use Eq. (2) to calculate a particle volume (andfrom that a diameter) corresponding to maximum probabil-ity, it is necessary to know H0 as well as HobssHmax. Thevalues of Vmax and dmax should of course be independent oftemperature. The anisotropy constant K should also be inde-pendent of temperature for non-interacting particles. The val-ues of Hmax at 4 K (Table 3) and 77 K (Table 2) are enoughto calculate H0 for the three samples measured at 4 K. Thevalues are H0s491.7 kOe for A1-DP, H0s482.5 kOe for C1,and H0s471 kOe for E1. These values do not depend on thevalue of K as long as it is independent of temperature for agiven sample. It seems reasonable to assume that H0 is thesame for the first four samples of Table 5 (i.e. 491.7 kOe)and for samples C1 and D1 (482.5 kOe). These values of H0

have been used to calculate the dmax values reported in Table5. The dmax is about the same for the first four samples (4.5nm) and also about the same for the last three samples, 4.1nm, which is of course appreciably smaller than the dmax forthe first four. The values of dmax are for particle sizes ofmaximum probability and are in all cases well below the dB

values, which correspond to the upper limit of the sizedistributions.

There are a number of features of these calculations whichrequire discussion. The first is the fact that H0 is different foreach of the samples measured at 4 K, as calculated above. At4 K there cannot be much difference between Hmax and H0

according to Eq. (2) and in fact Hmax changes by about asmuch as H0. Since all samples are nanophase akaganeiteaccording to X-ray results, the hyperfine field of bulk materialshould be the same. Chambaere and De Grave [14] fittedtheir data for bulk akaganeite at low temperature to threesubspectra with varying hyperfine fields. At 4 K these fieldswere 491.5 kOe (40%), 477.4 (54%), and 472.4 (6%). TheHmax of A1-DP at 4 K agrees with that of the first sextet inthe above sample. However, it appears that bulk particles ofthe iron oxide core in samples C1 and D1 have a lower H0,which indicates a difference due to more than simply particlesize.

Another surprising result is that the E1 sample has such alow TB compared to the others (Table 5) even though itsMCD values are comparable and certainly not appreciablysmaller (Table 4). One possible explanation for this is thatthe anisotropy constant K is smaller for sample E1 than forthe others. Thus, the values for dB and dmax in Table 5 wouldbe underestimated. One possible reason for the different Kcould be in the nature of the dextran coating, as this samplehas much less X-ray background scatter (Fig. 4). Also, thissample has a higher Cl/Fe ratio (Table 1).

Regardless of the exact values for K and H0, the trends inTable 4 must be qualitatively correct. All the distributionshave a peak of maximum probability with diameter dmax

between 3.9 and 4.5 nm, but the distributions fall into twogroups, the first four having dmax;4.5 and the last threedmax;4.1 nm. Samples C1 and D1 have core sizes whichextend to much larger values (dB;5.5 nm according to thiscalculation) than the others. Their distributions are thus

spread towards larger particles, which accounts for a largeraverage diameter as measured by X-ray diffraction.

In reality, the sextet for a given size particle does notcollapse at one temperature, but becomes broadened and ill-defined over a range of temperatures [15]. This accounts forthe fact that the distributions of hyperfine fields at 77 and 100K are skewed towards low values of H (Fig. 1) and includevalues much too small to apply Eq. (2) for calculating aparticle volume. The distributions at 4 K are much sharper(Fig. 3) and more symmetric (Table 3). It should be notedthat the method used for fitting the spectra to a distributionof fields is model independent. The interpretation of the dis-tribution does, of course, depend on a model. In the modelused, relaxation effects are neglected for simplicity.

5. Conclusions

Williams et al. [12] have fitted their low temperature(F10 K) spectra for ferritin to a model based on a general-ized form of Eq. (2) in order to extract a particle size distri-bution. However, their analysis neglects results at highertemperature where doublet and sextet coexist, and thusignores the use of Eq. (1). The above analysis is simple toperform and gives a consistent qualitative description of thecore size distribution in a variety of iron dextran complexes.The analysis clearly shows that large variations exist in thisdistribution for various samples of commercially availableiron dextran. The iron core distributions of the samples fromsuppliers A and B are essentially the same, while those forsamples C1 and D1, and sample E1 are quite different. Thismay be due to the way in which the bulk drugs are prepared.Even though the cores are all nanophase akaganeite, they fallinto three distinct groups of size distribution.

Acknowledgements

While this work was in the review stage, Dr Larry Bowendied suddenly. Larry was a gentleman and a scholar of thehighest level and one of the leading scientists in his field. Hewill be missed by his many friends and co-workers.

R.D.B. would like to acknowledge a referee who offeredvery specific and very helpful comments that were incorpo-rated directly into the revised manuscript.

Research samples of iron dextran products were providedby the Hospital Products Division of Abbott Laboratories,sponsored by Dr Ann Stelmach. The liquid heliumMossbauer¨spectra were obtained at the University of Ghent with thepartial support of a NATO Collaborative Research Grant, No.960075, to L.H.B. and E.D.G.

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Documents

Comparison of the core size distribution in iron dextran complexes using Mössbauer spectroscopy and X-ray diffraction