glycine crystallization during spray drying: the ph effect on salt and polymorphic forms

Glycine Crystallization during Spray Drying:The pH Effect on Salt and Polymorphic Forms

LIAN YU, KINGMAN NG

Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, Indiana 46285

Received 9 January 2002; revised 11 March 2002; accepted 17 May 2002

ABSTRACT: Spray drying of aqueous solutions of glycine revealed a strong pH effect onthe salt and polymorphic forms of the resulting powders. Adjusting pH by aqueous HClor NaOH between 1.7 and 10.0 caused the glycine solutions to crystallize as two poly-morphs (a and g) of the neutral glycine (þH3NCH2CO2

�) and as three salts (diglycineHCl, þH3NCH2CO2

� � þH3NCH2CO2H �C1�; glycine HCl, þH3NCH2CO2H �C1�; andsodium glycinate, H2NCH2CO2

� �Naþ). Although a-glycine crystallized from solutionswithout pH adjustment (pH 6.2), changing the pH to 4.0 and 8.0 caused g-glycine tocrystallize as the preferred polymorph. This phenomenon is attributed to the pH effecton the dimeric growth unit of a-glycine. The formation of a-glycine by spray dryingsolutions of neutral glycine contrasts the outcome of freeze drying, which yieldsb-glycine. Because g-glycine is thermodynamically more stable than a-glycine, the crys-tallization of g-glycine by pH adjustment provides away to improve the physical stabilityof glycine-containing formulations. Spray drying at low pH yielded various mixtures ofneutral glycine and its HCl salts: pH 3.0, g-glycine and diglycine HCl; pH 2.0, diglycineHCl; and pH 1.7 (the natural pH of glycine HCl), diglycine HCl (major component) andglycine HCl (minor component). Spray drying glycine HCl solutions (pH 1.7) yielded thesame diglycine HCl/glycine HCl mixture as did spray drying neutral glycine solutionsacidified to pH 1.7. Obtaining diglycine HCl by spray drying glycine HCl solutionsindicates a 50% loss of HCl during processing. The extent of HCl loss could be altered bychanging the inlet temperature of the spray drier. Spray drying glycine solutions at pH9.0 and 10.0 gave predominantly g-glycine and an additional crystalline product, possi-bly sodium glycinate. The glycine powders spray dried at different pH had differentparticle morphologies and sizes, which may influence their suitability for pharma-ceutical formulations. � 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association

J Pharm Sci 91:2367–2375, 2002

Keywords: glycine; glycine HCl; diglycine HCl; sodium glycinate; spray drying;crystallization; polymorphism; glycine cyclic dimer

INTRODUCTION

Polymorphism, the ability of a molecule to crys-tallize in multiple crystal forms that differ inmolecular packing and/or conformation, is a fre-

quently encountered phenomenon in pharmaceu-tical sciences.1,2 Molecules containing ionizablegroups (amino, carboxyl, etc.) can crystallize notonly in the neutral form but also as salts withcounter ions. The development of high-qualitypharmaceutical products requires a good under-standing and control of polymorphism and saltformation, because different solid forms lead todifferent physical properties (e.g., solubility, dis-solution rate, hygroscopicity, and particle morpho-logy). This need is greater in developing dosage

JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 91, NO. 11, NOVEMBER 2002 2367

Correspondence to: Lian Yu (Telephone: 317-276-1448;Fax: 317-277-2154; E-mail: [email protected]); K. Ng(Telephone: 317–276-9558; Fax: 317-277-3164;E-mail: [email protected]

Journal of Pharmaceutical Sciences, Vol. 91, 2367–2375 (2002)� 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association

forms for advanced drug delivery, which rely oncareful optimization of physical properties to achi-eve intended functions.3

Glycine, the simplest amino acid, is a commonpharmaceutical excipient. As a bulking agent infreeze drying, glycine provides a high eutecticmelting point with ice (ca.�38C), enabling the useof high drying temperatures to improve processefficiency. The crystallization and polymorphismof glycine have been studied during freeze dryingas a function of pH, salt form, and ionic strength.4

Low-temperature X-ray diffraction (XRD) hasbeen used to monitor the crystallization of glycinefrom frozen solutions and during freeze drying.5

Although glycine crystallization during freezedrying has been well studied, the same is not truefor glycine crystallization during spray drying,a common drying technique operated at highertemperatures for generating fine particles ofcontrolled morphologies.6,7 The interest in study-ing glycine crystallization during spray dryingarises in part from the use of this technique toproduce protein powders.8,9 As in the freeze dry-ing of proteins,10 different excipients are addedas stabilizers or bulking agents to optimize thephysicochemical properties of the formulation.11

Low-molecular-weight amino acids such as gly-cine and leucine have been used recently in theformulation of spray-dried protein powders forpulmonary delivery.12,13

The crystallization of glycine is complicated bythe existence of different ionic forms (Fig. 1). Theneutral glycine is a zwitterion, þH3NCH2CO2

�,in aqueous solution. The carboxyl group can beprotonated to give þH3NCH2CO2H at low pH

(pKa 2.35), and the amino group can be depro-tonated to give H2NCH2CO2

� at high pH (pKa

9.78). Accordingly, glycine can crystallize as thezwitterion,14–16 the protonated cation,17,18 ordeprotonated anion. This behavior suggests astrong pH effect on glycine crystallization, whicharises from the pH effect on the distributionof glycine species and from the interference be-tween structurally similar molecules during crys-tallization.

Polymorphism further complicates the crystal-lization of glycine. The neutral glycine can crys-tallize as at least three polymorphs (a, b, and g)of different densities, morphologies, and space-group symmetry.14–16 Although g-glycine is themost stable at room temperature,19 a-glycine usu-ally crystallizes from an aqueous solution, unlessthe solution is acidified or made alkaline.16

b-Glycine is the least stable at any temperature,whereas the relative stability of a- and g-glycinedepends on temperature (enantiotropism), with gbeing more stable at low temperature but lessstable at high temperature (g-to-a transformationobserved at ca. 1658C).16,19

Given the variable outcome of glycine crystal-lization, our objective was to determine system-atically how pH affects the salt and polymorphicforms of spray-dried glycine and ensuing physicalproperties. The crystallization of glycine duringspray drying was compared with that from room-temperature and frozen solutions. The resultswere interpreted on the basis of pH-dependentdistribution of glycine species, elemental unit ofcrystal growth, and conditions of spray drying.

EXPERIMENTAL

Materials

A 9.0mg/mL stock solution of glycine wasmade bydissolving glycine (a polymorph, 99.9% purity;Fisher Scientific) in Sterile Water for Irrigation(Abbott Laboratories). The pH of this solution was6.2. Additional solutions were prepared at pH 1.7,2.0, 3.0, 4.0, 8.0, 9.0, and 10.0 by adding appro-priate amounts of 5 N HCl or 5 N NaOH. Thevolume changes resulting from pH adjustment,though sizeable at low and high pH extremesowing to the buffering capacity of glycine, did notcause significant changes of the glycine concen-tration (ca. 9.0 mg/mL). A 9.0 mg/mL glycineHCl solution was prepared by dissolving glycineHCl (99þ% purity; Sigma) in Sterile Water for

Figure 1. The distribution of the neutral and ionizedforms of glycine as a function of pH. The curves werecalculated from the pKa of glycine (2.35 and 9.78).

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JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 91, NO. 11, NOVEMBER 2002

Irrigation (Abbott Laboratories). The pH of thissolution was 1.7. A sample of sodium glycinatewas obtained from Sigma (99% purity) and char-acterized as received. When dissolved in water(9.0 mg/mL), the sodium glycinate yielded a pHof 11.1.

Spray-Drying

Glycine powder was prepared using a laboratoryscale spray dryer (model 191; Buchi, Switzer-land). The volume of glycine solution spray driedwas mostly 100 mL and occasionally 200 mL.Compressed nitrogen was used for both atomiza-tion and drying. A typical inlet temperature of1208C was used with glycine solution pumped at aconstant rate of 2.8 mL/min resulting in an outlettemperature of 75–808C. Selected solutions werespray dried several times, each time with freshsolution, to test reproducibility: n¼ 4 for pH 6.2,n¼ 2 for pH 1.7 (glycine HCl), and n¼ 2 forpH 8.0. To study the effect of different dryingtemperatures on the loss of HCl, two other inlettemperatures were used: 508 and 1808C, which re-sulted in outlet temperatures of 308 and 1088C,respectively.

XRD

A Siemens D5000 X-ray diffractometer was used,which was equipped with a CuK source (l¼1.54056 A) operating at a tube load of 50 KV and40 mA. The divergence slit size was 1 mm, thereceiving slit 1 mm, and the detector slit 0.1 mm.Data were collected by a Kevex solid-state (SiLi)detector. Each sample was scanned between 48and 358 (2 y) with a step size of 0.028 and a maxi-mum scan rate of 3 s/step.

Thermal Gravimetric Analysis (TGA)

TGA was conducted at 108C/min in Al pans usinga TA SDT2960. The temperature was calibratedusing indium and the weight using manufacturersupplied standard weights and verified againstsodium tartrate desolvation.

Differential Scanning Calorimetry (DSC)

DSC was conducted at 108C/min in crimped Alpans using a TA DSC2920 under nitrogen purge(50 mL/min). The temperature and heat flow werecalibrated using indium.

Scanning Electron Microscopy (SEM)

SEM analysis was conducted using a PhilipsXL30 ESEM with a thermally assisted field emis-sion source. Images were acquired in conventionalhigh-vacuum mode using an Everhart-Thornleysecondary electron detector. A 30-mm final aper-ture was used. Images were collected using a 7.0–10.0 kV acceleration voltage and spot size of 3.0.Working distances ranged from 9.8 to 15.2 mm.Powder samples were prepared for analysis byair-dispersion onto an adhesive conductive car-bon tab attached to a 13-mm-diameter specimenmount. Samples were coated with a Denton DeskII sputter coater using a gold target. The coatingwas applied for 120 s at 50 mTorr and 45 mAcurrent.

RESULTS

The pH range chosen for this study, 1.7 to 10.0,covers the pKa’s of glycine (2.35 and 9.78) andtherefore allows the formation of all three gly-cine species in Figure 1, namely þH3NCH2CO2

�,þH3NCH2CO2H, and H2NCH2CO2

�. The glycinepowders spray dried at different pH showedstrong XRD (Fig. 2), indicating that they contain-ed crystalline components. DSC revealed no glasstransitions, but pronounced melt/decompositionendotherms, indicating that the amorphous con-tents of these powders, if any, were low. Except forthe powder spray dried at pH 10.0 (see below),TGA showed no weight loss up to 1108C, indicat-ing that the powders contained insignificantamounts of residual water.

XRD was used to identify the crystallinecomponents (salts and polymorphs) in differentspray-dried samples. For clarity, we have redis-played the XRD data (Fig. 2a) in three parts:Figure 2b, pH 1.7, 2.0, and 3.0; Figure 2c, pH 3.0,4.0, 6.2, 8.0, and 9.0; and Figure 2d, pH 9.0 and10.0. The relevant library patterns for phaseidentification are also shown in Figure 2b,c, andd. Table 1 summarizes the glycine phases identi-fied by XRD, along with the corresponding cry-stallographic records.

Figure 2c shows the effect of increasing anddecreasing the pH of glycine solutions on poly-morphic outcome. At 6.2 (no pH adjustment),a-glycine was the preferred polymorph.a Upon

aOnly one of the four independent runs yielded b-glycine as aphase impurity.

POLYMORPHS AND SALTS OF SPRAY-DRIED GLYCINE 2369


changing the pH to 3.0, 4.0, 8.0, and 9.0, g-glycinebecame the main crystalline component. Al-though phase impurities could be identifiedat pH 3.0 (diglycine HCl, see below), pH 8.0(a-glycine), and pH 9.0 (sodium glycinate, seebelow), the main effect of pH adjustment wasthe inhibition of a-glycine crystallization and thepromotion of g-glycine crystallization.

This pH effect is consistent with a previousreport16 that pH adjustment with acetic acid orammonium hydroxide causes g-glycine to crystal-lize from aqueous solutions at room temperature,instead of the usual a-glycine. This effect can beexplained by examining the crystal structure ofa-glycine and its elemental growth unit.20 Thea-glycine crystal consists of sheets of glycine

Figure 2. The XRD patterns of powders obtained by spray drying glycine solutions atdifferent pH. (a) Overview of all data. (b) pH 1.7 to 3.0 showing the emergence ofdiglycine HCl and glycine HCl salt forms. (c) pH 3.0 to 9.0 showing the effect of pHincrease or decrease from the natural pH (6.2) on the polymorphism of glycine. (d) pH 9.0and 10.0 showing the formation of a poorly crystalline phase, likely sodium glycinate.

2370 YU AND NG


molecules arranged perpendicularly to the b axis.Each sheet consists of hydrogen-bonded mole-cules of a single chirality; two mirror-relatedsheets are linked by hydrogen bonds to a bilayer.Within a bilayer, every molecule in one sheet islinked to a mirror-related molecule in the othersheet by two hydrogen bonds, forming a ‘‘cyclicdimer’’ (Fig. 3). The bilayers stack together by vander Waals forces along the b axis to produce thea-glycine crystal.

By analyzing in situ growth and dissolution ofa-glycine, Gidalevitz et al. 20 proposed that thegrowth of a-glycine proceed through the assemblyof the cyclic dimers, termed ‘‘the elemental growthunits,’’ rather than individual glycine molecules.

Because g-glycine does not have the bilayeredstructure, comprising instead non-centrosym-metric hydrogen-bonded helices, the cyclic dimeris unlikely the elemental growth unit. It followsthat conditions destabilizing the cyclic dimershould inhibit the formation of a-glycine andpromote the crystallization of g-glycine. b-glycine,the only other polymorph presently known, isunstable and transforms to a- or g-glycine overtime, especially at elevated temperature and hu-midity. Thus, if the crystallization of a-glycine isinhibited, g-glycine is the likely spray-dryingproduct.

Figure 1 shows that glycine molecules areneutral zwitterions at the natural pH (6.2) andbecome positively and negatively charged with pHincrease and decrease, respectively. The concen-trations of the charged glycine molecules risesharply below pH 4 or above pH 8. Because thecyclic dimers are formed by linking head-to-tailtwo neutral zwitterions (Fig. 3), charged mole-cules are expected to inhibit the dimer formationbecause of the repulsion of like charges or in-compatible bonding geometries.

Figure 2b compares the results of spray dryingat pH 1.7, 2.0, and 3.0. In this pH range, thecrystallization of diglycine HCl was detected. Thiscrystal form was not reported by Akers et al. 4 asa freeze-drying product at pH 3. At pH 2.0, di-glycine HCl was essentially the only crystallineproduct, whereas at pH 3.0 and 4.0, g-glycine alsocrystallized. At pH 1.7 (i.e., the pH of glycine HClsolutions), the spray-dried powder was a mixtureof diglycine HCl (major component) and glycineHCl (minor component). The same mixture wasobtained by spray drying solutions of pure glycineHCl (9 mg/mL, pH 1.7).

Table 1. Crystallographic Data of Polymorphs and Salts of Glycine Identified in This Study

Polymorph/Salt Crystallographic Dataa Ref.PDFCodeb

a-Glycine: þH3NCH2CO2� P21/n, a¼ 5.1054 A, b¼ 11.9688 A,

c¼ 5.4645 A, b¼ 111.697814 32-1702

b-Glycine: þH3NCH2CO2� P21, a¼ 5.077 A, b¼ 6.268 A,

c¼ 5.380 A, b¼ 113.2815 2-171

g-Glycine: þH3NCH2CO2� P32, a¼ 7.037 A, c¼ 5.483 A 16 6-230

Glycine HCl: þH3NCH2CO2H �Cl� P21/c, a¼ 7.117 A, b¼ 5.234 A,c¼ 13.745 A, b¼ 97.258

17 22-1744

Diglycine HCl: þH3NCH2CO2� � þH3NCH2CO2H �Cl� P212121, a¼ 8.0988 A, b¼ 18.015 A,

c¼ 5.3086 A18 32-1649

aIf one structure has been determined several times, the newest data are given.bThe Powder Diffraction File. International Centre for Diffraction Data, 12 Campus Blvd., Newtown Square, PA 19073-3273.

Figure 3. Molecular model of the cyclic dimer in a-glycine that has been postulated as the elemental unitof crystal growth. The two molecules are related bymirror reflection and linked by two hydrogen bonds(broken lines). The a-glycine crystal is formed by link-ing the cyclic dimers by lateral hydrogen bonds into abilayer and stacking the bilayers along the b axis.



The crystallization of diglycine HCl from a gly-cine HCl solution indicates a 50% loss of the HClduring spray drying, as shown by the followingmass balance equation:

2ðglycine HClÞ ðsolutionÞ !diglycine HCl ðcrystalÞþHCl "

To investigate the HCl loss during spraydrying, the solutions of glycine HCl were spraydried at different inlet temperatures (Tinlet¼ 508,1208, and 1808C). The XRD data of these samples(Fig. 4) show that the Tinlet¼ 1808C powder wasdiglycine HCl, the Tinlet¼ 508C powder glycineHCl, and the Tinlet¼ 1208C powder a mixtureof diglycine HCl and glycine HCl. Thus, as Tinlet

increased, the amount of glycine HCl in theproduct decreased and the amount of diglycineHCl increased. This result demonstrates the pos-sibility of losing HCl during spray drying and ofaltering the degree of HCl loss by changing theinlet temperature.

The DSC and TGA data of the Tinlet¼ 1808Cpowder (diglycine HCl) revealed a melt/decom-position endotherm near 1908C (Fig. 5c, onsetlabeled gg). The DSC and TGA data of the Tinlet¼

508C powder (glycine HCl) displayed a melt/decomposition endotherm near 1608C (Fig. 5a,onset labeled g). The glycine HCl sample obtain-ed from Sigma displayed similar DSC and TGAcharacteristics as the Tinlet¼ 508C powder (datanot shown). The Tinlet¼ 1208C powder showedboth g and gg endotherms and an intermediateTGA loss between those of the Tinlet¼ 508CandTinlet¼ 1808C powders (Fig. 5b). This confirmsthe XRD finding that the Tinlet¼ 1208C powderwas a mixture of diglycine HCl and glycine HCl.Thus, the thermal data verify that it is possible toremove HCl when spray drying low-pH glycinesolutions and to alter the degree of HCl loss bychanging the inlet temperature.

Figure 5 shows that the crystals of diglycineHCl have higher thermal stability than thecrystals of glycine HCl (compare endotherms gand gg). This result helps explain why the spraydrying of a glycine HCl solution causes at most50% loss of the HCl, even at the highest inlettemperature used (1808C). If diglycine HCl crys-tallizes, only half of the HCl contained in glycineHCl can escape, with the other half being retainedby the diglycine HCl crystals. Were the diglycineHCl crystals less stable than the glycine HClcrystals, further loss of HCl would occur; the lossof all HCl would lead to the crystallization of theneutral glycine. A practical relevance of the re-stricted HCl loss is the prevention of drastic pHincrease during spray drying. For example, if all

Figure 4. The XRD patterns of powders obtained byspray drying glycine HCl solutions at different inlettemperatures.

Figure 5. The DSC and TGA characteristics of thepowders obtained by spray drying glycine HCl solutionsat different inlet temperatures. (a) 508C, (b) 1208C, and(c) 1808C. The solid curves are the DSC data and thebroken curves the TGA data. For clarity, the curveshave been shifted vertically. Symbols g and gg indicatethe onsets of melting/decomposition of glycine HCl anddiglycine HCl, respectively.

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HCl escaped from a solution of glycine HCl, thepH would increase from 1.7 to 6.2. But if only 50%of the HCl could escape because of the formationof diglycine HCl, the molar concentration ofthe protonated glycine would be no less than thatof the zwitterionic glycine, or [þH3NCH2CO2H] �[þH3NCH2CO2

�]. The latter condition leads topH � 2.35 because of the relationshippH¼pKa�log([þH3NCH2CO2H]/[þH3NCH2CO2

�]),where pKa¼ 2.35 (the pKa of the carboxyl group).As a result, the pH increase caused by HCl lossis limited to 2.35 because of the crystallization ofdiglycine HCl.

Figure 2d shows the XRD patterns of the gly-cine powders spray dried at pH 9.0 and 10.0.g-Glycine was identified in both powders. In addi-tion, both powders displayed weaker and broaderXRD features that resemble those shown by asodium glycinate sample from Sigma (Fig. 2d,

second from bottom). This similarity indicatesthat the pH 9.0 and 10.0 samples contained acrystalline component that existed in the com-mercial sample of sodium glycinate. Unfortuna-tely, like the spray-dried powders, the commercialsodium glycinate also contained g-glycine as aphase impurity and was not an authentic sampleof sodium glycinate. This makes the identifica-tion of phases less certain. Both the commercialsodium glycinate and the powder spray dried atpH 10.0 contained approximately 10% water byTGA. This high water content indicates that it ismore difficult to spray dry glycine solutions athigh pH than at neutral or acidic pH. For thisreason, no attempts of spray drying at even higherpH was made.

Figure 6 shows typical SEM images of glycineparticles spray dried at different pH. Particlesproduced at pH 6.2 (Fig. 6d) were discrete and

Figure 6. Representative SEM images of the glycine powders spray dried fromsolutions of different pH: (a) 2.0, (b) 3.0, (c) 4.0, (d) 6.2 (natural pH), (e) 8.0, and (f) 9.0.



uniformly sized (average size 2–3 mm), character-istics desirable for aerosol formulations. Decreaseof pH to 4.0 did not significantly change theparticle characteristics (Fig. 6c). Further decreaseof pH to 3.0 caused severe agglomeration of parti-cles (Fig. 6b). This may be caused by the simul-taneous crystallization of g-glycine and diglycineHCl. The competition of the two crystallizationpathways may have retarded crystallization andproduced low-quality crystals. Particle character-istics improved (less agglomeration) when thepH was decreased to 2.0 (Fig. 6a), which may be aresult of single-component crystallization of digly-cine HCl.

Increase of pH to 8.0 yielded discrete particles(Fig. 6e) that were larger than those produced atpH 6.2. Further increase of pH to 9.0 causedgreater agglomeration of particles (Fig. 6f). At pH10.0, the spray-dried powder contained a signifi-cant amount of water (10%) and ‘‘fused’’ duringSEM analysis (not shown). The poor quality of theparticles obtained at pH 9.0 and 10.0 may beattributed to the concomitant crystallization ofg-glycine and sodium glycinate (Fig. 2d) and tothe low crystallinity of spray-dried sodium gly-cinate, as evidenced by its weak and broad XRDpeaks. These results demonstrate how pH-ind-uced changes of polymorphs and salt forms canalter particle properties.

It is informative to compare glycine crystalli-zation under freeze- and spray-drying conditions.The pH effect on polymorphic outcome revealedby this study (spray drying) correlates with theobservation by Akers et al.4 in their study offreeze-dried glycine. Similar to our results, theseauthors observed g-glycine as the preferred poly-morph at pH 3 and 10, but unlike our results, theyfound b-glycine as the preferred polymorph at thenatural pH. The latter observation was confirmedby Pyne and Suryanarayanan5 using in situ low-temperature XRD. The g-glycine observed byAkers et al. at pH 3 and 10 may be attributed tothe same pH effect on glycine dimers describedpreviously. It is unclear, however, why b- ratherthan a-glycine crystallizes from frozen solutionsat the natural pH. Among the possible causes arelow temperature, presence of ice, and adherenceto Ostwald’s Law of Stages.b Given the similarcrystal structures of a- and b-glycine (both con-tain hydrogen-bonded sheets perpendicular to the

monoclinic b axis), such change of polymorphicpreference seems reasonable. Another differencebetween glycine crystallization during freeze- andspray-drying is whether the loss of HCl occurs.Freeze drying being a less aggressive processthan spray drying, the HCl loss observed in thisstudy is unlikely to occur under freeze-dryingconditions.

Concerning the spray drying of glycine-con-taining formulations, this study provides severalguidelines for process optimization. First, the pHcontrol of polymorphic outcome may be used topromote the crystallization of g-glycine, the ther-modynamically most stable polymorph near theroom temperature,16,19 and inhibit the crystal-lization of the metastable a-glycine, thereby im-proving formulation stability against polymorphicconversions during storage. Second, high-qualityglycine particles, as defined by lack of agglomera-tion and uniformity of sizes, are obtained by spraydrying near the neutral pH (4.0 to 8.0). Aboveor below this range, particle agglomeration andfusion are pronounced. Third, if glycine is spraydried at low pH, a slight increase of pH caused bythe loss of HCl should be anticipated.

CONCLUSIONS

This study revealed a strong pH effect on glycinecrystallization during spray drying. By adjustingthe solution pH within the range 1.7 and 10.0, weobtained two polymorphs of the neutral glycine(a and g) and three salts (glycine HCl, digly-cine HCl, and sodium glycinate). The glycinesolution at the natural pH (6.2) yielded a-glycine,whereas the solutions adjusted to higher or lowerpH yielded g-glycine. This phenomenon can be ex-plained by the pH effect on the cyclic dimer, theelemental growth unit of a-glycine. Unlike freezedrying, spray drying of glycine solutions at low pHcaused significant loss of HCl, the extent of whichcould be adjusted by changing the inlet tempera-ture of the spray dryer. Spray drying the glycineHCl solution at an inlet temperature of 508Cyielded glycine HCl, whereas only diglycine HClwas obtained at a higher inlet temperature of1808C. The crystallization of glycine as differentpolymorphs and salt forms significantly changedparticle morphology and size. These changes mayinfluence the suitability of these powders forpharmaceutical formulations. Our results alsosuggest practical strategies for developing spray-dried formulations containing glycine.

bOstwald’s Rule of Stages (Ostwald W. 1897. Z Phys Chem22:289) states that when a system crystallizes from a super-saturated state, the first form to nucleate is the most soluble(the least stable).

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ACKNOWLEDGMENTS

We thank Johanna Liebetrau for collecting theXRD data, Ronald E. Vansickle for assisting withthe thermal analysis, and Eric Olson for perform-ing the SEM analysis.

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glycine crystallization during spray drying: the ph effect on salt and polymorphic forms

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