The role of vinyl sulfonic acid homopolymer in calcium oxalate crystallization

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Abstract

In this study, the effect of water soluble homopolymer of vinylsulfonic acid on spontaneous crystallization of calcium oxalate (CaOx) was investigated. CaOx crystals exhibiting different shapes and phase structures were produced in the presence of polymer. While the crystal growth of calcium oxalate was inhibited by homopolymer, the morphology of calcium oxalate transformed from monohydrate to dihydrate. Inhibition of calcium oxalate crystallization was provided by adsorption of homopolymer onto the active growth sites of crystals on account of the charge and hydrophilic effects. Polyelectrolyte effects were interpreted in terms of the adsorption of inhibitors onto the active growth sites on the crystal surface.

Introduction

The urolithiasis has become a vital health problem for the last century with an increase of kidney and ureteric stones. The majority of kidney stones contain calcium oxalate (CaOx) as the primary inorganic component [1], [2], [3]. As a clinical problem in human body, there are many studies regarding with calcium oxalate crystallization [4], [5]. The studies generally carried out in aqueous solution, diluted or undiluted urine and artificial urine were reported in literature [3]. Many methods have been used to prevent and treat urolithiasis [1].

The formation of kidney stones is a complex process since there are many parameters effecting nucleation and growth of calcium oxalate crystals. The level of supersaturation, the variety of urinary components, interactions with the kidney epithelium and kinetic factors (nucleation, growth and aggregation) play an essential role in the crystallization of CaOx [6], [7], [8], [9]. Though calcium oxalate is oversaturated in the human body, the formation of kidney stones occurs hardly since there are acidrich proteins causing strong inhibition in urine [10], [11].

In urinary stones, CaOx is defined as a mixture of an organic matrix of urinary macromolecules consisting proteins, lipids and carbohydrates [12]. Calcium oxalate monohydrate (COM) which is the most stable form in room temperature is transformed into calcium oxalate dihydrate (COD) by urinary proteins in order to avoid the development of urinary and kidney stones [13], [14], [15], [16]. Most of studies have provided that COM is more effective than COD in the formation of stones [17], [18], [19].

Some urinary proteins inhibit the formation of CaOx crystals with blocking the adhesion of crystals to cell [20] or providing the formation of COD crystals [21]. Glycosaminoglycans (GAGs) in urine and kidney stones are the most effective inhibitors. The main GAGs are chondroitin sulfate A (C4S) and chondroitin sulfate C (C6S) [3]. Understanding of the interactions between stone crystals and the component of the organic matrix is the main part of biomimetic synthesis of calcium oxalate [22]. It is obvious that the mechanism regarding with formation of urinary stones is not clear yet [3], [6].

One question of general interest is that of how much structural detail is necessary in order that synthetic polymers may mimic some of the functions of proteins. Polyelectrolytes are in principle substances that may serve for this purpose [4], [23], [24], [25]. As many biological molecules contain oxyanion functional groups polymers containing carboxylate, phosphate and sulfate esters groups can be used to form a sufficient number of coordinative bonds with the cations of the mineral surfaces [2], [26], [27]. It is known that surfactants, carboxylic acids [26], anionic polyelectrolytes and crystal-associated macromolecules were used to change the morphology and phase of calcium oxalate crystals [28], [29], [30]. Some synthetic and natural molecules, such as sodium diisooctyl sulfosuccinate, poly(ethyleneglycol)-block-poly(methacrylic acid), polypepdites crystallize COD crystals [14]. Ward et al. has focused on studying the interactions of polymers with pendant carboxylate groups with various crystallographic planes of the CaOx monohydrate crystals. It was found that local binding of anionic side chains to crystal surface sites governs growth inhibition rather than any secondary polymer structure [31], [32]. Oner and coworkers [33] found that environmentally friendly polysaccharide-based polycarboxylate, carboxymethyl inulin (CMI), was to be effective in directing calcium oxalate crystallization from COM to COD.

In the present work, the synthesis of vinylsulfonic acid homopolymer was carried out by radical polymerization and used to modify the nucleation, growth and morphology of calcium oxalate crystallization from aqueous solution. For health applications, using of sulfonic acid group is reported in literature [34], [35]. Sulfonic group was investigated in body environment since it causes apatite nucleation. The organic-inorganic hybrids consisting of organic polymer and the sulfonic group may be suitable for new bone-repairing materials [34]. Moreover, poly(vinylsulfonic acid) (PVS) was used in order to inhibit the cytopathicity of HIV-1 and HIV-2 at concentrations that are not toxic to the host cells [35].

Section snippets

Polymer synthesis

Radical polymerization of vinylsulfonic acid was carried out in the presence of potassium peroxodisulfate as an initiator agent to produce vinlysulfonic acid homopolymer. The vinylsulfonic acid (VS, sodium salt 30% in water) and potassium peroxodisulfate (KPS) used were purchased from the Aldrich Chemical Co. The synthesis was based on the homopolymerization of vinylsulfonic acid. The molecular weight of homopolymer was determined as 5000 by using Viskotek Gel Chromatography. The complete

Effect of polymer on calcium oxalate crystallization rate

Calcium oxalate crystallization has been investigated in the presence of vinylsulfonic acid (VS) homopolymer at different temperatures and reactant ratios. Table 1 summarizes the effect of added polymer on the crystallization rate. The effect of additives on crystal growth rate was evaluated by comparing growth inhibition of calcium oxalate crystallization in the absence and presence of polymer. The ability of polymer to act as inhibitor was evaluated by R0/Ri ratios. The higher R0/Ri values

Conclusions

Vinyl sulfonic acid homopolymer synthesized by radical polymerization was very good additive for inhibiting both nucleation and growth rate of the calcium oxalate crystallization. This polymer inhibited CaOx precipitation by adsorbing onto crystal surfaces, thus blocking sites for new crystal growth. The polymer backbone can then act as a “fence” on the crystal surface, thus forming an obstacle for propagating steps that lead to further crystal growth.

Addition of 1.0 ppm VS causes a distinct

Acknowledgements

We appreciate the support of YTUAF (Project No.: 22-07-01-01); TUBITAK (Project Nos.: TBAG-AY/236(101T098); TBAG-AY/250(101T174)) and DPT (Project No.: 24-DPT-07-04-01, 25-DPT-07-04-01) for the accomplishment of this work. The authors thank Prof. Dr. Sabriye Pişkin of Yildiz Technical University for use of the XRD.

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