Calcium Oxalate Monohydrate Precipitation at Phospholipid Monolayer Phase Boundaries
- PDF / 587,471 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 3 Downloads / 214 Views
W4.14.1
Calcium Oxalate Monohydrate Precipitation at Phospholipid Monolayer Phase Boundaries Isa O. BenÃtez and Daniel R. Talham* Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200 ABSTRACT The precipitation of calcium oxalate monohydrate (COM) was observed at biphasic phospholipid Langmuir monolayers with the aid of Brewster angle microscopy. COM appears preferentially at phase boundaries of a monolayer of 1,2-dipalmitoyl-sn-glycero-3phosphocholine (DPPC) in a state of liquid expanded/liquid condensed coexistance. However, when the phase boundary is created by two different phospholipids that are phase segregated, such as DPPC and 1,2-dimiristoyl-sn-glycero-3-phosphocholine (DMPC), crystal formation occurs away from the interface. It is suggested that COM growth at phase boundaries is preferred only when there is molecular exchange between the phases. INTRODUCTION Calcium oxalate and calcium phosphate are the principal crystalline materials found in urinary stones [1, 2]. The inorganic crystals are always mixed with an organic matrix composed of carbohydrates, lipids and proteinaceous materials that account for about 2% of the total mass, although a much larger percentage of the total volume [3, 4]. To better understand the process of stone formation, it is important to study interactions between the organic and crystalline components. We have previously performed a series of studies on calcium oxalate precipitation at an interface provided by phospholipid Langmuir monolayers that serve as models for the phospholipid domains within membranes [5-8]. We observed that the Langmuir monolayers can effectively catalyze the precipitation of calcium oxalate monohydrate (COM) and that the identity of the monolayer has a strong influence on the rate of crystal formation. Negatively charged monolayers (DPPG and DPPS) induce more extensive precipitation than a neutral monolayer (DPPC), implicating a mechanism whereby the calcium ions are concentrated at the interface promoting nucleation [5, 6]. Consistent with this mechanism is the observation that a large majority of crystals produced had their calcium-rich (10-1) face oriented towards the monolayer. Experiments performed with DPPC and DPPG at different degrees of monolayer compression revealed that the COM number density more than doubles when the surface pressure is decreased from 20 mN/m to 0.1-0.3 mN/m [6, 7]. These earlier studies also raised questions about the role of monolayer phase boundaries in the crystallization process. DPPC and DPPG can exist as LC, LE, or in a coexistence region. These effects are difficult to quantify if crystallization is monitored ex-situ with electron microscopy. We previously demonstrated that Brewster angle microscopy (BAM) can be used for in-situ observation of COM crystals growing at Langmuir monolayers [5, 7]. BAM has also been recently used to monitor calcium carbonate precipitation under fatty acid monolayers [9]. Advances in the commercial instrumentation now allow high quality images of the monolayer
Data Loading...
