The Epitaxial Growth of Cholesterol Crystals from Bile Solutions on Calcite Substrates

Epitaxial relationships between the surfaces of inorganic and bioorganic crystals can be an
important factor in crystal nucleation and growth processes in a variety of biological environments. Crystalline
cholesterol monohydrate (ChM), a constituent of both gallstone and atherosclerotic plaques, is often found
in association with assorted mineral phases. Using in situ atomic force microscopy (AFM) and well-characterized model bile solutions, the nucleation and epitaxial growth of ChM on calcite (101̄4) surfaces
in real-time is demonstrated. The growth rates of individual cholesterol islands formed on calcite substrates
were determined at physiological temperatures. Evidence of Ostwald's ripening was also observed under
these experimental conditions. The energetics of various (101̄4) calcite/(001) ChM interfaces were calculated
to determine the most stable interfacial structure. These simulations suggest that the interface is fully
hydrated and that cholesterol hydroxyl groups are preferentially positioned above carbonate ions in the
calcite surface. This combination of experimental and theoretical work provides a clearer picture of how
preexisting mineral seeds might provide a viable growth template that can reduce the energetic barrier to
cholesterol nucleation under some physiological conditions.

无机晶体与生物有机晶体表面间的外延关系，是多种生物环境中晶体成核与生长过程的重要影响因素。结晶一水合胆固醇（ChM）既是胆结石的组成成分，也是动脉粥样硬化斑块的组分之一，常与各类矿物相伴生。本研究借助原位原子力显微镜（AFM）与表征完善的模型胆汁溶液，实时证实了ChM在方解石(101̄4)晶面上的成核与外延生长过程。在生理温度条件下，测定了方解石基底上形成的单个胆固醇岛的生长速率。在此实验条件下还观测到了奥斯特瓦尔德熟化（Ostwald's ripening）现象。通过计算多种(101̄4)方解石/(001) ChM界面的界面能，确定了最稳定的界面结构。模拟结果表明，该界面完全水合，且胆固醇羟基优先位于方解石表面的碳酸根离子上方。这项结合实验与理论的研究，更清晰地阐明了预先存在的矿物晶种可作为可行的生长模板，在部分生理条件下降低胆固醇成核的能垒。