First Principles Modelling of Organic-Inorganic Interfaces in Renal Calculi

Urinary calculi (kidney stones) is a common ailment effecting around 10 % of the world’s population and resulting in more than 97,000 finished consultant episodes (FEC) each year in the UK alone.1 However, the key chemical interactions relating to stone crystallisation and aggregation are not fully understood. Urinary calculi are solid clusters of mineral that have precipitated from urine, and built up on the luminal surface of the endothelial cell surfaces of the microtubule in the kidney. There are three main types of kidney stone, calcium, struvite and uric acid. Moreover, kidney stones are often found complexed to organic matrices, such as proteins and amino acids. The fundamental chemistry underlying this observation is unknown. This research uses first principles modelling (CASTEP) to help elucidate the crystallisation phenomena, and unravel the chemistry behind stone composition. Urine analysis of kidney stone patient data has previously revealed that their urine contains higher amounts of phospholipids in comparison to healthy patients.2 In order to begin to understand the nucleation process, we have constructed surface models of calcium oxalate monohydrate and calcium oxalate dihydrate, and modelled stone growth, by simulating further calcium oxalate adsorption onto these surfaces. To investigate the interactions between urinary macromolecules and the growing crystal surfaces at an atomic level, we have performed ab initio molecular dynamics of phosphocholine adsorption on calcium oxalate surfaces. We have shown that the phosphocholine head groups become entrapped within the growing crystal and the lowest energy structures are those where the calcium oxalate dihydrate surfaces have attempted to reorganise their crystallographic structure. This could be because crystallographic waters that are driving the encapsulation of the phosphocholine head group.