# The Exceptional Solubility of Cyclic Trimetaphosphate in the Presence of Mg2+ and Ca2+

## TL;DR

This study explores how cyclic trimetaphosphate (cTmp) remains soluble in water with Mg2+ and Ca2+, unlike other phosphate compounds, which could be important for understanding early life chemistry.

## Contribution

The paper experimentally demonstrates that cTmp is uniquely soluble in prebiotic conditions with Mg2+ and Ca2+.

## Key findings

- cyclic trimetaphosphate (cTmp) remains soluble at high concentrations of Mg2+ and 200 mM Ca2+
- pyrophosphate and linear triphosphate precipitate at low divalent metal ion concentrations
- cTmp is the most soluble phosphate compound under Archaean seawater conditions

## Abstract

Studying the origin of life requires identifying chemical and physical processes that could have supported early self-replicating and evolving molecular systems. Besides the requirement of information storage and transfer, an essential aspect is an energy source that could have thermodynamically driven the formation and replication of these molecular assemblies. Chemical energy sources such as cyclic trimetaphosphate are attractive because they could drive replication with relatively simple catalysts. Here, we focus on cyclic trimetaphosphate (cTmp), and compare its solubility in water to linear triphosphate, pyrophosphate, and phosphite when Mg2+ or Ca2+ are present. These solubilities are important for facilitating the reactions under prebiotically plausible conditions. The results showed that cTmp was soluble even at molar concentrations of Mg2+ and little precipitation with 200 mM Ca2+. In contrast, pyrophosphate and linear triphosphate precipitated efficiently even at low divalent metal ion concentrations. The precipitation of phosphate was pH-dependent, showing similar precipitation with Mg2+ and Ca2+ at a prebiotically plausible pH of 6.5. Phosphite was soluble at high Mg2+ concentrations but started precipitating with increasing Ca2+ concentration. At conditions that model Archaean seawater, cTmp was the most soluble of these compounds. Together, this experimental overview may help to identify promising conditions for lab-based investigations of phosphate-based energy metabolisms in early life forms.