The conformational variability of linear peptides affects the strength and selectivity of receptor binding. In contrast, cyclopeptides adopt more rigid and conformationally constrained structures than their linear analogues, as the cyclization could lead to a restricted mobility of the peptide skeleton. Therefore, the cyclization is one of the important ways to improve the biological stability and physiological activity of the peptides. Furthermore, cyclopeptides are important building blocks in molecular material chemistry and life science, working in diverse areas such as physiological processes, nanoscale materials, and supramolecular chemistry. They are used to study the physiological process, construct transmembrane nanotube channels in order to transport molecules and ions, recognize molecules, anions, and cations. In recent years, the researchers pay more attention to the interaction of biological molecules and metal ions, especially with cyclopeptides as the models. In this paper, we review the research progress of the metal ion recognition functions of cyclopeptides. Alkali and alkaline earth metals, transition metals (such as Cu, Zn and Ni), heavy metals, and lanthanides are discussed. A major focus has been on the influencing factors to the metal ion affinity and selectivity. The challenges and prospects of the metal ion recognition are also discussed concisely.

Contents
1 Introduction
2 The research progress of the metal ion recognition functions of cyclopeptides
2.1 Alkali and alkaline earth metals
2.2 Transition metals
2.3 Heavy metals
2.4 Lanthanides
3 Outlook