G-quadruplex: Flexible conformational changes by cations, pH, crowding and its applications to biosensing

G-quadruplex (G4) is a non-canonical structure that is formed in G-rich sequences of nucleic acids. G4s play important roles in vivo, such as telomere maintenance, transcription, and DNA replication. There are three typical topologies of G4: parallel, anti-parallel, and hybrid. In general, metal cations, such as potassium and sodium, stabilize G4s through coordination in the G-quartet. While G4s have some functions in vivo, there are many reports of developed applications that use G4s. As various conformations of G4s could form from one sequence depending on varying conditions, many researchers have developed G4-based sensors. Furthermore, G4 is a great scaffold of aptamers since many aptamers folded into G4s have also been reported. However, there are some challenges about its practical use due to the difference between practical sample conditions and experimental ones. G4 conformations are dramatically altered by the surrounding conditions, such as metal cations, pH, and crowding. Many studies have been conducted to characterize G4 conformations under various conditions, not only to use G4s in practical applications but also to reveal its function in vivo. In this review, we summarize recent studies that have investigated the effects of surrounding conditions (e.g., metal cations, pH, and crowding) on G4 conformations and the application of G4s mainly in biosensor fields, and in others.