ASC-G4, an algorithm to calculate advanced structural characteristics of G-quadruplexes

ASC-G4 is an algorithm for the calculation of the ad-vanced structural characteristics of G-quadruplexes (G4). It allows the unambiguous determination of the intramolecular G4 topology, based on the oriented strand numbering. It also resolves the ambiguity in the determination of the guanine glycosidic configu-ration. With this algorithm, we showed that the use of the C3' or C5' atoms to calculate the groove width in G4 is more appropriate than the P atoms and that the groove width does not always reflect the space avail-able within the groove. For the latter, the minimum groove width is more appropriate. The application of ASC-G4 to 207 G4 structures guided the choices made for the calculations. A website based on ASC-G4 (http://tiny.cc/ASC-G4) was created, where the user uploads his G4 structure and gets its topology, the types of its loops and their lengths, the presence of snapbacks and bulges, the distribution of gua-nines in the tetrads and strands, the glycosidic con-figuration of these guanines, their rise, the groove widths, the minimum groove widths, the tilt and twist angles, the backbone dihedral angles, etc. It also pro -vides a large number of atom-atom and atom-plane distances that are relevant to evaluating the quality of the structure.