The average transmembrane helix turn propensity is calculated over a sliding amino acid window according to an experimental scale defined by Monné et al. (1999). The window size can be set to 3 or 5.
A Turn Propensity Scale for Transmembrane Helices. Magnus Monné, Marika Hermansson and Gunnar von Heijne. Journal of Molecular Biology 1999 vol. 288 (1) pp. 141-145 pubmed