Using geometric criteria to study helix-like structures produced in molecular dynamics simulations of single amylose chains in water

Amylose is a linear polymer chain of α-d-glucose units connected through α(1 → 4) glycosidic bonds. Experimental studies show that in non-polar solvents, single amylose chains form helical structures containing precise H-bond patterns. However, both experimental and computational studies indicate that these perfectly H-bonded helices are not stable in pure water. Nevertheless, amylose chains are observed to form helix-like structures in molecular dynamics (MD) simulations that exhibit imperfect H-bond patterns. In this paper, we study the structure of amylose chains in water using MD simulations to identify and characterize these “imperfect” helical structures. To this end we devise geometry-based criteria to define imperfect helical structures in amylose chains. Using this approach, the propensity of amylose chains to form these structures is quantified as a function of chain length and solvent temperature. This analysis also uncovers both short and long time helix-breaking mechanisms such as band-flips and kinks in the chain. This geometric approach to defining imperfect helices thus allows us to give new insight into the secondary structure of single amylose chains in spite of imperfect H-bond patterns.