After this initial "burst phase," more polar non-covalent interactions take over. Between 5 and 1000 milliseconds after protein folding initiation, three-dimensional structures of proteins, known as secondary and tertiary structures, are stabilized by formation of hydrogen bonds, in addition to disulfide bridges (covalent linkages). Through a series of small conformational changes, spatial orientations are modified so as to arrive at the most energetically minimized orientation achievable. The folding of proteins is often facilitated by enzymes known as molecular chaperones. Sterics, bond strain, and angle strain also play major roles in the folding of a protein from its primary sequence to its tertiary structure.