Chemical D, which prevents the binding of Ca2+ to troponin, could be a good choice for a muscle relaxer prior to surgery. The function of Ca2+ within the skeletal muscle cells is to activate muscle contraction through binding to troponin and displacing tropomyosin from active actin filament sites. When Ca2+ is released from the sarcoplasmic reticulum and binds to the troponin, the displacement of the tropomyosin occurs, causing exposure of the actin filaments, which the myosin then attaches itself to: thus, the actin-myosin complex is activated, and skeletal muscle contraction occurs.
Tropomyosin can be understood as a protein that regulates the contraction of skeletal muscles and thus can prevent muscles from contracting when it is positioned over actin filament sites. When Ca2+ binds itself to troponin, the troponin changes its structural shape, which allows the tropomyosin to move away from the myosin-binding sites on the actin filaments. As such, preventing the binding of calcium ions to the troponin would result in the tropomyosin continuing to block the myosin from attaching itself to the actin filament. The myosin-actin complex could not be activated, and the contraction of the skeletal muscle could not occur. Therefore, chemical D could be a good choice as a muscle relaxer.