Internal motions in myosin head: effect of ADP and ATP.

Internal flexibility of myosin heads in glycerinated muscle fibres in the presence of MgADP plus orthovanadate and after addition of Ca-ATP was studied using an isothiocyanate-based spin label attached to the reactive sulfhydryl sites of myosin. The spin labels were immobilized on the microsecond time scale and exhibited significant orientational order in rigor. In AM+.ADP.V(i) state a smaller fraction of ordered population was found showing distinct orientation from rigor; the larger population of heads was in dynamically disordered state. This new ordered population of heads was detected even in contracting fibres.