Indeed, a model involving the dynamics of Tm has been proposed, in which variations in the amplitude of tropomyosin vibrations drive the change in its radial position on actin. Numerous studies characterized various aspects of tropomyosin dynamics in solution. A fluorescent study showed that Tm is flexible in solution. We observed a similar timescale of motion for isolated Tm. Flexibility of Tm can be evaluated from its persistence length, a measure of lengthwise thermal bending fluctuations. More direct measurement of protein dynamics by NMR showed differential flexibility of Tm along its length with the C-terminus overlap region being unstructured and flexible. Molecular dynamics simulations gave further support to the idea of differential flexibility with C-terminus moving twice as fast as the N-terminal region. Thus, there is consensus of motional gradient in tropomyosin with the C-terminus of Tm being more dynamic that the rest; however all the above studies were performed for isolated Tm i.e. not associated with actin. As observed here, Tm interacting with actin in the reconstituted fibers exhibit different dynamics than in solution. There is still a gradient of mobility with the C-terminus of Tm being least mobile. The decrease in dynamics of the C-terminus region is not due to a local effect i.e. it is not due to the immobilization of the spin label by the proximity to actin surface. The four positions to which the spin label is attached are oriented in the same fashion with respect to actin. Additionally, because the label is placed on each of the two strands of Tm, at every position and in every state, one cysteine is pointing towards the actin and the other cysteine on the other strand is pointing away from F-actin. Therefore, if there is any local effect of spin label immobilization on the actin surface, we should observe two populations: one slow and one fast. We do not observe two populations of dynamics for any position and in any state studied indicating no local immobilization of spin label by actin. Labels at positions 153/157, 188/192, 268/272 should TM5275 sodium salt experience the same degree of steric restriction. Therefore, the NU6140 differences in mobility cannot be attributed to the differential interactions of the four Tm positions with actin.