The binding to purified calmodulin of five spin-labelled derivatives of chlorophenothiazine was investigated by e.s.r. spectrometry and by the antagonizing potency on the calmodulin-dependent activation of myosin light chain kinase. The results of a comparative study and the influence of pH and ionic strength on the binding support the occurrence of an electrostatic binding involving the terminal amino group of the side-chain of the chlorophenothiazine. These results are discussed in relation to the specificity of the interaction that holds the antipsychotic drug-calmodulin complex together.

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