Synthetic and binding studies on the calcium binding site I of bovine brain calmodulin. II. Synthesis and CD studies on the cyclic 20-31 sequence

The synthesis of the cyclic 20-31 sequence of bovine brain calmodulin corresponding to the loop of the hypothetical calcium binding site I of the protein has been accomplished by classical solution methods. The interaction of the synthetic cyclic fragment with calcium ions has been investigated by CD spectroscopy in water and in 98% trifluoroethanol solution. Calcium ions have no effect on the dichroic absorption of aqueous solution of the cyclic dodecapeptide in the wavelength range 200-250 nm. In 98% trifluoroethanol the CD spectrum of the cyclic compound in the absence of calcium ions is almost identical to that of the linear dodecapeptide in the presence of saturation concentrations of calcium. This result supports our previous hypothesis of a folding of the linear sequence upon interaction with the metal ion. The cyclic peptide also interacts with calcium ions in 98% trifluoroethanol forming a 1:1 complex.