Synthetic and binding studies on the postulated calcium binding site I of calmodulin

The synthesis of the dodecapeptide [sequence (20--31)] representing the hypothetical calcium binding site I of calmodulin by classical methods in solution is described. The interaction of this synthetic calmodulin fragment with calcium ions has been investigated by CD in water and in 98% trifluoroethanol solution. Calcium ions have no effect on the dichroic absorption of the dodecapeptide in aqueous solution in the wavelength range 200-250 nm. However, in 98% trifluoroethanol a linear variation of the CD properties has been observed as a function of the Ca2/peptide molar ratio. The CD results indicate the formation of a 1 : 1 calcium-dodecapeptide complex with a very high binding constant.