| Abstract
| - The interaction of apocalmodulin (apoCaM) with a peptide(Neurop) based on the primarysequence of the calmodulin-binding domain of neuromodulin has beenstudied by fluorescence spectroscopy.The 1:1 complex (12 μM) formed between apoCaM and theNeurop peptide is extremely sensitive to saltand is half dissociated in less than 0.1 M KCl, suggesting thatelectrostatic interactions contribute stronglyto complex formation. Ion pair interactions are frequentlysensitive to high hydrostatic pressure due toelectrostriction effects in the solvated ion state. Application ofhigh pressure to the apoCaM·Neuropcomplex causes a red shift of the Neurop tryptophanemission center of mass and a reduced residualanisotropy but with insignificant reduction in quantum yield. Thetransition is smooth, reversible, andapparently two-state with a midpoint pressure of approximately 0.8kbar. The residual anisotropy, quantumyield, and center of mass of the emission spectrum are consistent withthe movement of the tryptophanside chain to a more solvated, slightly less restricted environmentupon the pressure-induced transition.The pressure effect is independent of the concentration of thecomplex. These and other data are consistentwith the pressure-induced reorganization being a unimolecular event notrequiring dissociation of thecomplex. A volume change of approximately 66 mLmol-1 and a free energy change ofapproximately1.7 kcal mol-1 are associated with thereorganization. The residual interactions maintaining thecomplexunder high pressure are characterized by low standard molar volumeand/or high standard free energychanges upon disruption but are destroyed by 200 mM KCl. It ispostulated that the main effect of salton the complex at high pressure is to drive the collapse of thehydrophobic pocket to which the peptideis binding.
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