Internal Stark Effect Measurement of the Electric Field at the Amino Terminus of an α Helix

The strengths of electrostatic interactions in biological molecules are difficult to calculate or predict because they occur in complicated, inhomogeneous environments. The electric field at the amino terminus of an alpha helix in water has been determined by measuring the shift in the absorption band for a covalently attached, neutral probe molecule with an electric dipole moment difference between the ground and excited electronic states (an internal Stark effect). The field at the interface between the helix and the solvent is found to be an order of magnitude stronger than expected from the dielectric properties of bulk water. Furthermore, although the total electric dipole moment of the helix increases with length, the electric field at the amino terminus does not.