Half sphere exposure Half sphere exposure Half Sphere Exposure (HSE) is a protein solvent exposure measure that was first introduced by . Like all solvent exposure measures it measures how buried amino acid residues are in a protein. It is found by counting the number of amino acid neighbors within two half spheres of chosen radius around the amino acid. The calculation of HSE is found by dividing a contact number (CN) sphere in two halves by the plane perpendicular to the Cβ-Cα vector. This simple division of the CN sphere results in two strikingly different measures, HSE-up and HSE-down. HSE-up is defined
Ewald's sphere falling on the crystal has a wave vector formula_2 whose length is formula_3. The diffracted plane wave has a wave vector formula_4. If no energy is gained or lost in the diffraction process (it is elastic) then formula_4 has the same length as formula_2. The difference between the wave-vectors of diffracted and incident wave is defined as scattering vector formula_7. Since formula_2 and formula_4 have the same length the scattering vector must lie on the surface of a sphere of radius formula_3. This sphere is called the Ewald sphere. The reciprocal lattice points are the values of momentum transfer where