BioPHYSICAL chemistry

CHAPTER 15

15.1 To resolve the atomic structure the wavelength must be ≈1Å.

15.2 X-ray microscopes cannot be constructed because lenses suitable for

very small wavelengths cannot be made. The properties of materials

are dramatically different for wavelengths of less than 100 Å than

for visible light.

15.3 The scattering factor at zero angle is related to the number of elec-

trons present and hence the atomic number. The scattering factor

for all atoms drops off sharply with angle according to (sinθ)/λ.

15.4 (a)

(b)

15.5 A crystal is an array of repeating molecules that bind together in a

regular manner. The crystal can be thought of as a molecule form-

ing a structural motif built upon a lattice. The lattice can be classified

according to the symmetry that it possesses and the type of Bravais

lattice category. A protein crystal is a precise array of proteins that

has translation symmetry about the unit cell constants. At least one

protein is located at each of the lattice points of the space group

describing the crystalline array. Located in the regions surrounding

the proteins is solvent that comprises 30 – 80% of the total volume.

15.6 Salt crystals are composed only of the salts with the atoms at the

cell edge. A protein crystal is composed of only 30 – 60% of protein

• the rest is the crystallizing solution.
  15.7 In an X-ray diffraction experiment the intensity of each diffrac-
  tion point is measured. The square root of the intensity gives the
  amplitude of the structure factor. However, the structure factor is
  a complex number and all information about the phase is lost and
  must be determined through other measurements.
  15.8 The diagram shows how the vector analysis can be applied to deter-
  mine the phase associated with each structure factor.
  15.9 In isomorphous replacement the protein is modified with the addi-
  tion of a heavy metal whereas for anomalous dispersion the diffrac-
  tion from an existing metal is measured at different wavelengths.
  15.10 The Ramachandran plot shows whether the amino acid residues
  of a model fall into the allowed regions. Those that do not fall into
  the allowed region have improper geometry that has been altered,
  unless they are in a strained position in the protein.

sin

(. )

(. )

θ.

λ

== =→=θ

n

2 d

101

205

01

nm

nm

5 57.°

sin

(. )

()

θ.

λ

== = →=θ

n

2 d

101

25

001 0

nm

nm

..57°

ANSWERS TO PROBLEMS 475

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