2Crystals are made of infinite number of unit cells Unit cell is the smallest unit of a crystal, which, if repeated, could generate the whole crystal.A crystal’s unit cell dimensions are defined by six numbers, the lengths of the 3 axes, a, b, and c, and the three interaxial angles, , and .
3A crystal lattice is a 3-D stack of unit cells Crystal lattice is an imaginative grid system in three dimensions in which every point (or node) has an environment that is identical to that of any other point or node.
4Miller indicesA Miller index is a series of coprime integers that are inversely proportional to the intercepts of the crystal face or crystallographic planes with the edges of the unit cell. It describes the orientation of a plane in the 3-D lattice with respect to the axes.The general form of the Miller index is (h, k, l) where h, k, and l are integers related to the unit cell along the a, b, c crystal axes.
5Miller Indices Rules for determining Miller Indices: 1. Determine the intercepts of the face along the crystallographic axes, in terms of unit cell dimensions.2. Take the reciprocals3. Clear fractions4. Reduce to lowest termsAn example of the (111) plane (h=1, k=1, l=1) is shown on the right.
6Another example: Rules for determining Miller Indices: 1. Determine the intercepts of the face along the crystallographic axes, in terms of unit cell dimensions.2. Take the reciprocals3. Clear fractions4. Reduce to lowest terms
7Where does a protein crystallographer see the Miller indices? Common crystal faces are parallel to lattice planesEach diffraction spot can be regarded as a X-ray beam reflected from a lattice plane, and therefore has a unique Miller index.
8SymmetryA state in which parts on opposite sides of a plane, line, or point display arrangements that are related to one another via a symmetry operation such as translation, rotation, reflection or inversion.Application of the symmetry operators leaves the entire crystal unchanged.
9Symmetry Elements Rotation turns all the points in the asymmetric unit around one axis, the center of rotation. A rotation does not change the handedness of figures. The center of rotation is the only invariant point (point that maps onto itself).
12Symmetry Elements Translation moves all the points in the asymmetric unit the same distance in the same direction. This has no effect on the handedness of figures in the plane. There are no invariant points (points that map onto themselves) under a translation.
13Symmetry Elements Screw axes (rotation + translation) rotation about the axis of symmetry by 360/n, followed by a translation parallel to the axis by r/n of the unit cell length in that direction. (r < n)
16Symmetry ElementsInversion, or center of symmetryevery point on one side of a center of symmetry has a similar point at an equal distance on the opposite side of the center of symmetry.
17Mirror plane or Reflection Symmetry ElementsMirror plane or Reflectionflips all points in the asymmetric unit over a line, which is called the mirror, and thereby changes the handedness of any figures in the asymmetric unit.The points along the mirror line are all invariant points (points that map onto themselves) under a reflection.
18Symmetry elements: mirror plane and inversion center The handedness is changed.
19Symmetry Elements Glide reflection (mirror plane + translation) reflects the asymmetric unit across a mirror and then translates parallel to the mirror. A glide plane changes the handedness of figures in the asymmetric unit. There are no invariant points (points that map onto themselves) under a glide reflection.
22Symmetries in crystallography Crystal systemsLattice systemsSpace group symmetryPoint group symmetryLaue symmetry, Patterson symmetry
23Crystal systemCrystals are grouped into seven crystal systems, according to characteristic symmetry of their unit cell.The characteristic symmetry of a crystal is a combination of one or more rotations and inversions.
247 Crystal Systems orthorhombic hexagonal monoclinic trigonal cubic tetragonaltriclinicCrystal System External Minimum Symmetry Unit Cell PropertiesTriclinic None a, b, c, al, be, ga,Monoclinic One 2-fold axis, || to b (b unique) a, b, c, 90, be, 90Orthorhombic Three perpendicular 2-folds a, b, c, 90, 90, 90Tetragonal One 4-fold axis, parallel c a, a, c, 90, 90, 90Trigonal One 3-fold axis a, a, c, 90, 90, 120Hexagonal One 6-fold axis a, a, c, 90, 90, 120Cubic Four 3-folds along space diagonal a, a, ,a, 90, 90, 90
25LatticesAuguste Bravais( )In 1848, Auguste Bravais demonstrated that in a 3-dimensional system there are fourteen possible latticesA Bravais lattice is an infinite array of discrete points with identical environmentseven crystal systems + four lattice centering types = 14 Bravais latticesLattices are characterized by translation symmetry
26Four lattice centering types No.TypeDescription1PrimitiveLattice points on corners only. Symbol: P.2Face CenteredLattice points on corners as well as centered on faces. Symbols: A (bc faces); B (ac faces); C (ab faces).3All-Face CenteredLattice points on corners as well as in the centers of all faces. Symbol: F.4Body-CenteredLattice points on corners as well as in the center of the unit cell body. Symbol: I.
28Tetragonal lattices are either primitive (P) or body-centered (I) C centered lattice=Primitive lattice
29Monoclinic lattices are either primitive or C centered
30Point group symmetryInorganic crystals usually have perfect shape which reflects their internal symmetryPoint groups are originally used to describe the symmetry of crystal.Point group symmetry does not consider translation.Included symmetry elements are rotation, mirror plane, center of symmetry, rotary inversion.
33N-fold axes with n=5 or n>6 does not occur in crystals Adjacent spaces must be completely filled (no gaps, no overlaps).
34Laue class, Patterson symmetry Laue class corresponds to symmetry of reciprocal space (diffraction pattern)Patterson symmetry is Laue class plus allowed Bravais centering (Patterson map)
35Space groupsThe combination of all available symmetry operations (32 point groups), together with translation symmetry, within the all available lattices (14 Bravais lattices) lead to 230 Space Groups that describe the only ways in which identical objects can be arranged in an infinite lattice. The International Tables list those by symbol and number, together with symmetry operators, origins, reflection conditions, and space group projection diagrams.
36A diagram from International Table of Crystallography
37Identification of the Space Group is called indexing the crystal. The International Tables for X-ray Crystallography tell us a hugeamount of information about any given space group. For instance,If we look up space group P2, we find it has a 2-fold rotation axis and the following symmetry equivalent positions:X , Y , Z-X , Y , -Zand an asymmetric unit defined by:0 ≤ x ≤ 10 ≤ y ≤ 10 ≤ z ≤ 1/2An interactive tutorial on Space Groups can be found on-line in Bernhard Rupp’s Crystallography 101 Course:
38Space group P1Point group 1 + Bravais lattice P1
39Space group P1barPoint group 1bar + Bravais lattice P1
40Space group P2Point group 2 + Bravais lattice “primitive monoclinic”
41Space group P21Point group 2 + Bravais lattice “primitive monoclinic”,but consider screw axis
42Coordinate triplets, equivalent positions r = ax + by + cz,Therefore, each point can be described by its fractional coordinates, that is, by its coordinate triplet (x, y, z)
43Space group determination Symmetry in diffraction patternSystematic absencesSpace groups with mirror planes and inversion centers do not apply to protein crystals, leaving only 65 possible space groups.
46Asymmetric unitRecall that the unit cell of a crystal is the smallest 3-D geometric figure that can be stacked without rotation to form the lattice. The asymmetric unit is the smallest part of a crystal structure from which the complete structure can be built using space group symmetry. The asymmetric unit may consist of only a part of a molecule, or it can contain more than one molecule, if the molecules not related by symmetry.
47Matthew CoefficientMatthews found that for many protein crystals the ratio of the unit cell volume and the molecular weight is between 1.7 and 3.5Å3/Da with most values around 2.15Å3/DaVm is often used to determine the number of molecules in each asymmetric unit.Non-crystallographic symmetry related molecules within the asymmetric unit