X-RAY CRYSTALLOGRAPHY 95
radiation. Synchrotron radiation is tunable to wavelengths at or below 1 Å , an
advantage for the multiple - wavelength anomalous diffraction (MAD) method
for solving protein structures. A crystal may diffract shorter - wavelength radia-
tion more weakly, but the crystal suffers less radiation damage because it
absorbs less radiation at the shorter wavelength. A growing number of syn-
chrotron radiation facilities exist that are available to researchers wishing to
collect data and solve the structures of large proteins. A number of facilities
in the United States are associated with universities or national laboratories.
For instance, the Brookhaven National Laboratory (BNL) maintains multiple
synchrotron beamlines as do the Stanford Synchrotron Radiation Laboratory
(SSRL) and the Cornell High Energy Synchrotron Source. The European
Synchrotron Radiation Facility (ESRF) serves researchers internationally. The
Harima Institute ’ s Riken SPring - 8 Center has multiple beamlines serving
Japanese researchers. The Protein Crystallography Research Resource at BNL
( http://www.px.nsls.bnl.gov/ ) was running nine beamlines in late 2006, most
of which were carrying out MAD - based protein crystallography. MAD is
an acronym for multiple wavelength anomalous dispersion, as discussed
previously.
The next - generation X - ray sources are called X - ray free - electron lasers
(XFELs). XFELs promise X - ray beam lines that are vastly brighter, coherent
(which synchrotron radiation is not), capable of delivering higher energy and
shorter pulses than the synchrotron radiation used presently. Pulses as short
as 100 femtoseconds could be capable of producing movies of chemical reac-
tions — bond breaking and bond making — in progress. It is expected that
XFELS could determine structures of proteins and other biological macro-
molecules from noncrystalline materials, a great advance over the present
technology. The Linac Coherent Light Source (LCLS) at the Stanford Univer-
sity Linear Accelerator Center will be the world ’ s fi rst X - ray free electron laser
(XFEL) facility when it becomes operational in 2009 (see the website http://
www - ssrl.slac.stanford.edu/lcls/ for more information and updates).
X - ray detectors also come in several varieties: (1) single - photon counters
which yield accurate results but require up to several weeks to acquire the
10,000 – 100,000 (10^4 – 10^5 ) refl ections necessary to compile a complete data set
for a protein crystal; (2) image plates that operate much like photographic fi lm
but are 10 times more sensitive; (3) area detectors, electronic devices that
detect X - ray photons on a two - dimensional surface. Both fl uorescent - type
detectors, image plates and fast area detectors, are more sensitive at the
shorter wavelengths of X - ray radiation from synchrotron sources.
3.3.4 Descriptive Examples,
The following example of a calcium - ion - containing enzyme that hydrolyzes
adenosine triphosphate (ATP) is taken from Section 6.4.2 , where much more
detail on the enzyme will be found. In general, Ca 2+ - ATPases, especially those
found in the sarco(endo)plasmic reticulum (SR), are P - type ATPases that play
