OTHER INSTRUMENTAL METHODS 139
Putting together their M ö ssbauer analysis with previous ENDOR data,
EXAFS analyses, and X - ray crystallographic evidence, along with comparison
to a model Fe(II) complex with trigonal sulfur coordination, allowed the refer-
ence 41 authors to describe the M N center as having the oxidation state
arrangement of Mo 4+ – 3Fe 3+ – 4Fe 2+. As the preceding discussion of nitrogenase
metal – sulfur clusters indicate, analysis of complex bioinorganic systems
requires the use of multiple analytical techniques and the cooperative exchange
of data and ideas of many researchers. A more complete discussion of the
complex enzyme nitrogenase is found in Chapter 6 of the fi rst edition of this
text.^42
A more recent X - ray crystallographic study of the nitrogenase MoFe protein
at 1.16 - Å resolution shows a previously unrecognized ligand coordinated to
the six iron ions at the center of the FeMo cofactor (see Figure 3.28 ).^43 T h e
data are deposited in the protein data bank, PDB, with accession number
1M1N. Previous X - ray crystallographic structures, such as data deposited in
the PDB as 2MIN and 3MIN, 40b were solved at lower resolutions, and they
could not show a central ligand because the overwhelming electron density of
the iron and sulfur atoms surrounding it hid any electron density due to a light
(N, C, O) atom. The reference 43 authors, solving and refi ning the 1M1N struc-
ture, considered carbon, nitrogen, oxygen, and sulfur as plausible central atoms,
fi nally assigning it tentatively as a hexacoordinated nitrogen atom. Interested
readers may want to read the perspective on nitrogenase written by D. C. Rees
and J. B. Howard and published in late 2006 in the Proceedings of the National
Academy of Sciences, USA.^44 The article provides an overview of biological
nitrogen fi xation and introduces three other articles in the same issue that
address central aspects of the mechanism and assembly of nitrogenase. Before
leaving this topic, this author would like to address problems that arise in
accessing data in the protein data bank (PDB). It is important that PDB users
recognize the difference between the letter “ I ” and the number “ 1 ” when
entering accession numbers into the PDB ’ s search routine (found at http://
http://www.rcsb.org/pdb/home/home.do ). For instance, if the user enters 1MIN using
the letter “ I, ” the search routine returns 2MIN, the nitrogenase X - ray crystal-
lographic data that replaced 1MIN in the structure database (see reference
40b ). If the user enters 1M1N using the number “ 1, ” the search routine returns
information on the nitrogenase structure described in reference 43. The same
problem occurs when searching PDB accession numbers containing the letter
“ O ” and/or the number “ 0. ”
3.7 Other Instrumental Methods,
3.7.1 Atomic Force Microscopy,
Atomic force microscopy (AFM) is a microscopic method for chemists and
biologists offering the magnifi cation range of both the light and electron