BioPHYSICAL chemistry

Now consider the molecule with the isotopic substitution^13 C^16 O:

(11.42)

(11.43)

The change in frequency due to the isotope substitution is 45 cm−^1. This

is easily measured as infrared spectrometers have a resolution of 1 cm−^1.

Thus, by selectively substituting specific atoms of a molecule, the entire

assignment of the vibrational spectrum can be made unambiguously.

RESEARCH DIRECTIONS: HYDROGENASE

Various organisms contain the enzyme hydrogenase, which catalyzes

the simple reaction of two protons and two electrons, forming hydrogen

gas:

2H++2e−↔H 2 (11.44)

Representatives of most prokaryotic genera as well as

some eukaryotes contain hydrogenase, which was identified

originally in the 1930s. Despite the simplicity of this reaction

and the many years of research, scientists still are probing

its molecular mechanism to both better understand the bio-

logical processes involved and for application in the hydrogen

economy (Chapter 12). Hydrogenases are proteins that can

be divided into two classes: those that contain only iron and

those that have both nickel and iron. The iron-only enzymes

are highly evolved and can produce nearly 10,000 molecules

of hydrogen per second. Despite the simplicity of the reaction,

the molecular machinery is quite sophisticated. Hydrogenases

typically contain one or two protein subunits surrounding

the metal cofactors. Some of the metal cofactors are organized

into standard iron–sulfur cofactors that are aligned to direct

electrons from outside the protein (Figure 11.6). The protein

also has a potential pathway involving protonatable amino

acid residues for the transport of protons from the solvent into

the protein.

The most remarkable feature of hydrogenases is the active

site of the enzyme, known as the H or Ni–Fe cluster; which is

A

().

=

××−

−

−

1

23 10

1900

(^101) 119 10
1
π cm s^26
Nm
kgg
= 2122 cm−^1
μ=.

=

×

+

=×

mm

mm

CO m

CO

p

13 16

13 16

717 ( .1 67 10×=×−−^27 kg) 1 19 10.^26 kg

232 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY

Figure 11.6The general
structural features of the iron
hydrogenase showing the
positions of the iron–sulfur
and H cofactors.