2009_Book_FoodChemistry

48 1 Amino Acids, Peptides, Proteins
Fig. 1.13. Fluorescence detection of electrophoretically
separated DNA fragments obtained by using the
dideoxy method (according to Smith et al., 1986)
Fig. 1.15. Structure of an elongated peptide chain.
Carbon, ○ oxygen, ◦ nitrogen, hydrogen and
® side chain
density distributions of 2,5-dioxopiperazine
based on various degrees of resolution are
presented in Fig. 1.14. Individual atoms are
well revealed at 0.11 nm. Such a resolution
has not been achieved with proteins. Reliable
localization of the C α -atom of the peptide chain
requires a resolution of less than 0.3nm.
1.4.2.1 Extended Peptide Chains
Fig. 1.14. Electron density distribution patterns for
2,5-dioxopiperazine with varying resolution extent.
a 0.11 nm, b 0.15 nm, c 0.20 nm, d 0.60 nm (after Perutz,
1962)
1.4.2 Conformation
Information about conformation is available
through X-ray crystallographic analysis of
protein crystals and by measuring the distance
(≤30 nm) between selected protons of the peptide
chain (NH i –NH i+1 ,NH i+1 –C α H i ,NH i+1 –C β H i ,
C α H i –C α H i+1 ,C α H i –C β H) by means of H-NMR
spectroscopy in solution. This assumes that, in
many cases, the conformation of the protein in
crystalline form is similar to that of the protein in
solution. As an example the calculated electron
X-ray structural analysis and other physical measurements
of a fully extended peptide chain reveal
the lengths and angles of bonds (see the “ball
and stick” representation in Fig. 1.15). The peptide
bond has partial (40%) double bond character
with π electrons shared between the C ′ O
and C ′ N bonds. The resonance energy is about
83.6kJ/mole:
(1.91)
Normally the bond has a trans-configuration, i. e.
the oxygen of the carbonyl group and the hydrogen
of the NH group are in the trans-position;
a cis-configuration which has 8 kJ mol −1 more
energy occurs only in exceptional cases (e. g. in