Food Chemistry

48 1 Amino Acids, Peptides, Proteins

Fig. 1.13.Fluorescence detection of electrophoreti-
cally separated DNA fragments obtained by using the
dideoxy method (according toSmithet al., 1986)

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 (afterPe-
rutz, 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 (NHi–NHi+ 1 ,NHi+ 1 –CαHi,NHi+ 1 –CβHi,
CαHi–CαHi+ 1 ,CαHi–CβH) 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

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 Cα-atom of the peptide chain

requires a resolution of less than 0.3nm.

1.4.2.1 ExtendedPeptideChains

X-ray structural analysis and other physical mea-

surements of a fully extended peptide chain re-

veal the lengths and angles of bonds (see the “ball

and stick” representation in Fig. 1.15). The pep-

tide bond has partial (40%) double bond char-

acter withπelectrons 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 hydro-

gen 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