Theory and Calculation of Stopping Cross Sections of Nucleobases for Swift Ions - Radiation Damage in Biomolecular Systems - page 198

Table 12.3 The mean
excitation energy in the
length representation,

I 0

(eV)

Cytosine 69.60

I 0 ,

Uracil 73.13

for the nucleobases [ 14 ]

Thymine 70.00

Adenine 69.06

Guanine 71.58

Fig. 12.3 Stopping cross
sections of the nucleobases
for protons as a function of
projectile velocity
[green-dash-uracil;
red-dot-cytosine;
yellow-dash-dot-thymine;
blue-long-dash-adenine; lila

Table 12.4 Calculated mean excitation energies of the amino acids [ 13 ]

I 0 (eV) I 0 (eV) I 0 (eV)

alanine 67.5 glutamine 68.8 phenylalanine 60.7

arginine 65.3 glycine 71.2 serine 71.3

asparagine 71.0 isoleucine 61.9 threonine 68.5

aspartic acid 73.9 leucine 61.9 tyrosine 63.2

cysteine 84.0 lysine 62.7 valine 63.3

glutamic acid 71.3 methionine 76.3 H 2 O 72.2

The mean excitation energies and shell correction coefficients from Table 12.1
were then used in the ( 12.11 ) to determine the stopping cross sections for the five
nucleobases. These cross sections for a singly charged projectile ion are plotted in
Fig. 12.3 .
The principal quantity determining the stopping properties of the nucleobases is
the mean excitation energy. As the nucleobases all have similar mean excitation
energies, it would not be expected that they would differ much in stopping
properties. This expectation is borne out in Fig. 12.3 . In fact, the mean excitation
energies of the nucleobases are very close to calculated mean excitation energies
[ 13 , 23 , 40 ] of water (72.2 eV) and those of the amino acids (mean of 68.4 eV for 17
amino acids, see Table 12.4 ). Thus, the energy absorbing characteristics of water,
amino acids, and nucleobases are all very similar.