Biomedical Engineering Reference
In-Depth Information
orbital has been mentioned as a potential site of
electron attachment to the backbone [
49
,
116
] and has been invoked to explain
observations of low-energy dissociative attachment to the dibutyl and triethyl esters
of phosphoric acid [
46
]. On the other hand, as discussed by Burrow and coworkers
[
117
], computational studies question the conventional picture of the P-O bond
and thus the existence of such a
orbital conjugate to it. This
orbital; and indeed, their electron-transmission
spectra for several phosphate-containing compounds do not show evidence of it.
To explore the possible role of the phosphate group in attaching low-energy
electrons, we studied the simplest model compound, phosphoric acid (H
3
PO
4
), and
its three methyl esters, mono-, di-, and trimethylphosphate [
118
]. Although two of
the larger DNA fragments discussed above (Sects.
5.3.1.2
and
5.3.2.1
) included a
phosphate group, letting methyl stand in for deoxyribose avoids obscuring details
specific to the phosphate and, more importantly, allows us to carry out more accurate
calculations that include polarization effects for H
3
PO
4
and monomethylphos-
phate, against which static-exchange results for the di- and trimethyl esters can
be calibrated.
Our integral elastic cross sections for all four molecules are qualitatively
similar. While broad shape resonances are visible at higher energy, the calculations
including polarization indicate that the lowest resonance lies at about 7 eV, much
too high to be a
shape resonance. Other scattering calculations on phosphoric
acid reach similar conclusions: that of Tonzani and Greene [
98
] places resonances
at 7.7 and 12.5 eV, while the recent study of Bryjko and coworkers [
119
] indicates
a broad shape resonance at about 7 eV as well as nearby Feshbach resonances,
but no lower-energy resonances. Thus none of the cross-section calculations gives
evidence of a low-energy
resonance in H
3
PO
4
. For trimethylphosphate, Simons
and coworkers [
120
] reached essentially the same conclusion using bound-state
methods. They found that, although a clear P-O
orbital exists in a trivalent
phosphorus compound, CH
3
-O-PO, there is no identifiable P-O
orbital in the
pentavalent trimethyl ester. Likewise, absolute measurements of the dissociative
attachment cross section of trimethylphosphate indicate it to be very small [
31
].
Still, the situation at low energy is not completely clear. Although the scattering
calculations [
98
,
118
,
119
] and our own estimates based on minimal-basis-set orbital
energies [
118
] concur that there should be no shape resonances below about 7
eV in phosphoric acid or its alkyl esters, the electron transmission spectrum of
trimethylphosphate does show two resonances, albeit weak ones, at 2.1 and 4.6 eV,
and a procedure of empirically scaling virtual orbital energies also suggests shape
resonances in the 2-5 eV range [
117
]. Moreover, while dissociative attachment to
dibutyl phosphate yields a number of peaks in the 0-3 eV range, in various ion
channels, dissociative attachment to triethylphosphate appears to be much weaker
[
46
], suggesting that, as was discussed for sugars in Sect.
5.3.2.1
, the hydroxyl group
may play a significant role in promoting very-low-energy dissociative attachment.
Electron-stimulated desportion measurements [
45
] on the monosodium salt of
phosphoric acid, NaPO
2
(OH)
2
, found peaks in the ion yield only between 7 and
9 eV; however, only light ions (H
,O
,OH
) were detected in that work, while the
anions seen at low energy in the gas-phase measurements on dibutylphosphate were
