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HO2*, usually termed either hydroperoxyl radical or perhydroxyl radical, is the protonated form of superoxide; the protonation/deprotonation equilibrium exhibits a pK(a) of around 4.8. Consequently, about 0.3% of any superoxide present in the cytosol of a typical cell is in the protonated form. This ratio is rather accurately reflected by the published literature on the two species, as identified by a PubMed search; at the time of writing only 28 articles mention "HO2," "hydroperoxyl" or "perhydroxyl" in their titles, as against 9228 mentioning superoxide. Here it is argued that this correlation is not justifiable: that HO2*'s biological and biomedical importance far exceeds the attention it has received. Several key observations of recent years are reviewed that can be explained much more economically when the participation of HO2* is postulated. It is suggested that a more widespread appreciation of the possible role of HO2* in biological systems would be of considerable benefit to biomedical research.

_____________________________________________
   T           Cp          S        delta-Hf
 _____        ________________      ________
   K             cal/mol K          kcal/mol
_____________________________________________
   
  200         7.99       51.51        3.22
  298         8.34       54.75        3.00
  300         8.35       54.81        3.00
  400         8.89       57.28        2.80
  500         9.47       59.33        2.63
  600        10.00       61.10        2.50
  700        10.43       62.68        2.40
  800        10.77       64.09        2.31
  900        11.06       65.38        2.23
 1000        11.38       66.56        2.16
 1100        11.63       67.65        2.11
 1200        11.86       68.68        2.07
 1300        12.08       69.63        2.04
 1400        12.29       70.54        2.02
 1500        12.48       71.39        2.00
 1600        12.67       72.20        1.99
 1700        12.85       72.98        1.99
 1800        13.01       73.72        2.00
 1900        13.17       74.42        2.01
 2000        13.32       75.10        2.03
 2100        13.46       75.76        2.05
 2200        13.60       76.39        2.07
 2300        13.72       76.99        2.10
 2400        13.84       77.58        2.14
 2500        13.95       78.15        2.17
 2600        14.05       78.70        2.21
 2700        14.15       79.23        2.25
 2800        14.24       79.74        2.29
 2900        14.32       80.25        2.33
 3000        14.40       80.73        2.37

• The standard enthalpy of formation and standard entropy values at 298 K were provided by Martin J. Rabinowitz of NASA Lewis Research Center (marty@lerc.nasa.gov)
• HO2 molecule in action