CHEMICAL TRANSFORMATION DERIVATIZATION OF O6-METHYL(HYDROXYETHYL)GUANINE AND O6-(HYDROXYETHYL)GUANINE FOR DETECTION BY GC-EC MS

In this project we set out to make an important class of DNA adducts, comprising O6-alkyl and O6-(hydroxyalkyl)guanines, susceptible to sens itive detection by GC-EC/MS. While existing literature indicated that pentafluorobenzylation would be useful for the ring NH site on these c ompounds, how to best overcome the polarity of the exocyclic NH2 and O H groups, without losing the O6-alkyl moiety, was less clear. Working with O6-methylguanine and O6-(2'-hydroxyethyl)guanine as representativ e analytes, we found that the NH2 group could be converted into fluoro without loss of the O6 substituent. For the OH group, a comparison of several derivatives (OR') led to R' = tert-butyl as the best choice a t this stage. The latter work, especially via NMR, also allowed exact structural assignments to be made for the N7 and N9 pentafluorobenzyl isomeric derivatives that formed. Of these R' derivatives, the N7 isom ers migrated slower on silica-TLC, had higher GC retention times, had lower responses by GC-EC/MS, and were preferentially destroyed as the GC column aged. However, the N9 isomer was slower on TLC when the OH w as not derivatized. This behavior was rationalized using a concept of ''polar footprint'' for the derivatives. The concept also seemed to ex plain the puzzling GC-EC/MS behavior of some related compounds in our laboratory. Apparently the polar footprint should be minimized in desi gning derivatives for trace detection by GC-EC/MS.