Gas Phase Acidities and Electron Affinities of the Amino Acids

6 GAS PHASE ACIDITIES AND ELECTRON AFFINITIES OF THE AMINO ACIDS 

The experimental and CURES-EC electron affinities of the radicals are glycine-H 3.37(10), 3.38 eV alanine-H 3.42(13), 3.40 eV and histidine-H 3.84(19), 3.90 eV. The value for histidine-H is one of the largest experimental or CURES-EC Ea for an amino acid radical. Its GPA is also among the lowest of the amino acids and is comparable to that of uracil, 14.4(2) eV. Glycine with a GPA of 14.7(2) eV is a weaker gas phase acid than histidine or cytosine. The Ea of the radical from aspartic acid and tyrosine are about 3.8 eV for AM 1(0000), giving them a GPA comparable to 

Very little experimental work using ECD has been done on heterocyclic molecules. Some data have been obtained at a single temperature and were used to obtain Ea. Some Ea have been measured with TCT and/or E ji techniques and can be used to predict the ECD or NIMS temperature dependence. The temperature dependence of the compounds with similar Ea can be predicted by assuming a single state and value of Qan. The ECD response for pyridine has been determined to be low. The response for substituted pyridines should be similar to those for substituted benzenes. The compounds with Ea less than about 0.6 eV should exhibit only a single region. The ECD temperature dependence of quinoline should be like that of benzaldehyde, while that for acridine should be like the temperature dependence of benz[a]pyrene. 

The ECD response for the esters of the amino acids should be similar to that of the acetates. The CURES-EC calculated value for the methyl ester of glycine is 0.2 eV, approximately the same as that for ethyl acetate. By forming the trifluoromethyl ester of glycine, the electron affinity is increased to over 1.5 eV, making the compound a maximum capturing analyte. The amino acids have been analyzed in the ECD by making such electronegative derivatives. Very low limits of detection can be achieved, similar to those of some pesticides. For these cases the temperature dependence should be low [43 -6]. 

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