The Absorption Spectra of Amino Acids in the Region Two Hundred to Two Hundred and Thirty Millimicrons* By

Although the ultraviolet absorption spectra of most of the amino acids have been reported many times, in nearly all instances the spectra were limited to the region above approximately 220 rnp. Below this wavelength the absorption curves rise steeply into what has been considered a region of general, or continuous, or end-absorption. Most investigators have failedto examine this region closely. A few studies, principally of the end-absorption of tyrosine (2,3) and tryptophan (4,2 were made and new bands around 220 rnp were found. In 1932 Ley an d Arends (5), using a fluorite vacuum spectrograph, studied the absorption of a few amino acids down to 177 rnp. No new maxima were revealed but only the continuing steep rise in absorption with decreasing wave-length. Nevertheless, this work, by its observation of the quantitative changes of this steeply,rising absorption with change in pH, was able to make a real contribut,ion to our understanding of ionic configuration of amino acids. Because of the general neglect of this region, it was the impression of the authors that spectra showing the low wave-length bands for phenylalanine, histidine, and possibly methionine had not been rep0rted.l Just before this paper was submitted for publication, however, a microfilm copy of a paper by de Gouveia, Pinto Coelho, and Schon (7) was obtained which reported maxima in the spectra of phenylalanine and histidine at about 213 rnp. No observations below 210 rnp were reported. It is the purpose of this investigation to report the spectra of all of the amino acids which commonly occur in proteins down to 200 my. These will be compared to peptide spectra in a separate report. The extinctions of 205 rnp have already been compared to the extinctions of proteins at the same wave-length (8).