New infrared spectra and the tautomeric studies of purine and αL-alanine with an innovative sampling technique

New infrared spectra of purine and alpha L-alanine in the mid-IR range (400-4000 cm(-1)) were obtained with an innovative infrared sampling technique. The technique was directed at maximizing the separation of solid sample molecules at room temperature through a procedure of dissolution, spray, and deposition. The notoriously broad and overlapped bands in the region 2500-3300 cm(-1), which are characteristic of the KBr pellet spectra of amino acids and nucleic acid bases, are now absent in all our new spectra. The spectra obtained from neutral and acidic aqueous solutions are different. For purine, these spectral differences are interpreted in terms of the existence of N(7)H or N(9)H tautomers; For alanine, the differences are ascribed to its two different structures, zwitterion and un-ionized neutral. It was found that with this new method, better resolved IR spectra of purine and alanine were obtained at room temperature. Furthermore, for the first time, a spectrum of zwitterionic alanine of largely monomeric form in the complete mid-IR range was obtained. IR spectra were predicted from self consistent reaction field (SCRF) molecular orbital calculations at the HF/6-311 + + G(d, p) level. The SCRF calculations were for purine and alanine present in a KBr continuum. Good agreement was found between the observed and calculated spectra. (C) 1999 Elsevier Science B.V. All rights reserved.