Ultraviolet resonance Raman enhancement of the carboxylate and guanidinium groups in amino acids

✍ Scribed by Peter Hildebrandt; Roman S. Czernuszewicz; Christine A. Grygon; Thomas G. Spiro

• Publisher: John Wiley and Sons
• Year: 1989
• Tongue: English
• Weight: 584 KB
• Volume: 20
• Category: Article
• ISSN: 0377-0486
• DOI: 10.1002/jrs.1250201003

✦ Synopsis

Carboxylate and guanidinium groups of amino acids have been studied by ultraviolet resonance Raman (UVRR) spectroscopy. On near-resonant excitation (192 nm) into the allowed n-n* transition of the carboxylate chromophore, the vibrational spectrum of aspartate displays only one band originating from the COO-symmetric stretching mode, consistent with an A-term enhancement mechanism. The UVRR spectra of the guanidinium ion reveals a complex vibrational pattern, and the bands have been assigned with the aid of a normal coordinate analysis. UV enhancement is observed for both asymmetric (E') and totally symmetric (A,') modes, and also for the overtone of the out-of-plane CN, bending mode, T(A,"). The E' enhancement is suggested to reflect Jahn-Teller splitting in the first allowed electronic transition A," + E", whereas the 2r(A,") enhancement requires a lowering of the force constant for out-of-plane bending in the excited state, consistent with its expected tendency toward pyramidalization. Similar enhancement patterns are seen in the UVRR spectra of the guanidinium chromophore in methylguanidinium and arginine, which show, in addition, splittings and intensity redistribution due to the substituents. The excitation profiles of guanidinium and arginine reveal a complex structure that may arise from an excited-state Jahn-Teller effect. The Raman enhancements are too low to permit detection of guanidinium or carboxylate in the UVRR spectra of proteins, even at wavelengths as low as 192 nm.