p-Cresol is a simple molecular model for the para phenolic side chain of tyrosine. Previously, Siamwiza and co-workers [(1975) Biochemistry 14, 4870-4876] investigated p-cresol solutions to identify Raman spectroscopic signatures for different hydrogen-bonding states of the tyrosine phenoxyl group in proteins. They found that the phenolic moiety exhibits an intense Raman doublet in the spectral interval 820-860 cm(-1) and that the doublet intensity ratio (I2/I1, where I2 and I1 are Raman peak intensities of the higher- and lower-wavenumber members of the doublet) is diagnostic of specific donor and acceptor roles of the phenoxyl OH group. The range of the doublet intensity ratio in proteins (0.30 < I2/I1 < 2.5) was shown to be governed by Fermi coupling between the phenolic ring-stretching fundamental nu1 and the first overtone of the phenolic ring-deformation mode nu(16a), such that when the tyrosine phenoxyl proton is a strong hydrogen-bond donor, I2/I1 = 0.30, and when the tyrosine phenoxyl oxygen is a strong hydrogen-bond acceptor, I2/I1 = 2.5. Here, we interpret the Raman and infrared spectra of p-cresol vapor and extend the previous correlation to the non-hydrogen-bonded state of the tyrosine phenoxyl group. In the absence of hydrogen bonding, the Raman intensity of the higher-wavenumber component of the canonical Fermi doublet is greatly enhanced such that I2/I1 = 6.7. Thus, for the non-hydrogen-bonded phenoxyl, the lower-wavenumber member of the Fermi doublet loses most of its Raman intensity. This finding provides a basis for understanding the anomalous Raman singlet signature (approximately 854 cm(-1)) observed for tyrosine in coat protein subunits of filamentous viruses Ff and Pf1 [Overman, S. A., et al. (1994) Biochemistry 33, 1037-1042; Wen, Z. Q., et al. (1999) Biochemistry 38, 3148-3156]. The implications of the present results for Raman analysis of tyrosine hydrogen-bonding states in other proteins are considered.

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Recently, laser-induced breakdown spectroscopy (LIBS) has been developed for the elemental analysis of geological samples for application to space exploration. There is also interest in using the technique for the analysis of water ice and ice/dust mixtures located at the Mars polar regions. The application is a compact instrument for a lander or rover to the Martian poles to interrogate stratified layers of ice and dusts that contain a record of past geologic history, believed to date back several million years. Here we present results of a study of the use of LIBS for the analysis of water ice and ice/dust mixtures in situ and at short stand-off distances (< 6.5 m) using experimental parameters appropriate for a compact instrument. Characteristics of LIBS spectra of water ice, ice/soil mixtures, element detection limits, and the ability to ablate through ice samples to monitor subsurface dust deposits are discussed.