We have grown a novel crystal of hydrated gycylglycine
(Gly-Gly), determined its crystal structure, and
measured its Raman tensor at 488, and 244 nm, close to resonance.
This crystal has 8 molecules of (Gly-Gly) and
12 molecules of water per unit cell. The Gly-Gly hydrogen-bonds in
a β-sheet-like structure where all of the amide
planes are parallel. We report here the development of a general
method to use preresonance single-crystal Raman
measurements to determine the direction of the molecular electronic
transition moments. We utilize the UV (244
nm) excited Raman tensor and relationships derived here to accurately
determine the transition moment orientation
of the amide and carboxylate group π→π* transitions and the
orientation of a charge transfer transition from the
carboxylate group to the amide group. The amide π→π*
(NV1) transition moment is found to be in the
peptide
plane at an angle of −46° ± 3° to the amide carbonyl bond.
The carboxylate π→π* (NV1‘) transition moment
is
oriented almost parallel to a line connecting the carboxylate oxygens
(4.6° off), as theoretically expected. The
charge transfer band from the carboxylate to the amide chromophore is
found at an angle of −83° ± 3° to the amide
carbonyl bond, almost along the line of intersection of the amide and
carboxylate planes.
A new laser has been developed which generates hundreds of milliWatts of cw UV power below 260 nm. The laser consists of a small-frame Ar+-ion laser which is intracavity doubled with the use of BBO nonlinear optical crystals. More than 300 mW are available at 244 and 257 nm, while 180, 100, and 30 mW are available at 248, 238, and 228.9 nm, respectively. This laser is an ideal source for UV Raman spectroscopy since it avoids the nonlinear and saturation problems common with the typical pulsed laser excitation sources. It also minimizes thermal sample degradation. We demonstrate the increased spectral signal-to-noise ratios possible due to the ability to focus the cw laser into a small-volume element that can be efficiently imaged into the spectrometer. We demonstrate the ability of this laser to excite Raman spectra of solid samples such as coal-liquid residuals, and point out the utility of the 228.9-nm line for studying aromatic amino acids in proteins. We also demonstrate the ability to selectively study pyrene intercalated into calf thymus DNA.
We demonstrate the utility of a new 206.5-nm continuous-wave UV laser excitation source for spectroscopic studies of proteins and CVD diamond. Excitation at 206.5 nm is obtained by intracavity frequency doubling the 413-nm line of a krypton-ion laser. We use this excitation to excite resonance Raman spectra within the π → π amide transition of the protein peptide backbone. The 206.5-nm excitation resonance enhances the protein amide vibrational modes. We use these high signal-to-noise spectral data to determine protein secondary structure. We also demonstrate the utility of this source to excite CVD and gem-quality diamond within its electronic bandgap. The diamond Raman spectra have very high signal-to-noise ratios and show no interfering broad-band luminescence. Excitation within the diamond bandgap also gives rise to narrow photoluminescence peaks from diamond defects. These features have previously been observed only by cathodoluminescence measurements. This new continuous-wave UV source is superior to the previous pulsed sources, because it avoids nonlinear optical phenomena and thermal sample damage; Photoluminescence.
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