## 2 Vibrational Spectroscopy
\begin{table}
\begin{tabular}{c c} \hline \hline
**Wavenumber (cm\({}^{-1}\))** & **Raman Peak Assignments** \\ \hline
621 & C-C twisting mode of Phenylalanine (proteins) \\
642 & C-C twisting mode of Tyrosine and Phenylalanine \\
670 & Thymine, Guanine (DNA/RNA) \\
720 & C-N stretching in Adenine and lipids \\
750 & symmetric breathing of Tryptophan (protein) \\
782 & Uracil, Thymine, Cytosine (ring breathing modes in the DNA/RNA) \\
827 & PO\({}_{2}\) stretching in DNA, Tyrosine \\
854 & Ring breathing in Tyrosine and Proline (proteins) \\
935 & C-C stretching mode of Proline and Valline \\
1003 & C-C aromatic ring stretching in Phenylalanine \\
1030 & C-H bending mode in Phenylalanine, C-N stretching in proteins \\
1080 & PO\({}_{2}\) symmetric stretching (DNA/RNA) \\
1085 & C-O stretching \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Tentative peak assignments [4] for typical Raman cellular spectrum shown in Figure 3.  
Figure 3: Typical cellular Raman spectrum. Raman and IR spectroscopy are complementary techniques. IR spectroscopy involves the absorption of radiation whereas Raman spectroscopy is a scattering technique, whereby the incident radiation couples with the vibrating polarisation of the molecule and generates or annihilates a vibration. These different underlying mechanisms give rise to the complementarity of the two techniques and they each have different selection rules. For a vibration to be active in IR spectroscopy, a change in dipole is required, whereas a change in polarisability is required for a vibration to be Raman active. Vibrations of asymmetric, polar bonds tend to be strong in IR spectra, whereas symmetric, nonpolar groups are strong in Raman spectra. For example, O-H vibrations of water are very strong in IR spectra, whereas they are relatively weak in Raman spectra. This feature means that Raman spectroscopy may be a more suitable technique for biological applications. Another difference is that Raman spectroscopy can be employed in the UV, visible or near IR regions of the electromagnetic spectrum whereas IR spectroscopy monitors the absorption of IR radiation. Therefore, Raman spectroscopy has a higher spatial resolution (\(<\)1 \(\upmu\)m for Raman, \(\sim\)5-10 \(\upmu\)m for IR). Raman spectroscopy is, however, inherently weak with only a small percentage of photons undergoing Raman scattering (\(\sim\)10\({}^{-6}\)). Longer spectral acquisition times are therefore needed to achieve adequate signal to noise in Raman compared to IR spectroscopy. IR and Raman spectra of biomedical samples such as cells and tissues are a superposition of all the contributions from each individual biochemical component such as DNA, RNA, proteins, lipids and carbohydrates. Thus vibrational spectroscopy methods can provide a molecular diagnosis by providing a \"biochemical fingerprint\" of the complete genome, proteome and metabolome of the cell or tissue [5]. This can be provided in a rapid, label free, non-destructive manner allowing additional analyses to be performed subsequently, such as staining, immunocyt_etc._ Standardised methods and procedures for collecting IR spectra and images from biological samples have recently been described [6]. In recent years, Raman and FTIR spectroscopy have been used in the detection of a variety of cancers, including breast, prostate, oesophageal, colon, lung, oral and cervical cancer, and excellent sensitivity and specificity values have been reported [7, 8, 9].