**2. Experimental details**

constituted by one carbon, one NH3

202 Raman Spectroscopy and Applications

expressed as in Eq. (1):

interaction. An eigenstate of *H*0 is *ψi*

*R* = CH3 for alanine, *R* = CH2-OH for serine, etc.

+

vibrational and structural properties of amino acid crystals [3–42].

group, one CO2

−

The radical (*R*) characterizes the different types of amino acids; for example, *R* = H for glycine,

As it is well established, every cell of the living being on the Earth uses a set of 20 amino acids to produce all kinds of proteins [1, 2]. The 20 standard amino acids can be classified as (i) unpolar (alanine, valine, leucine, isoleucine, methionine, proline, phenylalanine, and tryptophan) and (ii) polar (glycine, arginine, asparagine, cysteine, glutamine, lysine, aspartic acid, glutamic acid, serine, threonine, tyrosine, and histidine). Additionally, some compendia include selenocysteine and pyrrolysine as belonging to the group of proteinogenic amino acids, but this is not unanimity yet. In the last years, a series of studies have investigated the

One of the main techniques to investigate vibrational properties of materials, whatever it is, is Raman spectroscopy. The technique consists in the interaction of light from a laser source with the material and further scattering of the light. The scattered light carries information about the rotational, vibrational, and, eventually, electronic states of the material. Concerning the Raman scattering effect, a pivotal concept is the scattering cross-section, σ, which represents a likelihood of a scattering event to occur [43]. It is defined as the rate at which energy is removed from the incident photon by the scattering substance, divided by the rate at which energy in the incident photon crosses a unit area perpendicular to its direction of propagation [43],

In this equation, *h* represents the Planck constant, ωI represents the angular frequency of the incident light, 1/*τ* is the transition rate of the scattering process, and *I*I is the surface power density, given in units of energy/[time × area]. 1/*τ* is calculated through time-dependent perturbation theory from quantum mechanics. Obviously, if we are using quantum theory we need to define a Hamiltonian representing the system, as well as the states of the system. It is possible to define a Hamiltonian composed of two parts: (i) one unperturbed (*H*0), represented by the contribution of the scattering medium and by the radiation field and (ii) one perturbed (Hp), with contributions from the electron-radiation interaction and electron-phonon

photons, phonons, and about the electrons. The process involves the excitation of the medium due to the incident photon, leading the system to an intermediate state, creating an electronhole pair. Such pair is scattered in a different intermediate state; finally, the electron-hole pair recombines, occurring the emission of a photon. This scattered phonon will have the informa-

tion about the system. Using time-dependent perturbation theory we can show that

group, and a group denominated as radical.

s pt *I I (= hw / 2 I )* (1)

that encompasses information about incident and scattered

(2)

In the present work, we have used two experimental set-ups: an instrument using Fourier transform mechanism and a dispersive (conventional) spectrometer. On the one hand, FT-Raman spectra were recorded using a Bruker RFS100/S FTR system and a D418-T detector, with the sample excited by means of the 1064 nm line of a Nd:YAG laser. In these cases, the spectral resolution was 4 cm−1. On the other hand, the conventional Raman spectra were excited with the 514.5 nm line of argon ion lasers and the scattered light was analyzed in a Jobin-Yvon T64000 spectrometer equipped with the nitrogen cooled CCD system. Typically, the spectral resolution in the conventional Raman experiments was 2 cm−1. Theoretically, the bands appearing in the Raman spectrum are independent of the excitation energy of the laser, although the intensity of the scattered light is proportional to *λ*−4 (*λ* is the wavelength of the laser) and, consequently, long-wavenumber excitation will produce a weaker Raman spectrum. However, in some cases, the exciting photons—mainly in the visible—are sufficient to excite the system into the lowest energy electronic states, and further relaxation to the ground state will appear as a broadband fluorescence in the Raman spectrum. Regarding the samples of amino acids, we have utilized two kinds: those used to obtain FT-Raman spectra were commercial reagents, while those where polarized Raman spectra are shown, were crystals obtained through aqueous solutions at several temperatures.
