Vibrational Raman spectroscopy of nanoscale needle shaped histidine V. Sonois *# , P. Faller # , N. Fazouan ** , A. Estève *** and W.S. Bacsa * # Laboratoire de Chimie de Coordination, 205 route de Narbonne, 31077 Toulouse Cedex 4 ** Faculté des Sci. et Tech., Lab. de Phys. et de Méc. des Matériaux, BP 523 23000 Beni Mellal, Marocco *** LAAS, 7 avenue du Colonel Roche, 31077 Toulouse Cedex 4 nMat group/ CEMES-CNRS, Univ. Toulouse, 29 rue Jeanne Marvig, BP 94347, 31055 Toulouse Cedex 4, wolfgang.bacsa@cemes.fr ABSTRACT Amino acids in aqueous solutions have low Raman signals and are difficult to detect at low concentrations. To increase Raman signals one often uses UV excitation. We find that nano-needels can be grown from droplets of aqueous histidine on silica substrates. We observe intense and narrow vibrational Raman bands of nanoscale needle shaped histidine using visible excitation sources. Scanning electron microscopy reveals a combined structure of folded flat leaves and regions with a dense array of needles with diameters in the 50-100nm range. The needles are formed mainly due to substrate heating, convection is found to be less important in their formation. The pH dependent measurements show their influence on the detected spectral bands. The observed spectral bands are compared with density functional calculations which taking into account of the peptide bond formation with neighbouring molecules. The C-H stretching at high frequency are identified and attributed to the imidazole ring and the back bone. C-H stretching mode of the back bone is found to be strongly conformational dependent. We show that the growth method can be generalized to other amino acids like glycine, valine or histamine. Keywords: nanoscale needles, histidine, vibrational bands, Raman spectroscopy, scanning electron microscopy 1 INTRODUCTION Raman spectroscopy is a powerful non-invasive tool to obtain information on structure, function and reactivity of biological targets. The vibrational spectra of amino acids in peptides and proteins depend sensitively on organisation and interaction with its environment. Histidine takes part in many biological processes such as the coordination of metal ions [1-2] or acid-base reactions and is a common residue in organisms (up to 3%). The imidazole side chain is often found as a coordinating ligand of metal ions like copper or zinc in metalloproteins. For example histidine is a very important amino acid in Alzheimer’s disease because it binds copper in the amyloids plaques [3] thus contributes to the production of reactive oxygen species and finally to the death of neurons. Raman detection of histidine at relatively low aqueous concentration (mM range) is challenging using visible laser excitation. UV Raman spectroscopy has been increasingly used to enhance the Raman response of proteins by resonance excitation [4]. Recently structural and vibrational properties of L-histidine oxalate crystals have been investigated [5-6]. We show here, using Raman spectroscopy with visible laser excitation, that intense and narrow Raman signals can be observed from histidine nano-needles grown on SiO 2 . Scanning electron microscopy reveals a structure of folded flat leaves and nano-needles in different orientations. We compare the experimental Raman spectra with ab-initio calculations taking into account two histidine molecules to include effects of neighboring molecules. 2 EXPERIMENTAL Histidine (Sigma Aldrich) was first dissolved in 1ml of de-ionised water at a concentration of 30mM and then single droplets (15μl) were deposited on SiO 2 plates. The droplet was dried under a 40W power lamp at a distance of 30cm or on a heating plate at variable temperatures (50°C - 80°C). Raman spectra were recorded (Dilor XY 2400 spectrometer) using 488nm excitation (Spectra Physics 2017 argon ion laser, 20mW). The solubilized histidine droplet forms when dried a deposit in the form of a ring. The ring consists of a thin film contracted into leaflets and regions with a high concentration of needles with diameters in the 50nm range and several micrometers long. Fig. 1.a shows the leaflet with a homogeneous thickness and a region with a dense array of nano-needles (Fig. 1.b). Fig. 1.c shows the nano- needels at high concentration. The Raman spectra of the histidine nano structures differ from histidine in aqueous solution and are also different from macroscopic histidine crystals or histidine in NSTI-Nanotech 2007, www.nsti.org, ISBN 1420061836 Vol. 2, 2007 37