Investigations of the hydrophobic and hydrophilic interactions in polymer–water systems by ATR FTIR and Raman spectroscopy P. Schmidt * , J. Dybal, M. Trchova ´ Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic Available online 21 July 2006 Abstract ATR FTIR and Raman spectra of polymers containing amide groups in the main chain and in the side chain and of the amide low-molecular- weight model compounds in water media were measured. The hydrophobic and hydrophilic interactions of the dissolved compounds with the neighboring water molecules are reflected in the wavenumbers of the CH 3 stretching and of the Amide I and II vibrations. The possibility of the existence of b-sheet-like structures in polypeptides surrounded by water molecules is also discussed. # 2006 Elsevier B.V. All rights reserved. Keywords: Infrared and Raman spectra; Polymers with amide groups; Amide model compounds; Hydrophobic and hydrophilic interaction; b-Sheet 1. Introduction Polymers exhibiting the reversible phase separation defined by a lower critical temperature in aqueous solution are of the common interest. They are soluble at low temperature, but they segregate on heating above the transition temperature T t . According to their chemical nature, T t can be changed by external influence, e.g. by changes of concentration, of pH, of salt concentration or of lighting [1]. As all these materials show enormous potential for future use in different technological fields and biomedicine, a deeper understanding of their structural changes is of great importance. Until now, at IMC in Prague the following peptide polymers have been studied by Raman, IR or NMR spectra: poly(GVGVP), poly(AVGVP), [0.3(VPGEG)–0.7(VPGVG)], genetically prepared [(PGVGV) 2 –(PGEGV)–(PGVGV) 2 ] n with n = 9, 15, 75 (G holds for glycine, V for valine, P for proline, A for alanine, E for glutamic acid residue). This type of polymers acquires after crosslinking a favorable elastic modulus and a remarkable biocompatibility [2–6]. The synthetic amide-type polymers are presently also investigated, namely poly(N-isopropylacrylamide) – poly(Ni- PAA) and poly(N-isopropylmethacrylamide) – poly(NiPMAA), both of them developed as possible drug carrier systems [7–11]. The aim of the present study was to obtain more information about the structure of amide compounds in aqueous media. Because of this the infrared and Raman spectra both of polymers (poly(AVGVP), poly(NiPMAA), polyglycine (PG), D,L-polyalanine (PA)), and of the model compounds (N- methylacetamide (NMAA) and N-acetylglycine N-methyl amide (AGMA)) were investigated. Their chemical structures are shown in Fig. 1. 2. Experimental Infrared spectra were measured on FTIR spectrometers NEXUS NICOLET 870 and BRUKER IFS 55 using a horizontal ATR unit Golden Gate having a diamond internal reflection element. Raman spectra were obtained using a spectrometer BRUKER IFS 55 with Raman module FRA 106 (Nd + YAG laser) and a Renishaw in Via Raman microscope with HeNe laser excitation. For all the vibrational measurements the spectral resolution was 4 cm À1 . The investigated compounds were measured as 5–10% water solutions or in water saturated by LiBr, spectra of bulk solvents were subtracted. As an example, the measured ATR infrared and Raman spectra of 5% water solution of poly(AVGVP), of water and their differences are shown in Fig. 2. The bands in ATR infrared spectra usually show small shifts in position compared to their optically unaffected values obtained in the transmission mode. Our test measurements with NMAA have shown that these shifts do not www.elsevier.com/locate/vibspec Vibrational Spectroscopy 42 (2006) 278–283 * Corresponding author. Tel.: +420 2 20403381; fax: +420 2 35357981. E-mail address: schmidt@imc.cas.cz (P. Schmidt). 0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2006.05.005