IRRAS studies on chain orientation in the monolayers of amino acid amphiphiles at the air-water interface depending on metal complex and hydrogen bond formation with the headgroups.
Monolayers of N-octadecanoyl-L-alanine at the air-water interface on pure water and metal ion containing subphases have been studied using polarized infrared reflection-absorption spectroscopy (IRRAS). The metal complex and hydrogen bond formation with the headgroups give rise to a change in chain order depending on metal ion in the subphase. On pure water and Ag(+)-/Pb(2+)-containing subphase, the antisymmetric CH(2) stretching band intensity [nu(a)(CH(2))] undergoes a slower increase than the symmetric one [nu(s)(CH(2))] below the Brewster angle, so the intensity ratios of nu(a)(CH(2))/nu(s)(CH(2)) are less than 1 in the cases of Ag(+) and Pb(2+). Beyond the Brewster angle, the nu(a)(CH(2)) band intensities are substantially reduced in comparison with the nu(s)(CH(2)) ones in the cases of pure water and Ag(+), but the nu(a)(CH(2)) bands still remain negative-oriented in the presence of Pb(2+). These unusual spectral features indicate that the alkyl chains take a preferential orientation with their C-C-C planes parallel to the water surface. The parallel packing of the alkyl chains results from the intermolecular hydrogen bonds C=O...H-N between the neighboring amide groups, strengthened by the metal complex of covalent interaction. On the Ca(2+)-/Cu(2+)-containing subphase, the corresponding polarized spectra display a usual behavior. The alkyl chains are roughly estimated to be inclined around 35-40 degrees from the surface normal on the assumption of chain segment orientation for the monolayers in the liquid-expanded phase. The chain conformation and tilt are closely related to the formation of intramolecular hydrogen bonds and the ionic interaction of the metal complex in the cases of Ca(2+) and Cu(2+).
Du
Xuezhong
X
Key Laboratory of Mesoscopic Chemistry (Nanjing University), Ministry of Education, and Department of Chemistry, Nanjing University, Nanjing 210093, People's Republic of China. xzdu@nju.edu.cn
Miao
Wangen
W
Liang
Yingqiu
Y
D000042:Absorption; D000388:Air; D000596:Amino Acids / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry; D002244:Carbon / Q000737:chemistry; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003300:Copper / Q000737:chemistry; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D007477:Ions; D008670:Metals / Q000737:chemistry*; D008956:Models, Chemical; D013053:Spectrophotometry / Q000379:methods; D013055:Spectrophotometry, Infrared / Q000379:methods*; D014867:Water / Q000737:chemistry*
D000596:Amino Acids; D007477:Ions; D008670:Metals; D014867:Water; D002244:Carbon; D003300:Copper; D002118:Calcium