Polar groups, angles between

When the filming amine condenses, the hydrophilic polar radical of the molecule (the head) adsorbs onto the metal surface and the hydrophobic, long chain (the tail) is directed at a 90° angle of inclination away from the metal surface. Provided the feed rate is adequate, the critical concentration is eventually reached and a continuous monomolecular surface film is formed. At this stage, the physical size of the interstices between the polar groups is smaller than the molecules of water, carbon dioxide, or oxygen, and these molecules are thus physically prevented from reaching the metal surface. 

The calculated Euler angles (a = 50°, /3 = 60°, and y = 40°), which determine the relative orientation between the principal-axis system of the rotational diffusion tensor and that of the moment of inertia tensor, indicate a significant shift between the two tensors. This result is expected because of the fact that molecule 31 contains a number of polar groups and hydrogen-bonding centers, leading to strong intermolecular interactions. 

This technique was employed to monitor the B —> A transition of DNA as a function of the relative humidity (Pilet and Brahms, 1973 Pohle et al., 1984). The investigated bands are those which reflect the vibrations of the phosphate groups. As shown by Fig. 4.7-3, which presents the polarized infrared spectra of a salmon sperm DNA hydrated film with 93% RH (top, B form) and 58% RH (bottom, A form), the dichroism of the two phosphate bands changes. The B form of the antisymmetric PO2 stretching vibration around 1230 cm is non-dichroic, while that of the A form is perpendicular. The B form of the symmetric PO2 stretching vibration around 1090 cm is perpendicular, while that of the A form is parallel. A simple computation, for instance for the latter band, shows that the value of the angle between the transition dipole moment of this vibration and the double helical axis varies between 68 ° (B form) and 49 ° (A form). This parameter is an extremely sensitive indicator of a B A transition and may also be employed to show the inhibition of a B —> A transition by various classes of molecules, such as proteins (Liquier et al., 1977 Taillandier et al., 1979) or drugs (Fritzsche and Rupprecht, 1990). 

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