UV resonance Raman determination of molecular mechanism of poly(N-isopropylacrylamide) volume phase transition.

Poly(N-isopropylacrylamide) (PNIPAM) is the premier example of a macromolecule that undergoes a hydrophobic collapse when heated above its lower critical solution temperature (LCST). Here we utilize dynamic light scattering, H-NMR, and steady-state and time-resolved UVRR measurements to determine the molecular mechanism of PNIPAM's hydrophobic collapse. Our steady-state results indicate that in the collapsed state the amide bonds of PNIPAM do not engage in interamide hydrogen bonding, but are hydrogen bonded to water molecules. At low temperatures, the amide bonds of PNIPAM are predominantly fully water hydrogen bonded, whereas, in the collapsed state one of the two normal CO hydrogen bonds is lost. The NH-water hydrogen bonding, however, remains unperturbed by the PNIPAM collapse. Our kinetic results indicate a monoexponential collapse with tau approximately 360 (+/-85) ns. The collapse rate indicates a persistence length of n approximately 10. At lengths shorter than the persistence length the polymer acts as an elastic rod, whereas at lengths longer than the persistence length the polymer backbone conformation forms a random coil. On the basis of these results, we propose the following mechanism for the PNIPAM volume phase transition. At low temperatures PNIPAM adopts an extended, water-exposed conformation that is stabilized by favorable NIPAM-water solvation shell interactions which stabilize large clusters of water molecules. As the temperature increases an increasing entropic penalty occurs for the water molecules situated at the surface of the hydrophobic isopropyl groups. A cooperative transition occurs where hydrophobic collapse minimizes the exposed hydrophobic surface area. The polymer structural change forces the amide carbonyl and N-H to invaginate and the water clusters cease to be stabilized and are expelled. In this compact state, PNIPAM forms small hydrophobic nanopockets where the (i, i + 3) isopropyl groups make hydrophobic contacts. A persistent length of n approximately 10 suggests a cooperative collapse where hydrophobic interactions between adjacent hydrophobic pockets stabilize the collapsed PNIPAM.

UV raman examination of alpha-helical peptide water hydrogen bonding.

UV resonance Raman spectra (UVRS) of an alpha-helical, 21 residue, mainly Ala peptide (AP) in the dehydrated solid state were compared to those in aqueous solution at different temperatures. The UVRS amide band frequencies of a dehydrated solid alpha-helix peptide show frequency shifts compared to those in aqueous solution due to the loss of amide backbone hydrogen bonding to water; the amide II and amide III bands of the solid alpha-helix downshift, while the amide I band upshifts. The shifts are identical in direction but smaller than those that occur for alpha-helices in aqueous solution as the temperature increases; water hydrogen bonding strengths decrease as the temperature increases. The UV Raman amide band frequency shifts can be used to monitor alpha-helix hydrogen bonding.

cis-Dioxocyclam.

Molecules of 1,4,8,11-tetraazacyclotetradecane-5,7-dione, or cis-dioxocyclam, C(10)H(20)N(4)O(2), lie across mirror planes in space group Cmca; the crystal structure reveals interleaved columns of cis-dioxocyclam molecules along the 2(1) screw axis parallel to the crystallographic b axis. The columns are interconnected in a chain-like arrangement by an amido hydrogen-bonding network (N* * *O = 2.816 A) and an amino hydrogen-bonding network (N* * *N = 3.193 A). The intracolumn spacing is 9.02 A.