Far ultraviolet optical rotatory properties of poly-L-tryptophan. I

A block copolymer [γ-Et-DL -Glu]_m [L -Trp]_n was prepared using N-carboxy anhydrides (NCA) of L -tryptohan and γ-ethyl DL -glutamate. The block copolymer, dissolved in trifluoroethanol (TFE)–dichloroacetic acid (DCA) mixtures, exhibited a sharp change in the specific rotation at 546 mμ when the solvent composition reached 70–75% DCA content. Optical rotatory dispersion (ORD) and circular dichroism (CD) measurement were carried out in TFE solution in the spectral range 180–350 mμ. Indole side-chain chromophores were found to be optically active in the polymer. On the other hand, these groups exhibit very small optical activity in the model compound C₆H₃? CH₂? O? CO? (L -Trp)₂? O? CH₃. Indole groups therefore appear to be in a dissymmetric environment only in the polymer. From these data it was concluded that poly-L -Trp is in some type of helical conformation in TFE. Strong overlapping of CD bands from side-chain chromophores and peptides chromophores in the wavelength range 185–240 mμ does not allow definite conclusions to be drawn about the type of helical conformation which exists in poly-L -Trp in TFE solution.     

The optical rotatory properties of poly- -D-glutamic acid

Measurement of the optical rotatory dispersion spectra of poly-γ-D -glutamic acid (obtained from Bacillus anthracis) dawn to 200 nm wavelength reveal difference between the unionized and ionized froms. The profile of the unionized polyacid shows similarities to those obtained for α-helical polypeptides, although with displaced frequencies of the respective maxima and minima. It is suggested that the relative position and magnitudes of the Cotton effect are consistent with a helical structure such as proposed by Rydon (J. Chem. Soc., 1964 , 1328). The optical rotatory dispersion spectrum of the ionized from resembles those obtained from the β-chain from of α-L polypeptides. From model-building studies an extended chain similer to the β-from would seen the most reasonable structure for the ionized poly-γ-acid to adopt, since the charged groups in such a conformation would be at their maximal distances from each other. Such an ordered structure for a polymer is consistent with the hypotheses put forward in the recent literature that charged polypeptides adopt ordered rather than random-coiled conformations.     

Nanosecond optical rotatory dispersion spectroscopy: application to photolyzed hemoglobin-CO kinetics.

A standard technique for static optical rotatory dispersion (ORD) measurements is adapted to the measurement of ORD changes on a nanosecond (ns) time scale, giving approximately a million-fold improvement in time-resolution over conventional instrumentation. The technique described here is similar in principle to a technique recently developed for ns time-resolved circular dichroism (TRCD) spectroscopy, although the time-resolved optical rotatory dispersion (TRORD) technique requires fewer optical components. As with static ORD, TRORD measurements may be interpreted by empirical comparisons or may be transformed, via the Kramers-Kronig relations, to more easily interpreted TRCD spectra. TRORD can offer experimental advantages over TRCD in studying kinetic processes effecting changes in the chiral structures of biological molecules. In particular, the wider dispersion of ORD bands compared with the corresponding CD bands means that ORD information may often be obtained outside of absorption bands, a signal-to-noise advantage for multichannel measurements. Demonstration of the technique by its application to ns TRORD and the transform-calculated TRCD of carboxy-hemoglobin (Hb-CO) after laser photolysis is presented.     

Optical properties of hyaluronic acid. Ultraviolet circular dichroism and optical rotatory dispersion

The molar optical rotation at 220 nm and ellipticity values at 210 nm of both sodium hyaluronate and hyaluronic acid are greatly enhanced in comparison to the values for the monomeric units and oligosaccharides indicating a degree of preferred order. With increasing hydrogen ion concentration, there is no appreciable change in the 210 nm circular dichroic band, but the second circular dichroic band below pH 4 changes abruptly to the positive side and reaches a maximum value at pH 2·5. This positive circular dichroic band of hyaluronic acid is temperature and concentration dependent. The major change in sign and position of the second circular dichroic band of hyaluronic acid below pH 4 is attributed to the conformational change of a single polysaccharide chain or to a chain-chain interaction. The results indicate that increase in concentration or decrease in temperature and in the ionization of carboxyl group promotes the formation of ordered cross-link regions. The conformational changes found in solution have been interpreted as an order-disorder transition in the crosslink regions based on the interconversion of random coil and double helix.     

Conformations and optical rotatory properties of poly-O-acetylthreonine and poly-O-acetylallothreonine

Optically active poly-O-acetylthreonine and poly-O-acetyl-allothreonine exist in the β form in pyridine, in 2-chloruethanol–dichloromethane mixed solvent., and in the solid state. The optical rotatory dispersion of the β form of poly-O-acetyl-L -threonine in a 2-ehloroethanol–dichlomethane mixture showed a Cotton effect in the 230 mμ region. The ORD data in the visible spectral region fit, the Moffitt. equation. The absolute values of b₀, as measured in pyridine for polypeptides with a molecular weight above 17000, varied in the range 100–200°, the sign depending on the absolute configuration of the side chain.