Optical rotatory dispersion , solvent

The UV spectrum of a complex conjugated molecule is usually observed to consist of a few broad band systems, often with fine structure, which may be sharpened up in non-polar solvents. Such a spectrum can often be shown to be more complex than it superficially appears, by investigation of the magnetic circular dichroism (MCD) spectrum, or by introduction of dissymmetry and running the optical rotatory dispersion (ORD) or circular dichroism (CD) spectrum. These techniques will frequently separate and distinguish overlapping bands of different symmetry properties <71PMH(3)397). 

Werbowyj and Gray (79) examined the relationships between the cholesteric pitch and optical properties of HPC in water, CH3COOH and CH3OH. The reciproc pitch varied as the third power of the HPC concentration. Optical rotatory dispersion results show HPC has a right-handed superhelicoidal structure regardless of structure. As will be discussea below, a change in solvent can reverse the handedness of other cellulose derivatives. 

There are many more solvent effects on spectroscopic quantities, that cannot be even briefly discussed here, and more specialized works on solvent effects should be consulted. These solvent effects include effects on the line shape and particularly line width of the nuclear magnetic resonance signals and their spin-spin coupling constants, solvent effects on electron spin resonance (ESR) spectra, on circular dichroism (CD) and optical rotatory dispersion (ORD), on vibrational line shapes in both the infrared and the UV/visible spectral ranges, among others. 

Thermal and charge induced random coil to a-helix transitions of polyll-glutamic acid) (PGA) are measured by optical rotatory dispersion in various solvents. The data of PGA in 0.1 M NaCI are analyzed by the Zimm-Rice theory. The initiation parameter, o, of the Zimm-Rice theory is given by a value of 5 ( 1) x 10 3. Random copolymers of L-glutamic acid and L-alanine containing 10, 30, and 40 molar percents of alanyl residue are analyzed as well. 

Methoxyquebrachamine (VIII) has the rotation, —103° in dioxane, compared with — 111° found for quebrachamine (I) in the same solvent, and the optical rotatory dispersion (ORD) curves are very similar. It may therefore be deduced that (— )-quebrachamine has the same absolute configuration at position 5 as does aspidospermine. 

Palytoxin is a white, amorphous, hydroscopic solid that has not yet been crystallized. It is insoluble in nonpolar solvents such as chlorophorm, ether, and acetone sparingly soluble in methanol and ethanol and soluble in pyridine, dimethyl sulfoxide, and water. The partition coefficient for the distribution of palytoxin between 1-butanol and water is 0.21 at 25°C based on comparison of the absorbance at 263 nm for the two layers. In aqueous solutions, palytoxin foams on agitation, like a steroidal saponin, probably because of its amphipathic nature. The toxin shows no definite melting point and is resistant to heat but chars at 300°C. It is an optically active compound, having a specific rotation of -i-26° 2° in water. The optical rotatory dispersion curve of palytoxin exhibits a positive Cotton effect with [a]25o being -i-700° and [a]2,j being +600° (Moore and Scheuer 1971 Tan and Lau 2000). 

More recently, Doty and co-workers (Doty, 1959) have found that a large number of globular proteins are directly soluble in the solvent 2-chloroethanol. While the apparently unique solubilizing power of this substance is probably attributable to the HCl present in the unstable solvent, some very significant measurements of the optical rotatory dispersions of proteins in a pure weakly protic nonaqueous solvent were made as a result of this discovery. In all cases studied, proteins exhibited larger, often substantially larger, values of —ho in 2-chloroethanol than in HjO (Table VII). These effects are reversible with change of solvent. 

A solvent which has been foimd to be of great interest in connection with protein conformation studies is ethylene glycol. Sage and Singer (1958, 1962) have investigated in some detail the properties of RNase in pure ethylene glycol, containing added neutral electrolyte. They examined the ultraviolet absorption spectrum, the ionization behavior of the tyrosine residues by spectrophotometric titration experiments, and the optical rotatory dispersion of the system. 

Figure 12. Optical rotatory dispersion dependence on cellulose concentration solvent composition, 28.0/72.0 DP, 210 storage time at 25 C, 30 days.

The initial decrease in optical rotation found in aqueous solutions of /3-lactoglobulin and ovalbumin is not, however, sufficient to differentiate globular proteins from simpler synthetic polypeptides in their transition behavior, for neither ribonuclease nor human serum albumin appear to exhibit it. The specific rotation of ribonuclease in water-2-chloroethanol mixtures becomes steadily less levorotatory as the proportion of nonpolar solvent increases (Weber and Tanford, 1959). In the case of human serum albumin Bresler (1958) and Bresler el al. (1959) find that only progre.ssive increases in specific rotation occur as the concentration of 2-chloroethanol is increased and that this change is accompanied by a steady rise in viscosity and the corresponding axial ratios characteristic of the formation of rodlike particles. If these proteins do have some initial helical content in water, as can be argued from their optical rotatory dispersion, then it appears that hydrophobic forces are not required for the stability of these regions. 

Since Werner s pioneering work on optical activity in complex inorganic compounds there have been many important developments in the field. One of the more interesting of these is known as the Pfeiffer effect which is a change in the optical rotation of a solution of an optically active substance e,g, ammonium d-a-bromo-camphor-T-sulfonate) upon the addition of solutions of racemic mixtures of certain coordination compounds (e,g, D,L-[Zn o-phen)z](NOz)2, where o-phen = ortho-phenan-throline). Not all combinations of complexes, optically active environments and solvents show the effect, however, and this work attempts to apply optical rotatory dispersion techniques to the problem, as well as to determine whether solvents other than water may be used without quenching the effect. Further, the question of whether systems containing metal ions, ligands, and optically active environments other than those already used will show the effect has been studied also,... 

In this work the authors have attempted to expand the scope of the Pfeiffer effect to other systems and solvents and to determine unambiguously the source of the effect. To this end they applied optical rotatory dispersion techniques as a tool in their study. 

In aqueous solution potassium tris(oxalato)chromate(III) racemizes faster than the analogous cobalt compound but more slowly than the corresponding iron compound. Many studies of the racemization rate have been made under various conditions. - The rate is dependent on the nature of the solvent it is lowered by addition of organic solvents to aqueous solutions. - Both enantiomers have been the subject of numerous optical rotatory dispersion studies.No exchange has been shown to occur between oxalate ion containing radioactive carbon and the tris(oxalato)-chromate(III) ion. ... 

ORD data, however, can be obtained for a number of systems of interest where little or no data are possible by circular dichroism, or when the absorption bands cannot be reached due to the high absorbance of a solvent or because the absorption bands of interest are beyond the usual 180-190 nm limit of solution studies. Also, most of the optical rotation data in the literature prior to the mid-1960s are reported in terms of optical rotatory dispersion. Additionally, when correcting the distortions in the CD spectra of biomembranes (see below), it is convenient to have the ORD curve of what is referred to as the pseudoreference state. 

---