Molecular weight, spectrophotometric determination

Results and Discussion. The 2-ethyl polyaniline concentration in the silica gel film was determined by constructing a Beer s law calibration curve from solutions of known concentration. Assuming an average molecular weight of 5000, the 2-Et PANi concentration in the silica gel was found to be 9.6 x 10 4 M. The refractive indices of CS2 and 2-Et PANi SiC>2 were estimated to be 1.6 and 1.4 at 1.06 im, respectively. The emeraldine base doped silica gel was found to have low losses due to scatter, and exhibited good transparency at 1.06 im. Spectrophotometric measurements at 1.06 fim yielded absorption coefficients of 0.1 cm-1 (> 99% T over 1 mm pathlength) for the CS2 reference and 4 cm 1 (96% T over 1 mm pathlength) for the 2-Et PANi doped silica film. 

Figure 8. Renaturation of low molecular weight urokinase observed in samples collected from continuous flow refolding. UK was injected in 9.3 M urea into 2.0 M urea, 20 mM Bis-Tris, pH 7.8 buffer with a gradient from 2.5 mM reduced glut-athione (GSH) to 2.5 mM oxidized glutathione (GSSG). Upper panel Data for GSSG concen-tration are from direct absorbance measurements at the secondary UV detector data for GSH were determined by DTNB titration of collected fractions. Lower panel urokinase activity in collected fractions measured by spectrophotometric assay using S-2444 (12) in a Molecular Devices titerplate reader.

The conventional spectrophotometric techniques (UV-visible, IR, NMR) are of limited use in structural determination of melanins. Consequently, an array of degradation techniques that yield easily identifiable, low molecular weight fragments has been developed. Many of these methods were developed in the 1950s and 1960s and are documented by Nicolaus (7). The degradation methods are classified as reductive, oxidative, pyrolytic, and photochemical, and recent findings are described below. 

One practical alternative to the radiometric methods is provided by murexide, which forms a complex with free calcium (46). This complex can be determined spectrophotometrically and used for the calculation of unbound calcium. Bound calcium calculated by difference, enables one to construct binding plots. This method has two advantages in special cases it is very rapid and therefore can be used for labile binding proteins, and since no dialysis or ultrafiltration is needed it can be used with low molecular weight peptides. About 1 mg of protein is required for the murexide method. 

Molecular weight determination by the method of partial substitutional and spectrophotometric measurements of picrates gave values of about 1,360 for the hydrochlorides of both colistin A and B . Quantitative amino acid analysis showed the presence of six moles of L-a,y-diaminobutyric acid, two moles of L-threonine, one mole of L-leucine and one mole of D-leucine in both colistin A and By partial acid hydrolysis as well as by enzymatic... 

If an unknown compound can be treated to form a derivative in which a chromo-phore of known e value is incorporated, the molar concentration of the chromophore may be obtained spectrophotometrically. This provides a simple method for determining molecular weights. Although the molar absorptivity of the absorption band remains constant in all the derivatives, the absorbance A) will depend upon the molar concentration and hence on the molecular weight of the molecule of interest. The molecular weight (M) may be determined spectrophotometrically from the relation... 

Picric acid and the picrate salts of amines absorb at 380 nm with a molar absorptivity of 13,400. An accuracy of +2% was obtained for the spectrophotometric determination of molecular weights of amines [30]. Molecular-weight determinations have been reported of sugars from the absorption spectra of their osazones... 

Variations in drug concentration from 0.1 to I percent, and chitosan membrane thicknesses of 40 to 150 micron were used in the study of the permeabilities of isoniazid and amitriptyline hydrochloride. No changes in release rate (measured by ultraviolet spectrophotometric determination) into water were observed with either concentration or membrane thiekness, but rates for the two drugs were very different and were related to the respective molecular weights of the materials. 17 refs. BRAZIL... 

This silation spectrophotometric method is capable of determining down to 0.1% of hydroxy groups in a polyethylene glycol (PEG) with a molecular weight of 3000. 

Sweetser and Bricker determined a variety of cations by spectrophotometric titrations with EDTA. Bruckenstein determined water in acetic acid by an ultraviolet photometric titration. C4 and higher molecular-weight olefins in automobile exhausts have been determined by spectrophotometric titration, using the high molar absorptivity of free tribromide ion at 290 m/x. 

Spectrophotometric determinations of molecular weight have been made by Paquot el al. Silverman, and Houk... 

Sloneker, J. H., J. B. Mooberry, P. R. Schmidt, J. E. Pittsley, P. R. Watson, A. Jeanes, Spectrophotometric determination of high molecular weight quaternary ammonium cations with picric acid application to residual amounts in polysaccharides. Anal. Chem., 1965, 37, 243-246. 

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