Archibald method

Three experimental methods are used to evaluate the term dlnc/dr in Eq. (11.4) the Archibald method, the van Holde and Baldwin method, and the Yphantis method. All are designed to determine the molecular weight of polymers (particularly biological polymers). 

At the beginning of the run, the material enters the column of solution at the meniscus and the bottom. It soon becomes saturated, so that the material can no longer transport to the meniscus and bottom. A simple criterion is the plot of the experimental data in the form of (l/rc)(dc/dr) versus r. If the plot produces a horizontal line, then equilibrium is attained. 

Archibald suggested that we need focus only on the top and the bottom of the cell during the process of equilibrium sedimentation to determine molecular weight we do not have to wait for equilibrium to be reached. Hence, the method is also called approach to equilibrium. Equation (11.4) can be rearranged for 

The value of cq is obtained from a synthetic boundary cell run, which we describe later. The term Cp refers to the position at the plateau region. 

Let us consider the equilibrium at meniscus. To determine the value of M, we need the values of four paramenters (in addition to other constants) (9c/0r), v. Cm, and cq. Using the Schheren system, a typical graph in the photographic plate is obtained in the following form  

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U -OPCp -ONSu Mn by Archibald method X-ray diffraction studies CD measurements, IR... 

Thus BLs can be obtained from the limiting slope of a plot of 1/Mw pp° vs. Co. The Archibald method is restricted to sector-shaped centerpieces. 

Evaluation of Mweq. Sedimentation Equilibrium Experiments and the Archibald Method. At constant temperature the condition for sedimentation equilibrium is that the total potential jZi for each associating species i (i = A, B, or AnBw) be constant at every radial position r in the solution column of the ultracentrifuge thus... 

Equations 15 and 16 were first obtained by Kegeles and his co-workers (16, 20, 21) and they are valid only for cells with sector-shaped center-pieces. Because of convective disturbances in the solution column of the ultracentfifuge cell during the transient state when nonsector-shaped cells are used, it is customary to do Archibald experiments in cells with sectorshaped centerpieces which avoid this problem. For details on the Archibald method—pitfalls, extrapolation to zero time—one should consult the papers of LaBar (17) and Fujita et al. (22). 

Analysis by the Archibald Method or by Sedimentation Equilibrium Experiments at Different Speeds. Instead of using Mweq here, one uses Mwa, the apparent weight-average molecular weight. For the Archibald experiment one obtains Mwa,t at rm or r6 by the application of Equations 13-16. The extrapolation of Mwa>t to zero time gives Mwo. For sedimentation equilibrium experiments at different speeds, one can evaluate Mwa by two different methods here one uses either Equations 17 or 18. For a mixed association such as A + B AB, the basic sedimentation equilibrium equation can be written as... 

Received January 17, 1972. Work supported by grants GM 15551 and GM 17611, National Institutes of Health. It is based on the MS theses submitted by Allen H. Pekar (nonideal, sedimentation equilibrium experiments and Archibald method) and by Peter J. Wan (light scattering) to the Graduate School of the Illinois Institute of Technology. 

Values kindly determined by Mr. D. M. Brown and Prof. E. L. Smith on a sample prepared in this laboratory. The values have been obtained, respectively, by the Archibald method of sedimentation equilibrium and by sedimentation-diffusion, assuming a partial specific volume of 0.721 for the protein. 

Determined from light-scattering data and the Archibald method. 

Figure 3 a) Molecular weight determination by equilibrium ultracentrifugation b) Comparison of molecular weights determined by the Archibald method and by sedimentation equilibrium. 

Molecular weight determined by ultracentrifugation (Archibald method). 

It often takes an inconveniently long time for sedimentation equilibrium to be established. In 1947 the Canadian physicist William J. Archibald developed a method based on the approach to sedimentation equilibrium. The so-called Archibald method now is used commonly by biologists and biochemists. For details of the various methods used for studying sedimentation the student is referred to Chapter 11 and to texts listed at the end of this chapter. The interested reader is also referred to a useful new technique in sedimentation studies, density gradient centrifugation (see... 

Dissolved in hot DMF and measured at room temperature before the precipitation of polymer. Determined by Archibald method on the analytical ultracentrifuge. 

The molecular weight, about 6,200, is calcrdated from the formula, and is in accord with a value of about 7,000 estimated by the Archibald method. From the results of studies of the thermolytic peptides, it was suggested that the protein has a structural feature in which the central part of the molecule is very rich in arginine and at least half of the half cystine residues are located in the more terminal parts. 

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