A bead model to determine the electrophoretic mobilities and translational diffusion constants of weakly charged peptides is developed that is based on a approximate structural model of peptides and is also grounded in electrohydrodynamic theory. A peptide made up of X amino acids is modeled as N=2X beads with 2 beads representing each amino acid in the chain. For the two beads representing a particular amino acid in a peptide, the radius of one bead is set to one-half the nearest neighbor Calpha-Calpha distance, and the radius of the other bead is chosen on the basis of the diffusion constant of the free amino acid. Peptide conformations, which are defined by a set of psi-phi dihedral angles, are randomly generated by using the transformation matrix approach of Flory (Flory, P. Statistical Mechanics of Chain Molecules; John Wiley: New York, 1969) and rejecting conformations which result in bead overlap. The mobility and diffusion constants are computed for each conformation and at least 100 independent conformations are examined for each peptide. In general, the mobility is found to depend only weakly on peptide conformation. Model and experimental mobilities are compared by examining the data of Janini and co-workers (Janini, G.; et al. J. Chromatogr. 1999, 848, 417-433). A total of 58 peptides consisting of from 2 to 39 amino acids are considered. The average relative error between experimental and model mobilities is found to be 1.0% and the rms relative error 7.7%. In specific cases, the discrepancy can be substantial and possible reasons for this are discussed. It should be emphasized that the input parameters of the peptide model are totally independent of experimental mobilities. It is hoped that the peptide model developed here will be useful in the prediction of peptide mobility as well as in using peptide mobilities to extract information about peptide structure, conformation, and charge. Finally, we show how simultaneous measurements of translational diffusion and mobility can be used to estimate peptide charge.

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although examples of such effects in biradicals are rare.11 Alkyl radicals display distinct nucleophilic properties.12 In particular, methyl radicals abstract hydrogen from HC1 much faster than from hydrocarbons.12 If such behavior can be expected of the 7-alkyl radical site in the type II biradical, we can conclude that the proton character of the hydroxyl hydrogen is responsible for the large proportion of disproportionation. The transition state for disproportionation is probably stabilized by a charge-separated resonance form as shown.

Whitesides et al. / Reaction of Substituted Aryl Bromides with Magnesium23 1 clohexane of s for CH3Br-. and 3 X lo-* s for CH3CI-. (P. P. lnfelta and R. H. Schuler, J. phys. Chem., 76,987 (1972)). If these data are Correct, alkyl bromides and alkyl chlorides might react by different initial steps.(14) Radical chain reactions are usually detected by rate inhibitions resulting from addition of free-radical scavengers. Any reactions involving freeradical chains in this system must be fast to be compatible with the observation that the overall reaction is mass transport limited. Although it is possible to find a limited number of radical scavengers which are compatible with a reacting mixture of alkyl halide and magnesium, and although these scavengers produce no obvious effect on the reaction rate, it is difficult to guarantee that these scavengers would be effective in trapping the very short-lived radicals implied by the high rates observed.(15) Clear derivations of equations relating current density to stirring rate and kinematic viscosity for a rotating disk electrode are given by J. Albery. Abstract:Hammett constants for reaction of substituted aryl bromides with magnesium in several solvents follow: diethyl ether ( p = I , I ) ; di-n-butyl ether ( p = I ,9); 3: 1 v:v di-n-butyl ether-cyclohexane ( p = 2.0); tetrahydrofuran ( p N 0 ) ; 3: 1 v:v tetrahydrofuran-hexamethylphosphoramide ( p N 0); diethyl ether containing 0.8 M cyclopentylmagnesium bromide ( p N 0). In the polar solvents, reaction appears to be mass transport limited. The rate of reaction of aryl iodides with magnesium in diethyl ether is probably also mass transport limited ( p = 0). The rate of reaction of chlorobenzene with magnesium ( p N 2.0) is ca. IO4 slower than that of bromobenzene in diethyl ether, and is not mass transport limited. The reaction of aryl bromides with tri-n-butyltin hydride (AIBN, hu) has p = 1.3. Analysis of these values of p narrows the choice for the rate-determining step in the reaction of bromobenzene with magnesium in diethyl ether to one of three limiting possibilities: electron transfer from the metal to the aryl bromide (3), aryl radical formation by bromine abstraction from the aryl bromide by the metal ( 6 ) . or, less probably, insertion of a magnesium atom into the carbon-bromine bond (7).

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