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Conformation by electrochemistry
Using steady-state voltammetry, the ray data. The charge spacing for the coil diffusion-limited current (»D) for the reform is 0.89 nm, which is somewhat lower duction of Tl+ in an aqueous solution of Electrochemistry is rarely thought of as a than the 1.0 nm values from extended mo20 mM Na„K-car was determined at varitechnique for determining structures of lecular models. (J. Am. Chem. Soc. 19199 biomolecules. Nevertheless, because it is a ous temperatures. A change in iD over the 121,1617-18) 10-25 °C range, much simpler and less expensive method indicative of the than traditional structure-determining approaches such as NMR, MS, and X-ray dif- conformation change for the fraction, electrochemistry is certainly worth developing for conformational analy- polysaccharide, was observed. sis. This is exacdy what Janet G. OsterPlugging the young of North Carolina State University measured values and Malgorzata Ciszkowska of Brooklyn into the theoretiCollege are doing, using a theoretical model that relates measured diffusion coef- cal model a value of 0.38 ficients for counterions of highly charged for the uniform polyions to charge-spacing values. In this work, the researchers investigated spacing between the polyion the anionic polysaccharide K-carrageenan (K-car), which undergoes a transition from a charges in the double helix coil to a double helix conformation as the temperature decreases. The polysaccharide form was determined which consists of one charge per repeating disacNormalized reduction current of 0.3 mM TPwith K-car (O, A) and agrees with Xcharide unit. The probe ion is TV. circular dichroism values for the polysaccharide (*) versus temperature.
A dissection of cross section Because understanding the structural similarities between gas-phase and native conformations of proteins will aid in the study of protein folding, researchers have recently determined the cross sections for some protein and peptide ion systems. In a new paper, David E. Clemmer and colleagues at Indiana University report the cross sections for a series of
113 related peptides and describe the correlations between amino acid composition and cross section. The researchers quantify these relationships by determining the average intrinsic contributions to size for individual amino acid residues (referred to as "intrinsic size parameters"). To collect the data, the researchers used an ion mobility/iime-of-flight method (Anal. Chem. 1997,69, 728 A-735 A) that allows the cross sections and m/z ratios for mixtures of ions to be measured in a single experiment. Common proteins such as cytochrome c, myoglobin, and albumin were digested, resulting in peptides of die form [(Xxx) Lys + H]+ where Xxx is naturally offl 11"-
Intrinsic size parameters for individual amino acids from a series of peptides.
308 A
Analytical Chemistry News & Features, May 1, 1999
ring amino acid (except Lys Arg His orCys) and » = 4-9 The peptides were electrosoraved into the eas ohase and analyzed The intrinsic
size parameters were calculated by using linear regression to solve a system of equations that related the frequency of occurrence of each amino acid to the reduced cross section. Clemmer and colleagues found that the cross sections depended mainly on die chemical natures and physical sizes of the amino acid side chains. The largest contributions came from the nonpolar residues—Ala, Val, He, and Leu—while polar groups such as Asp, Glu, and Asn contributed less. Large polar aromatic groups, such as Trp and Tyr, contributed little. The size of the side chains was also important an increase in die length of aliphatic side chains led to a larger contribution to the cross section. The researchers report that theoretical values calculated on the basis of the intrinsic size parameters were within 2% of the experimental values for 90% of the peptides, adding that it seems likely that many of the deviations arose from differences in structure, which depend on sequence. Thus, they conclude, amino acid sequence does play a role in cross-section values, albeit a smaller one than chemical nature and physical size. (J. Phys. Chem. m 1919,103, 1203-07)
