Coloration of silver-stained protein bands in polyacrylamide gels is caused by light scattering from silver grains of characteristic sizes.

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Abstract
This study investigates the physical basis of color effects in the detection of proteins in polyarylamide gels by silver staining. Specifically, the hypothesis that different colors may correlate with the development of silver grains of characteristic sizes was investigated by electron microscopy. Protein bands that stained brown, yellow, and blue were excised from stained gels and prepared for electron microscopy by thin-sectioning. In each case, the size distributions of globular silver grains were determined directly from the electron micrographs. We found that blue bands have larger silver grains (with diameters of 40-100 nm) than yellow (21-39 nm) or brown bands (17-35 nm). On the basis of these and other observations, a general mechanisms is proposed whereby chemical specificity of electrophoretically separated proteins is expressed in color-specific silver staining.