Absorption Spectrum of the Peptide Bond

In an earlier report (1) it was shown that a band characteristic of the peptide bond appeared in the ultraviolet and that values of the molar absorption coefficient (em) at 205 mp could be used to characterize this band. It was shown, for a series of proteins, that the contribution of each peptide bond (ep) to the molar absorption coefficient is fairly constant, lying between 2500 and 2800. The large value of ep for glycylglycine (3300) was attributed to an interaction of the charges on the dipole ion which was assumed to affect the peptide bond, causing an increase in Q,. As a confirmation of this it was pointed out that, when the charges were separated further, as in triglycine, the value of Q, dropped to about 2500, and, where no dipole existed, as in acetylglycine, the value of Q, dropped even further. It was thought desirable to reexamine the effect of chain length and it has been found that, contrary to the earlier conclusion, the value of ep does vary with the number of amino acid units in the peptide chain.