We have determined the near-ultraviolet circular dichroic spectrum (235-315 nm) and the ultraviolet difference absorption spectrum of ribonuclease A (RNase A) and its peptic derivative, des-(121-124)-RNase A (RNase P), which is missing the four C-terminal residues and retains no more than a few per cent of the enzymic activity of RNase A. The circular dichroic spectra of both proteins were resolved into gaussian parameters and the rotational strengths of the resolved bands (due mainly to tyrosines and disulfides) were found to be very similar in the two proteins. Also, the denaturation difference spectrum was almost identical, thus indicating that in each native protein the same number of tyrosyl groups are inaccessible to solvent. The circular dichroic spectrum of RNase P between 195 and 235 nm was also obtained and compared to the known spectrum of RNase A. The gen-
The adsorption of protein and non-protein amino acids by Na-montmorillonite was studied at pH 3, 7, and 10, in order to determine whether clays could have played a part in selection of protein over non-protein amino acids in prebiotic times. Five pairs of amino acids, containing two to six carbons, were used at a concentration equal to 100% cation exchange capacity of the clay in adsorption experiments. The following pairs of protein and non-protein amino acids were used: glycine and sarcosine, alpha-alanine and beta-alanine, alpha-aminobutyric acid and gamma-aminobutyric acid, valine and norvaline, L-isoleucine and D-alloisoleucine. No selective adsorption of protein amino acids occurred at varying hydrogen ion concentrations. The one difference observed in the adsorption of amino acids in the mixtures was a three- and four-fold greater adsorption of beta- and gamma-amino acids, respectively, than their alpha-amino acid counterparts under acidic and neutral conditions. Strong and weak adsorption of amino acids on the clay were correlated with mechanisms such as cation exchange and hydrogen bonding. The results of this research are significant to understanding the role of clay in chemical evolution because they do not support the role of preferential adsorption of protein over non-protein amino acids by clays.
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