The influence of irradiation by longitudinally polarized protons on the differential decomposition of amino acid is investigated in a test of the Vester Ulbricht hypothesis that L-amino acids were produced in preference to the D isomers on the primitive earth due to the effects of parity violation in beta decay. Samples of DL-leucine were irradiated with protons of both positive and negative longitudinal polarization, then analyzed by gas chromatography. Despite advantages of higher polarization, lower velocity and higher ionization density of protons with respect to electrons, proton irradation is found to lead to no detectable asymmetries in DL-leucine degradation, even at 50% gross degradation.