Magnetic field dependent long-lived spin states in amino acids and dipeptides

Magnetic field dependence of long-lived spin states (LLSs) of the beta-CH2 protons of aromatic amino acids was studied. LLSs are spin states, which are immune to dipolar relaxation, thus having lifetimes far exceeding the longitudinal relaxation times; the simplest example of an LLS is given by the singlet state of two coupled spins. LLSs were created by means of the photo-chemically induced dynamic nuclear polarization technique. The systems studied were amino acids, histidine and tyrosine, with different isotopomers. For labeled amino acids with the alpha-CH and aromatic protons substituted by deuterium at low fields the LLS lifetime, T-LLS, for the beta-CH2 protons was more than 40 times longer than the T-1-relaxation time. Upon increasing the number of protons the ratio T-LLS/T-1 was reduced; however, even in the fully protonated amino acids it was about 10; that is, the long-lived mode was still preserved in the system. In addition, the effect of paramagnetic impurities on spin relaxation was studied; field dependencies of T-1 and T-LLS were measured. LLSs were also formed in tyrosine-containing dyads; a T-LLS/T-1 ratio of similar to 7 was found, usable for extending the spin polarization lifetime in such systems.