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

Magnetic field dependence of long-lived spin states (LLSs) of the β-CH 2 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 α-CH and aromatic protons substituted by deuterium at low fields the LLS lifetime, T LLS , for the β-CH 2 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 ∼7 was found, usable for extending the spin polarization lifetime in such systems. Long-lived spin states are formed in amino acids and dipeptides at low magnetic field by photo-chemically induced dynamic nuclear polarization.