Magnetic field dependence of long-lived spin states (LLSs) of the β-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 α-CH and aromatic protons substituted by
deuterium at low fields the LLS lifetime, TLLS, for the β-CH2 protons was more
than 40 times longer than the T1-relaxation time. Upon increasing the number
of protons the ratio TLLS/T1 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 T1 and TLLS were measured.
LLSs were also formed in tyrosine-containing dyads; a TLLS/T1 ratio of
[similar]7 was found, usable for extending the spin polarization lifetime in
such systems
