Spin−Lattice Relaxation of the Tryptophan Triplet State Varies with Its Protein Environment

Abstract

Spin−lattice relaxation (SLR) has been studied at 1.2 K in the photoexcited triplet state of tryptophan (Trp) in a number of protein sites in zero applied magnetic field. We find that there is a distribution of SLR rate constants in every case that indicates the partitioning into two populations, ns0 and nf0, with differing behavior. ns0 undergoes slow SLR with rate constants in the range found in crystalline matrixes, while the SLR of nf0 is at least 1−2 orders of magnitude faster. The former population has considerable spin alignment during optical pumping, decays nonexponentially, and gives rise to optically detected magnetic resonance (ODMR) responses at 1.2 K. The latter has negligible spin alignment, decays largely as a single exponential, and is ODMR silent. The rapid SLR of nf0 is attributed to coupling with disorder (tunneling) modes that modulate the electron−electron dipolar coupling, and whose efficiency varies with protein site. The less efficient SLR of ns0 is the result of direct interactio...