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, n(s)(0) and n(f)(0), with differing behavior. n(s)(0) undergoes slow SLR with rate constants in the range found in crystalline matrixes, while the SLR of n(f)(0) 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 n(f)(0) 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 n(s)(0) is the result of direct interaction with lattice phonons. We find no correlation of the distribution of populations with the heterogeneity of the protein site as determined by ODMR bandwidth. It is suggested that more efficient disorder mode-induced SLR is associated with decreased rigidity of the local environment.