292 Bio~imica etBiophvsica Acta, 719 (1982) 292 298 Elsevier Biomedical Press BBA21262 NMR FIELD-CYCLING RELAXATION SPECTROSCOPY OF BOVINE SERUM ALBUMIN, MUSCLE TISSUE, MICROCOCCUS L UTEUS AND YEAST 14N i H-QUADRUPOLE DIPS FRANZ WINTER and RAINER KIMMICH * Sektion Kernresonanzspektroskopie, Universiti~t Ulm, Postfach 4 066, D-7900 Ulm (F.R.G.) (Received April 5th, 1982) Key words: NMR," Albumin; Muscle," Yeast," (M. luteus) The frequency dependence of the proton spin lattice relaxation time of bovine serum albumin, muscle tissue, Micrococcus luteus and yeast has been measured by the aid of the field-cycling technique. In all systems 14NIH-quadrupole dips have been observed. The conclusion is that amide groups are the dominating relaxation centers up to approx. 10 7 az. This finding can be understood by the fact that protein backbone fluctuations and, if possible, tumbling of the whole molecule rather than side group motions are the relevant mechanisms in this frequency range. A proton relaxation scheme for cells and tissue is presented. Introduction In the last few years much attention has been paid to proton relaxation of biological systems. It was found that the proton spin lattice relaxation time (Tt) is remarkably sensitive to system proper- ties even under low resolution conditions. Thus, different protein solutions show different relaxa- tion behaviours [1-3]. Malignant tissue relaxes more slowly than normal tissue [4,5]. The cell partition cycle reveals itself in a cyclic variation of the proton spin lattice relaxation times [6]. The question is now, what do we measure by (unresolved) proton relaxation in this type of sys- tem? Clearly, even if the proton spectra could be completely resolved, the answer would not be easier, because spin diffusion (or cross-relaxation) at least partially 'mixes' the relaxation features of the diverse molecular groups [1,7-9]. * To whom correspondence should be addressed. 0000-0000/82/0000-0000/~a02.75 © 1982 Elsevier Biomedical Press In a previous paper [10] we suggested that ~4NIH-groups act as relaxation sinks in a frequency range where side-group motions are less effective. This mechanism could be verified by the detection of ~4N~H-quadrupole dips which have been mea- sured by the aid of proton field-cycling relaxation spectroscopy [11]. Preliminary reports concerning proteins and muscle tissue have been published in Refs. 12-14. In this paper we present additional experimental data and derive a relaxation scheme valid in the low-frequency regime. Theoretical background A two-spin ensemble consisting of I = 1/2, S = 1 spin pairs has /-spin relaxation features which strongly deviate from the behaviour described by the Bloembergen-Purcell-Pound formula [15]. This holds especially for the so-called low-field case. In a previous paper [14] we derived and discussed appropriate expressions. For instance, unlimited rotational diffusion leads to the following expres- sion for the spin lattice relaxation of the/-spins in