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The measurement and interpretation of relaxation data on macromolecules present special problems. The required instrumental conditions and extensions of relaxation theory are outlined and illustrated by the analysis of 13C and 1H relaxation data on a small protein (bovine pancreatic trypsin inhibitor).

We present evidence that cytoplasmic acidosis is a cause of meristematic death in hypoxic root tips of maize and pea seedlings. Usually, leakage of acid from the vacuole is responsible for cytoplasmic acidosis. Leakage of acid, which occurs earlier during hypoxia in pea root tips than in maize root tips, appears to account for the lower tolerance of peas for hypoxia. Cytoplasmic acidosis is accelerated in maize root tips that are either (i) deficient in alcohol dehydrogenase, so that lactic acid production continues throughout hypoxia, or (ii) exposed to external CO2 during hypoxia, or (iii) perfused slowly so that escape of CO2 produced during ethanolic fermentation is retarded. All three conditions decrease the length of time maize root tips can tolerate hypoxia; more rapid cytoplasmic acidosis is associated with more rapid death under hypoxia. Possible mechanisms by which cytoplasmic acidosis leads to death are suggested; the mechanism does not involve inhibition of glycolysis by low pH.

A specific temperature-dependent conformational transition of hen egg-white lysozyme, occurring between 20 degree C and 30 degree C in solution, has been detected by 13-C-nuclear magnetic resonance spectroscopy. Selective changes in the chemical shifts of aromatic residues, together with differences in the chemical shifts, and nuclear Overhauser enhancement in the carbonyl, carboxyl, and alpha-carbon regions of the spectrum point to the vicinity of subsites D and E as the primary locus of the structural change.

We have measured the 31P and 1H NMR parameters of fractionated double-stranded DNA fragments 600, 300, and 150 base pairs long. The results indicate that, inside an intact double helix, both the deoxyribose and the sugar-phosphate backbone of the helix fluctuate substantially from their equilibrium geometry. The time constant for those coupled motions is of the order of 1 nsec.

The solution structures of the trp-repressor from Escherichia coliin both the liganded (holo-) and unliganded (apo-) form, have been refined by restrained molecular dynamics with simulated annealing using the program XPLOR and additional experimental constraints. The ensemble of refined holorepressor structures have a root-mean-square deviation (r.m.s.d.) of 0·8 Å relative to the average structure for the backbone of the dimer core (helices A, B, C, A′, B′, C′) and 2·5 Å for the helix-turn DNA-binding domain (helices D and E). The corresponding values for the aporepressor are 0·9 Å for the backbone of the ABC-dimer core and 3·2 Å for the DE helix-turn-helix. The r.m.s.d. of the average structures from the corresponding crystal structures are 2·3 Å for the holorepressor ABC core and 4·2 Å for its DE region; 2·3 Å for the aporepressor core and 5·5 Å for its DE region. The relative disorder of the DNA-binding domain is reflected in a number of experimental parameters including substantially more rapid backbone proton exchange rates, exchange-limited relaxation times and crystallographic B-factors. The stabilizing effect of the l-Trp ligand is evident in these measurements, as it is in the higher precision of the holorepressor structure.

We show that a transient lactic fermentation provides the signal triggering ethanol production in hypoxic maize root tips. The signal is cytoplasmic pH. This interaction between lactic and ethanolic fermentation permits tight cytoplasmic pH regulation during hypoxia--cytoplasmic pH remaining near neutrality for several hours. Mutant roots unable to synthesize ethanol can neither regulate cytoplasmic pH nor maintain ATP levels during extended periods of hypoxia and, like vertebrate tissues, are less tolerant of hypoxia than normal maize. This indicates that cytoplasmic pH regulation is an important factor in survival under hypoxia.