Cassini radio science investigations will be conducted both during the cruise (gravitational wave and conjunction experiments) and the Saturnian tour of the mission (atmospheric and ionospheric occultations, ring occultations, determinations of masses and gravity fields). New technologies in the construction of the instrument, which consists of a portion on-board the spacecraft and another portion on the ground, including the use of the Ka-band signal in addition to that of the S-and X-bands, open opportunities for important discoveries in each of the above scientific areas, due to increased accuracy, resolution, sensitivity, and dynamic range.

Mars Observer radio science investigations focus on two major areas of study: the gravity field and the atmosphere of Mars. Measurement accuracies expressed as an equivalent spacecraft velocity are expected to be of the order of 100 μm/s (for both types of investigations) from use of an improved radio transponder for two‐way spacecraft tracking and a highly stable on‐board oscillator for atmospheric occultation measurements. Planned gravity investigations include a combination of classical and modern elements. A spherical harmonic (or equivalent) field model of degree and order in the range 30–50 will be obtained, while interpretation will be in terms of internal stress and density models for the planet, using the topography to be obtained from the Mars Observer laser altimeter. Atmospheric investigations will emphasize precision measurement of the thermal structure and dynamics in the polar regions, which are regularly accessible as a result of the highly inclined orbit. Studies based on the measurements will include polar processes, cycling of the atmosphere between the poles, traveling baroclinic disturbances, small‐scale waves and turbulence, the planetary boundary layer, and (possibly) the variability and altitude of the ionosphere. As the radio occultation is insensitive to dust in the atmosphere per se and measures only the resulting change in thermal structure, it is expected that the radio technique can contribute to understanding of dust storm phenomena. Mutual observations of the atmosphere by means of radio occultation and by the pressure modulator infrared radiometer and the thermal emission spectrometer are expected to strengthen the reliability and accuracy of all three investigations.

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The paramagnetic resonance of a spin-label attached to a protein in solution, or in a single crystal, depends on static as well as on the dynamic features of its molecular environment.' This conformation-dependent paramagnetic resonance offers the possibility of relating structural properties of proteins in single crystals to those in solutions. The present paper is a brief report of a study of this type.Here we compare the paramagnetic resonance spectra of spin-labeled horse hemoglobin in solution, and in single crystals, for the carbonmonoxyand acid met (ferric) derivatives. The present work was stimulated by an earlier, puzzling observation that the paramagnetic resonance spectra of spin-labeled horse oxy-and acid methemoglobin in solution are significantly different,2 whereas Perutz and co-workers have shown that these two molecules must have very similar secondary and tertiary structures, and identical quaternary structures in the crystalline state. 3 4 Materials and Methods.-The spin-label used in the present work is N-This label, analogous to the five-membered nitroxide ring iodoacetamide label described previously,2 was prepared as follows: Commercially available 2,2,6,6-tetramethyl-4-aminopiperidine (I) was acetylated at O0 in acetic anhydride.-The resulting 2,2,6,6-tetramethyl-4-acetamidopiperidine (II) was treated with hydrogen peroxide in the standard manner6' 7 to give the corresponding nitroxide (III) in 65% yield (based on the starting material I) after recrystallization from toluene, mp 139-141' (uncorrected). Hydrolysis of III with refluxing 10% sodium hydroxide gave a 72% yield of hygroscopic 2,2,6,6-tetramethyl-4-aminopiperidine-1-oxyl (IV), which was purified by distillation, bp 61-63°(1 mm). The product crystallized in the receiver. Coupling of (IV) with iodoacetic acid that was freshly recrystallized from benzene-hexane was accomplished with pure dicyclohexylcarbodiimide in ethyl acetate.8 Chromatography on neutral alumina followed by recrystallization from toluene-hexane gave label V in 60% yield, mp 118.5-120.5o (uncorrected).Horse hemoglobin was prepared by the method of Benesch and Benesch.9 The hemoglobin was labeled with the iodoacetamide V, using procedures similar to those described previously.2 The crystals were prepared with a recipe kindly given to us by Dr. M. F. Perutz:4 Hemoglobin crystals were obtained from a series of vials, each containing 2 ml of 2% labeled carbon monoxide hemoglobin (HbCO), or methemoglobin (met Hb), and between 2 and 3 ml of solution "A." Solution "A" contained 2 vol of 4 M (NH4)2SO4 and 1 vol of 2 M (NH4)2HP04. After crystals were formed they were "cured" by addition of ca. 0.3 ml of a 1.8 M NH4H2PO4 solution to 4-5 ml of the suspension medium. This brought the final crystal pH to 6.9 or 7.0. No significant changes in the resonance spectra of the single crystals were detected in the range pH 6.8-7.2. In the present study we used crystals of HbCO rather than oxyhemoglobin (HbO2) in order to reduce the likelihood of contamination by met Hb. 77"Downloaded...

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