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A newly constructed pulsed nozzle, Fourier-transform microwave spectrometer utilizes a Fabry-Perot cavity consisting of spherical resonators having diameters of only 10 cm. Tests of this very compact-cavity system show that its sensitivity is only slightly lower than that of a comparably configured system of the Balle-Flygare design having resonators with diameters of 36 cm. With a volume 50 times smaller than in conventional systems, the compact cavity also requires a much smaller vacuum chamber which can be pumped by a relatively small 6 in. diffusion pump. The system includes an integral ceramic nozzle which can be heated to temperatures above 1000 "C. Spectrometer characteristics have been demonstrated by means of experiments on OCS isotopomers in ground and excited vibrational states, ArOCS complexes, and chloroketene, a reactive intermediate formed by pyrolysis of chloroacetylchloride. 0 1995 American Institute qf Physics.

Calmodulin is an EF hand calcium binding protein. Its binding affinities to various protein/peptide targets often depend on the conformational changes induced by the binding of calcium. One such target is melittin, which binds tightly to calmodulin in the presence of calcium, and inhibits its function. Chemical cross-linking combined with Fourier transform ion cyclotron resonance mass spectrometry has been employed to investigate the coordination of calmodulin and melittin in the complex at different concentrations of calcium. This methodology can be used to monitor structural changes of proteins induced by ligand binding, and study the effects these changes have on non- covalent interactions between proteins. Cross-linking results indicate that the binding place of the first melittin in the calcium free calmodulin form is the same as in the calcium loaded calmodulin/melittin complex.

state and gas-phase structures recently noted for (CH3)3N-S0227 (0.21 Á) is encouraging in this respect, as it demonstrates the possibility of a substantial "dynamic range" in N-S bond lengths. ConclusionMicrowave spectroscopy has been used to observe the gas-phase adduct formed between CH3CN and BF3. The structure has been determined to be intermediate between the limits normally expected for van der Waals and covalent interactions and is significantly different from that observed in the crystalline material. The correlation between the boron-nitrogen bond length and the N-B-F angle in a series of BX3 species with nitrogen donors provides the first gas-phase example of a structure correlation analogous to those used frequently to map reaction paths in crystalline systems.Acknowledgment. The support of the National Science Foundation (Grant No. CHE-8807895) and the McKnight Foundation is gratefully acknowledged. We are also grateful to Ms. Robin Jurgens-Lutovsky and Prof. Jan Alrnlof for making the results of their ab initio study known to us prior to publication and to Prof. R. L. Kuczkowski for providing us with a preprint of his work on (CH3)3N-S02.