The effect of high pressure on the crystal structures of -, -and -glycine has been investigated. A new polymorph, δ-glycine, is obtained from  glycine. δ-Glycine is monoclinic, P2 1 /a, a = 11.156(4), b = 5.8644(11), c = 5.3417(17) Å,  = 125.83(4)° at 1.9 GPa. The transition, which occurs between 0 and 0.8 GPa, proceeds from a single crystal of -glycine to a single crystal of δ-glycine, resulting in an equal number of NH…O hydrogen bonds, but an increase in the number and strength of CH…O hydrogen bonds, which act to close-up of 'holes' which are formed within the layers of -glycine in the centers of R-type hydrogen bonded motifs. Trigonal -glycine begins to undergo a transition to another high-pressure phase, -glycine, at 1.9 GPa, but the transformation is destructive; it is essentially complete at 4.3 GPa. The structure is monoclinic Pn, a = 4.8887(10), b = 5.7541(11), c = 5.4419(11) Å,  = 116.682(10)° at 4.3 GPa. The structure consists of layers similar those observed in -glycine with inter-layer separations of 2.38 and 3.38 Å and CH…O interactions formed between the layers. Monoclinic -glycine is known to be stable to 23 GPa, and we have obtained a single crystal structure of this polymorph at 6.2 GPa. Super-short NH…O hydrogen bonds are not formed up to 6.2 GPa, and they only shorten significantly if they are formed parallel to CH…O hydrogen bonds which strengthen, or vectors across holes which close-up, under pressure.

Methane hydrate is thought to have been the dominant methane-containing phase in the nebula from which Saturn, Uranus, Neptune and their major moons formed. It accordingly plays an important role in formation models of Titan, Saturn's largest moon. Current understanding assumes that methane hydrate dissociates into ice and free methane in the pressure range 1-2 GPa (10-20 kbar), consistent with some theoretical and experimental studies. But such pressure-induced dissociation would have led to the early loss of methane from Titan's interior to its atmosphere, where it would rapidly have been destroyed by photochemical processes. This is difficult to reconcile with the observed presence of significant amounts of methane in Titan's present atmosphere. Here we report neutron and synchrotron X-ray diffraction studies that determine the thermodynamic behaviour of methane hydrate at pressures up to 10 GPa. We find structural transitions at about 1 and 2 GPa to new hydrate phases which remain stable to at least 10 GPa. This implies that the methane in the primordial core of Titan remained in stable hydrate phases throughout differentiation, eventually forming a layer of methane clathrate approximately 100 km thick within the ice mantle. This layer is a plausible source for the continuing replenishment of Titan's atmospheric methane.

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