Covalently linked hydroxycinnamic acids

In addition to ester-linkages, hydroxycinnamic acids can also be covalently linked to cell wall components via phenyl ether bonds. While this has only been demonstrated for wheat (Triticum aestivum L.) straw lignin... 

The Photoactive Yellow Protein (PYP) is thought to be the photoreceptor responsible for the negative phototaxis of the bacterium Halorhodospira halophila [1]. Its chromophore, the deprotonated 4-hydroxycinnamic (or p-coumaric) acid, is covalently linked to the side chain of the Cys69 residue by a thioester bond. Trans-cis photoisomerization of the chromophore was proved to occur during the early steps of the PYP photocycle. Nevertheless, the reaction pathway leading to the cis isomer is still discussed (for a review, see ref. [2]). Time-resolved spectroscopy showed that it involves subpicosecond and picosecond components [3-7], some of which could correspond to a flipping motion of the chromophore carbonyl group [8,9]. 

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