Sentence:  Because negative pressure dialysis gave better recovery of proteins , permitted detection of <WSD>proteins</WSD> concentrated from very dilute solutions and was a gentler procedure , it was used in all but the earliest experiments . 
Meaning of the word marked with <WSD> tags:  any of a large group of nitrogenous organic compounds that are essential constituents of living cells; consist of polymers of amino acids; essential in the diet of animals for growth and for repair of tissues; can be obtained from meat and eggs and milk and legumes