  We use an instrument combining optical (surface plasmon resonance) and
acoustic (Love mode acoustic wave device) real-time measurements on a same
surface for the identification of water content in collagen and fibrinogen
protein layers. After calibration of the surface acoustic wave device
sensitivity by copper electrodeposition, the bound mass and its physical
properties -- density and optical index -- are extracted from the complementary
measurement techniques and lead to thickness and water ratio values compatible
with the observed signal shifts. Such results are especially usefully for
protein layers with a high water content as shown here for collagen on an
hydrophobic surface. We obtain the following results: collagen layers include
70+/-20 % water and are 16+/-3 to 19+/-3 nm thick for bulk concentrations
ranging from 30 to 300 ug/ml. Fibrinogen layers include 50+/-10 % water for
layer thicknesses in the 6+/-1.5 to 13+/-2 nm range when the bulk concentration
is in the 46 to 460 ug/ml range.
